Research Narrative

Each year ASP awards appointments as postdoctoral fellows or as graduate fellows to a number of new scientists. These appointments fulfill several purposes: to permit scientists near the beginning of their careers to work with groups at the forefront of the science, to ensure that there will be in the university and government communities scientists who are familiar with NCAR's capabilities, and to add energy and creativity to the NCAR research programs. While at NCAR, fellows work directly within ongoing research programs. The efforts of some of the graduate fellows and postdoctoral fellows are described below.

Also described below are efforts of the scientists of the Geophysical Turbulence Program (GTP), the Senior Research Associates (SRA's) and the NCAR Aerosol Program (NAP).

NCAR Graduate Fellows Research

During the fiscal year, the graduate program changed direction to bring in new graduate fellows focused in one area of research based on the 2002 colloquium topic. While in residence at NCAR, each graduate fellow carries out research based on his or her thesis proposal, which must be endorsed jointly by the university thesis advisor and an NCAR scientist. Listed below are research summaries from graduate fellows finishing appointments during this fiscal year.

Judith Berner (CGD) studied to what degree nonlinearities in the governing equations of atmospheric motion play a role for the dynamics on interannual timescales. Together with Grant Branstator (CGD) she described non-Gaussian features in the probability density function (PDF) of the state vector. In addition, they investigated the use of trajectories through phase space as an alternative approach. They find that distinct signatures of nonlinearity can be seen in the distribution of mean phase space tendencies for states from an extended integration of CCM0. By fitting a nonlinear stochastic model with a drift vector equal to the mean tendencies of the General Circulation Model (GCM), they find, that the non-Gaussian features in the GCM's PDFs can be attributed to the nonlinearities in the mean tendencies.

Another part of Berner's work addressed the question on which timescales a deterministic system with finitely correlated noise -- such as the physical processes modeled by a GCM-- can be described by a stochastic process. If such a timescales exists, the stochastic model can be used to describe higher-order moments of the distribution of states as they evolve with time. She demonstrated that for the GCM such a timescale exists and that a nonlinear stochastic model is able to model essential non-Gaussian behavior in the temporal evolution of the GCM states.

Amanda Cox (MMM) continued evaluating the impact of different calibration schemes with algorithms on the data quality of the Airborne Imaging Microwave Radiometer (AIMR) which flies on the NCAR C-130. Recent work focuses on uncertainty reduction and image quality improvement through the application of calibration algorithms to reduce image streaking. She developed an active microwave beacon as a geolocation aid for validation of the Advanced Microwave Scanning Radiometer (AMSR) sea-ice products. This beacon provides a single bright pixel in passive microwave image data which has a very well-known location obtained using a survey-grade GPS receiver. The bright pixel is useful for precise geo-registration in geophysical applications where standard methods of selecting ground control points using features identifiable in different image types are problematic. The planned deployment for this device is in the AMSR validation effort in Spring of 2005. She also collaborated with James Randa, David Walker and Robert Billinger of NIST to study the errors resulting from the reflexivity of calibration targets. This research is guided by Larry Radke (ATD) and Judith Curry (Georgia Tech).

So-Young Ha (COSMIC) has been working on the data assimilation of ground-based GPS water vapor measurements through and after her Ph.D. thesis job. She succeeded in assimilating ground-based GPS slant-path water vapor measurement, a new kind of observing data, into the numerical forecast model and demonstrated that the variational data assimilation of the data can potentially improve the short-range rainfall prediction in the severe weather system.

Postdoctoral Fellows Research

ASP Postdocs work with all science and technology divisions in NCAR. Listed below, by broad topic area, are some examples of projects listed by Fellow (with division or program affiliation). Links to corresponding activities are highlighted.

Meteorology

Beres's work provides information needed to implement a source spectrum parameterization of convectively generated gravity waves based on the properties of underlying convection. Preliminary results of this work show that the gravity wave spectrum forced by convection at 100 mb has a strong latitudinal dependence as shown in Figure Y1. In the tropics gravity waves are generated with a broad range of phase speeds and carry both eastward and westward momentum. Such a distribution is caused by deep convection in an environment with little vertical wind shear. At higher latitudes however, where tropospheric winds are mostly eastward and convection is shallower, gravity waves are generated with low phase speeds and carry mostly westward momentum flux. This implies that gravity waves forced by mid-latitude convection are an unlikely source of eastward momentum in the mesosphere needed to produce the cold summer mesopause. Beres is currently exploring other sources of the mesospheric forcing.

Judith Berner (CGD) is working on the development of two approaches to stochastic parameterization. The first, more common approach, is to use models or measurements that resolve the process that shall be parameterized. Estimates of the spatial and temporal moments are used to guide the stochastic parameterization of this process in a larger-scale model. The second approach tries to solve the inverse problem by asking: Which effective forcing is needed to get the correct moments in the resolved variables?

George Bryan (MMM) utilizes numerical simulations to understand the structure of mesoscale convective systems. Unexpected findings from these studies contradicts the long-standing conclusion that precipitation from squall lines increases as environmental vertical wind shear increases. Two approaches are being used to challenge this common belief. First, a numerical model intercomparison is being performed to address whether these findings are dependent on the numerical model that is used. Bryan is using a numerical model he developed at Penn State University, along with the Weather Research and Forecasting Model (with Jason Knievel of MMM) and the Advanced Regional Prediction System (with Matthew Parker of the University of Nebraska). Preliminary results show that numerical techniques specific to individual models can affect the total precipitation in the simulation. Consequently, some of the models predict less precipitation as wind shear increases-a result contradictory to previous research. The second approach has been to repeat the simulations using significantly higher resolution. The supercomputers of the Scientific Computing Division (SCD) were utilized for this purpose. The higher resolution allows for convective organization that could not be resolved previously. In particular, it is found that long-lived convective plumes are a common feature in low-shear environments, which partially explains the unexpectedly high precipitation in these conditions.

Huaqing Cai (ATD) worked on a detailed case study of a dryline from the International H2O Project (IHOP). His objective in this study was to document the detailed kinematic and thermodynamical structure of the dryline and try to understand why convection does not initiate along this particular portion of the dryline. Another research topic Cai pursued was to distinguish between tornadic and non-tornadic mesocyclones using fractal geometry. The preliminary results are very promising, and his future plans are to try to finish more cases so the conclusion can be more robust in a statistical sense.

David Dowell (MMM) compared the results of various methods of retrieving the wind and thermodynamic fields in convective storms from Doppler radar observations. He examined the results of both a "traditional" analysis method and a relatively new "ensemble Kalman filter" data-assimilation method, for both simulated cases and real data cases. This work has applications for both improving case-study analysis and initialization of high-resolution numerical forecast models.

Dowell also studied the tornadogenesis process in supercell storms, as revealed by mobile radar observations; modeled the centrifuging of particles in tornadoes and quantified the associated contamination of Doppler wind analyses. Together with SOARS protégé Amber Reynolds, studied the environments of bow echoes that do and do not produce swaths of damaging winds.

David Gochis' (RAP) research can best be described as focusing on hydrometeorological and hydroclimatological processes in regional climates, rainfall-runoff responses, land-atmosphere exchanges and terrain-induced convection. The study of convective and surface hydrological processes has taken David's research at NCAR along two interdisciplinary courses. The first path has been to explore improved methods for representing surface hydrological processes in the NCAR suite of land surface models. This work has culminated in the implementation of a physically based, explicit, surface and subsurface routing methodologies in the community Noah land surface model, which is the primary land surface parameterization for the WRF, MM5 and NCEP Eta weather forecast models. With Dr. Fei Chen (RAP), Dr. Gochis has completed offline development and testing of a hydrologically enhanced version of Noah (now called 'Noah-router') and has summarized this adaptation in a recently submitted NCAR Science and Technical Report.

The second course of Gochis' research interest resides in studying terrain-induced convection over western North America. The convective initiation process over complex terrain is central to the understanding of warm season climates such as the North American Monsoon (NAM). His past work in Mexico along with other recently published work has begun to identify certain commonalities in the warm season convective regime.

Ensemble forecasts can never be perfect because of errors in the forecast model. Useful probabilistic forecasts and ensembles for data assimilation purposes should be calibrated to account for the model error. This is normally achieved with climatological error statistics, but error is highly flow-dependent. One particular class of model error, deficient spatial variance properties, results in under-dispersive ensembles that cannot envelope the true evolution of the atmosphere. A collaboration between Joshua Hacker (MMM/CGD) and David Baumhefner (CGD) produced a simple scale- and flow-dependent calibration that corrects for this type of error. It depends only on the current forecast case, and requires only one superior (typically more expensive) forecast to compute calibration coefficients in Fourier space. Its performance is summarized by the error-growth curves in Fig. 1 where curve damped model ensemble spread (DMP) is corrected to curve calibrated ensemble spread (COR), which agrees with the "correct" error growth in Weather Research Forecast Model (WRF) ensemble spread. The effect of other sources of model error, some of which cannot be corrected with this method, is measured as the residual error-growth differences after calibration. The calibration has applications to ensembles with limited-area models and ensemble-based data assimilation. A manuscript and presentation slides are available at http://www.mmm.ucar.edu/individual/hacker/.

Surface-layer (screen-height) observations, such as exist in a typical mesonet, are under-utilized in current data assimilation (DA) algorithms because of weak coupling with the free atmosphere aloft. But simulation and short-range forecasts of near-surface conditions could benefit from these data. The Ensemble Kalman Filter data assimilation algorithm, which uses anisotropic and flow-dependent covariance information to spread the influence of an observation, is appropriate for this task. Collaboration between Hacker and Chris Snyder (MMM) used a column PBL model to successfully assimilate simulated observations. The results (click here for pdf) showed significant error reduction for temperature (a), wind (b), and moisture (c). This places an upper bound on the advantage of using surface observations to specify the state of the Planetary Boundry Layer (PBL). Hacker and Snyder also performed parameter estimation experiments to mitigate the negative effects of simulated model error, showing that the observations can be useful to correct erroneous parameters. The successful assimilation suggests that the potential exists for surface observations to improve numerical simulations and forecasts of air-pollution events, convective outbreaks, and cyclogenesis where PBL preconditioning is important.

Jan Kazil (HAO) models the aerosol formation in the troposphere and lower stratosphere in the presence of atmospheric ions, using new laboratory thermochemistry data of negative sulfuric acid/water clusters buildup. Ion production rates due to Galactic Cosmic Rays (GCR), covering different atmospheric regions, are modeled using the GEANT software (Detector Description and Simulation Tool) developed at the European Laboratory for Particle Physics (CERN). This approach allows to investigate and compare the aerosol production due to binary and ion-induced nucleation in various conditions and at different phases of the solar cycle. This work is done in collaboration with Edward R. Lovejoy (NOAA Aeronomy Lab, Boulder) and Laurent Desorgher (University of Bern, Switzerland). At the same time, Kazil works on a parametrization of ion induced aerosol formation for use in General Circulation Model (GCM) based on model results of ion induced nucleation. This work is performed in collaboration with Edward R. Lovejoy (NOAA) and Sachchida Tripathi (Indian Institute of Technology, Kanpur, India).

In collaboration with Donald H. Lenschow (MMM) and with Dave Leon and Gabor Vali (both from the University of Wyoming), Marie Lothon (MMM) worked on the feasibility of estimating the turbulence characteristics in marine stratocumulus using the Wyoming Cloud Radar (WCR, http://www-das.uwyo.edu/wcr/), mounted on the NCAR C-130 during DYCOMS-II (DYnamics and Chemistry Of Marine Stratocumulus, http://www.atmos.ucla.edu/ bstevens/dycoms/). The aim of Marie Lothon is to delineate turbulence structure as a function of height throughout the drizzling marine boundary layer, using the Doppler velocity measurements. As the spectral width was not stored, she used the fine-structure of the Doppler velocity field to deduce the turbulence characteristics, especially turbulence dissipation and integral scales. One essential step for this study was to appraise the fluctuations in Doppler velocity due to the fluctuations in terminal fall velocity of hydrometeors. Using microphysics probe measurements, Marie Lothon estimated this contribution from the spatial distribution of the drop counts in each bin. She found that the counts observed by the probes have a lognormally varying Poisson distribution and that the fluctuations in reflectivity-weighted fall velocity have a weak effect on the Doppler velocity variance. As a second important step, Marie Lothon also studied the effect of the velocity averaging within the pulse resolution volume. She still need to correct the dissipation estimates for this effect, but she obtained profiles of the integral scales within the boundary layer, which show how `squashed' the turbulence is at the top and bottom of the boundary layer in these summer anticyclonic conditions.

The jet stream in the mid-latitudes is known from observations to be an important source of inertia-gravity waves (IGW). However, the mechanisms responsible for the generation of IGW from balanced motions such as a jet are yet poorly understood, let alone quantified. They are relevant to parameterizations of gravity wave sources in climate models including a middle atmosphere (the Whole Atmosphere Community Climate Model, for example), as well as to fundamental questions on the limitations of validity of balanced models. Riwal Plougonven (MMM) contributed to the study of these themes using theoretical and numerical tools as well as observations. In collaboration with H. Teitelbaum and V. Zeilin (Laboratoire de Meteorologie Dynamique, Paris, France), an analysis of inertia-gravity waves generated by the jet over the North Atlantic was conducted, using observations (radiosondes) collected during the Fronts and Atlantic Storm-Tracks Experiment (FASTEX). Configurations of the jet most favorable to gravity wave generation and detailed case studies of wave generation events were obtained. The issue of how operational models describe large-scale inertia-gravity waves that are under-resolved was addressed. Possible applications for observational studies were given. Observational studies have provided a number of case studies of events of gravity wave generation by the jet. It is necessary to make progress in the understanding of the dynamical mechanism responsible for the waves to be able to quantify this generation mechanism.

Wen-wen Tung (MMM) worked on employing multifractals to characterize the multiscale convective systems in the tropics. The complex systems from diurnal to synoptic time scales are described with multifractal dimensions. A multiplicative cascade model was used to reproduce the dimensions. The techniques and results can be used to objectively judge how skillful a model can realize the tropical convection, particularly its ability to capture extreme events. The research results have been written into a paper which has been submitted to J. Climate with coauthors Mitchell Moncrieff (MMM) and Jian-Bo Gao (University of Florida). Similar techniques as well as wavelet analysis will be applied to climate-scale datasets including outgoing longwave radiation, convective heating and drying derived from the ECMWF Reanalysis, (ERA) as well as model realizations of NCAR CAM (GCM).

During the same time, Tung collaborated with George Kiladis (NOAA) and studied the climatological structures of dynamic and thermodynamic fields associated with tropical disturbances including the Madden-Julian Oscillation (MJO) in the ERA. Previous studies suggested that the representation of the moist process is crucial in realizing MJO in GCMs; therefore, the work will be focused on how the aforementioned two fields are coupled. In addition, with Moncrieff and Wojciech Grabowski (MMM), Tung will investigate how well state-of-the-art cloud-resolving cloud parameterization captures these structures. Since the 40-year ERA is now available, one of the side projects will be to address the tropical circulations in ERA-40 and the older ERA-15, as well as the different cloud types, convective heating, and convective moistening implied by the two reanalyses.

Chemical Modeling

Kimberly Mace's (ACD) work focuses on the sources and fate of water-soluble organic nitrogen in the atmosphere. She works with international and local scientists to determine the role of biomass burning, dust, and confined animal operations in the delivery of water soluble nitrogen to and from the global atmosphere. Her work also focuses on using analytical methods to determine the specific compounds comprising the water-soluble atmospheric organic nitrogen fraction.

The chemical and physical conditions required for the formation of new particles in the atmosphere from the gas-phase is not well understood. While classical binary nucleation of sulfuric acid and water vapors can explain some ambient observations, observed rates often vary from theory by orders of magnitude. Much recent work has explored the potential for additional species (e.g. ammonia) to play significant roles in new particle formation. Techniques for the physical observation of new particle formation events in the atmosphere are reasonably well-established and new particles that have grown in size to 3 nm can be consistently detected. However, techniques to determine the chemical composition of these ultrafine particles are mostly indirect, relying upon the particle’s physical response to changes in humidity or volatilization, for example, to characterize the species involved. Recently a new instrument ­ the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) ­ was developed at NCAR to directly measure the chemical composition of newly formed and ultrafine atmospheric particles. Katharine Moore (ACD) has successfully deployed the TDCIMS for observations in Boulder, CO to determine the chemical composition of the local ambient "background" sub-20 nm particles and establish the quantitative basis for these results. These observations build on her earlier participation in the August 2002 Aerosol Nucleation and Real-time Characterization Experiment studying new particle formation events in Atlanta, GA (Atlanta-ANARChE). Her research also focuses on the continuing development, evaluation and characterization of the TDCIMS. This includes identifying the TDCIMS' sensitivity to variations in operational and instrumental parameters and improving the data interpretation methodology.
A more complete description of the TDCIMS and on-going projects can be found at http://acd.ucar.edu/~jimsmith/POP/.

Mark Potosnak (ACD) studies the exchange of reduced carbon between terrestrial ecosystems and the atmosphere, with particular emphasis on emissions from tropical ecosystems and how changing environmental conditions will affect future plant emissions at the leaf level. Potosnak made two field campaigns this year to study whole ecosystem emissions of isoprene (a volatile organic compound emitted by many plant species) from tropical rainforests. The first campaign was to the La Selva Biological Station in Costa Rica, where he joined Thomas Karl (ACD) and Alex Guenther (ACD), Jed Sparks (Cornell University), and John Walker and Jeff Herrick (both from the US EPA). The group measured leaf level and whole system isoprene emissions, and also focused on NOx and ozone chemistry. The second field campaign was to a site in Brazil that is part of the Large scale Biosphere-atmosphere experiment in Amazonia (LBA) run by the group of Steven Wofsy (Harvard University). In collaboration with, Luciana Vanni Gatti and Simone Avino (both from the University of Sao Paulo), he measured whole system fluxes of isoprene with the Fast Isoprene Sensor, which was developed at NCAR by Alan Hills (RAP). These two field campaigns were made during their respective sites dry seasons. Measurements were compared to results from previous campaigns conducted during the wet season by other members of the Biosphere-Atmosphere Interactions group at NCAR led by Alex Guenther. The results show that precipitation regime controls whole system isoprene emissions in the tropics, and this research was presented at the American Geophysical Union's Fall Meeting.

Potosnak also continued research concerning the effects of elevated carbon dioxide concentrations on leaf level and whole system isoprene emissions. Collaborating with researchers Todd Rosenstiel, Russ Monson and Ray Fall (all from the University of Colorado) and Kevin Griffin (Columbia University), a mechanism was proposed to explain the effect of inter-cellular carbon dioxide concentration on leaf level isoprene emissions. This research was extended to measurements made at the Free Air Carbon Enrichment (FACE) experiment in the Duke Forest as part of the Chemical Emission Loss, Transform and Interactions within Canopies (CELTIC) campaign led by Alex Guenther (ACD) and Chris Geron (US EPA). The FACE experiment subjects small patches of a pine plantation forest to elevated carbon dioxide concentrations. Leaf level measurements of isoprene emissions from a understory species, sweetgum, showed no clear effect of carbon dioxide concentration on isoprene emissions, but there was a strong relationship between isoprene emission rates and light environment. Since light regimes are strongly affected by carbon dioxide concentration, a modeling study will be conducted to determine how whole system isoprene emissions are influenced by elevated carbon dioxide concentration.

Craig Stroud (ACD) worked on two numerical modeling projects this past year and participated in the NCAR-led Chemical Emission, Loss, Transformation and Interactions within Canopies (CELTIC) field program. The first project involved developing a chemically-based model to simulate organic aerosol formation within smog chambers. He successfully coupled an explicit gas-phase organic mechanism with gas-aerosol equilibrium partitioning theory. Model-measurement comparisons showed an interesting dependence on existing particle mass concentration and suggested that the equilibrium assumption in the model was breaking down at low aerosol loadings. He also addressed the question of the relative importance of the non-volatile to the semi-volatile fraction in the aerosol. His modeling results suggested that the aerosol-phase was composed largely of a few non-volatile species.

Stroud's second project involved modeling ozone photochemistry in the arctic free troposphere. His research supported the theory that the springtime ozone maximum in the arctic free troposphere is driven by in situ photochemistry rather than transport from other regions. His research illustrated the importance of pernitric acid as a temporary sink for odd nitrogen in the arctic. His research also uncovered significant model-measurement differences for CH3CHO concentrations, suggesting possible heterogeneous pathways not yet discovered for this species.

Stroud also participated in the CELTIC field program at Duke Forest, North Carolina and acquired measurements of canopy-level radiation, particle size distributions and cloud condensation nuclei concentrations. He plans to incorporate these results into a canopy-scale chemical-transport model to assess the importance of canopy-level photochemistry to trace gas and particle emissions from the forest surface layer.

Learning Processes and Societal Impacts

Solar influences

The ionospheric and magnetospheric origins of ions flowing out of the Earth's system are a hot topic in the Geospace Environment Modeling community right now. The acceleration mechanisms and geophysical conditions under which ion outflow occurs are key aims of current investigations. Gang Lu (HAO) and Kavanagh began work on incoherent scatter radar measurements from the April 2002 geomagnetic storm. These radars are particularly well suited to observing the ionospheric portion of upwelling ions and the aim is to combine these observations with satellite data to build a consistent picture of how the ions flow to magnetospheric altitudes.

Despite the importance of overshoot at the base of the solar convection zone for the storage of strong toroidal magnetic field produced there by the solar dynamo, the uncertainties concerning the depth and mean subadiabatic stratification remain large. Overshoot models based on the non-local mixing-length theory generally produce a shallow weakly subadiabatic region with a sharp transition to the radiative interior, whereas several numerical simulations lead to significantly subadiabatic overshoot with penetration depth of more than a pressure scale height.

Matthias Rempel (HAO) developed a semi-analytical model for the solar convection zone and underlying overshoot region, based on the assumption that the convective energy flux is governed by downflow structures with a low filling factor. This approach allows for modeling both the parameter regime addressed by non-local mixing-length approach as well as the regime addressed by numerical simulations. It turns out that the main differences between the non-local mixing-length approach and numerical simulations (nearly adiabatic versus strongly subadiabatic overshoot) are caused by the much larger energy flux used in numerical simulations due to numerical constraints. The depth of the overshoot region is determined predominantly by the mixing between downflows and upflows in the convection zone. Furthermore the model shows that the sharp transition between the nearly adiabatic overshoot and radiative interior, a typical result of the non-local mixing-length approach, can be avoided by assuming an ensemble of downflows with different strength (Rempel, ApJ, submitted and undergoing review).

For the solar magnetic cycle the storage and stability of strong toroidal magnetic field in the solar tachocline are of crucial interest. The equilibrium and stability of toroidal magnetic field configurations in combinations with the solar differential rotation has been studied at HAO by Mausumi Dikpati and Peter Gilman using the magnetohydrodynamic (MHD) shallow-water approach. In these studies the magnetic curvature stress was balanced by an equatorward hydrostatic pressure gradient created by extra mass on the poleward side of the toroidal magnetic field band.

Rempel generalized in collaboration with Dikpati the MHD shallow-water approach to include equilibria in which the poleward magnetic curvature stress is balanced by the Coriolis force associated with a prograde jet inside the band and analyzed the relation of the MHD shallow-water approach to a full MHD description (Rempel & Dikpati, 2003, ApJ, 584, 524). In collaboration with Dikpati and Gilman the stability of banded toroidal magnetic field in the solar tachocline was investigated, showing that the presence of the jet stabilizes the magnetic field for a field strength above 40kG and suppresses significantly the tipping instability found in previous studies (Dikpati, Gilman, & Rempel 2003, ApJ, 596, 680).

Climate

Philippe Naveau (University of Colorado) and Hee-Seok Oh (University of Alberta, Canada) and Ammann have generated and applied new statistical tools to extract external forcing fingerprints from climate time series. Using discrete wavelet decomposition on decadal to century time scales, the detection was successful in isolating potential solar influence in different multi-proxy reconstructions and simplify the detection in the coupled model simulations. For the very short-lived volcanic effects, a state-space model approach was chosen that allows quantification of the volcanic cooling with associated posterior probability. Efforts are currently undertaken to expand these methods into a spatio-temporal framework. One sideline of this research has led Ammann and Naveau to study the occurrence intervals of large eruptions. Using the ice core data, the frequency of large tropical events was found to follow a rather regular fluctuation of currently unknown origin (http://www.cgd.ucar.edu/ccr/ammann/76).

Judith Berner collaborated with Grant Branstator (CGD) to address the effects of a limited sample size for the estimation of the probability density functions of observed states. Their works demonstrates that multiple modes found in GCM datasets comparable in length to that of observed states are artifacts of the short sample size and disappear when more data are used.

Richard Cullather (CGD) has been working with Climate Analysis Section scientists in the Climate and Global Dynamics Division on topics related to the climate and tropospheric circulation in the polar regions. As part of his Ph.D. studies, Cullather diagnosed the annual cycle of sea level pressure over the Arctic Basin using observations and gridded analyses. Above the Canada Basin/Laptev Sea side of the Arctic, the annual cycle of surface pressure was dominated by the first harmonic, which has amplitude of about 5 hPa and maximum pressure occurring in March. Along the periphery of northern Greenland and extending to the North Pole, a weak semiannual cycle was found in surface pressure with maxima in May and November. The presence of the semiannual variation over time was found to be highly variable. The progression of the annual cycle was further characterized through an examination of the divergent atmospheric mass field, which indicated a transfer from Eurasia and into the Canadian Archipelago in spring and the reverse condition in autumn. Over the central Arctic Basin, springtime pressure increases result from an enhanced poleward mass transport from Eurasia. An increase of equatorward transport over the Canadian Archipelago in May and June results in central Arctic pressure decreases into summer. A less distinct temporal separation between the poleward Canadian transport and the equatorward Eurasian transport results in the weaker second pressure maximum in autumn. Additional dissertation material was presented in May 2003 at the Seventh American Meteorological Society Conference on Polar Meteorology and Oceanography in Hyannis, Massachusetts. This presentation reviewed computations of atmospheric static stability parameters from gridded analyses and twentieth century rawinsonde observations in the Arctic basin and surrounding land surfaces. The geographic relation between large values of convective available potential energy (CAPE) and the summertime baroclinic zone surrounding the basin, known as the Arctic Front, were examined. Long-term trends towards more frequent convective activity that has been determined in Arctic native-knowledge studies were unconfirmed, although a trend towards more frequent instances of small values of CAPE in Siberian locations was noted.

In coordination with James Hurrell (CGD), Cullather evaluated the Community Climate System Model, Version 2, using a feature-tracking algorithm applied to synoptic sea level pressure fields. The focus of the study was an assessment the model storm tracks in comparison to observation-based analyzed fields from a unique perspective. The study was made using a control simulation of the fully coupled model and an integration of the atmospheric model forced with observed 20th Century sea ice and sea surface temperature fields. Principal results highlighted the North Atlantic storm track which was found to have a more zonal depiction in both coupled and uncoupled simulations than is observed, and the character of anticyclones over the central Arctic Basin, which are generally weaker and shorter lived in the model as compared with observations. These results were presented at the June 2003 CCSM Workshop in Breckenridge, and are being prepared for publication.

Carrie Morrill (CGD) studied climate variations during the present interglacial and last glacial time periods. Her current research is focused on two areas. First, she is using the NCAR Climate System Model to study the influence of the El Niño-Southern Oscillation on climate in the extratropics since the last glacial maximum. Second, she is using published paleoclimate proxy records to test several hypotheses about the spatial patterns, timing and causes of abrupt climate changes that occurred between 4000 and 6000 years ago.

Cloud radiative forcing has important roles in global climate. As a contribution to understanding the possible impact of altered climate regimes on marine clouds, variations of ship observed marine clouds and precipitation frequency (FQ) associated with ENSO are examined. Marine cloud variations associated with ENSO could be roughly grouped into the following three categories: 1) storm track cloud variations in association with modulated mid-tropospheric stability, 2) marine boundary layer (MBL) cloud variations in association with modulation of MBL static stability, and 3) deep convective cloud variations in association with modulated mid-tropospheric stability. Sungsu Park (CGD) also found significant ENSO signals in ship observed visible sky conditions over the northeastern Atlantic and western Mediterranean Sea (WM) region during late summer and autumn. Analysis of atmospheric flows indicates that the strong ENSO-WM correlation in the August through October (ASO) season arises from two components: a previously unreported quasi-stationary Rossby wave propagating eastward from the western equatorial Pacific, and an anomalous component of the Asian wet monsoon circulation.

A new single column model is being developed to study mechanisms responsible for the properties of stratocumulus clouds in the upper part of the MBL. The key feature of the model is an algorithm for evaluating decoupling parameters used to specify the statistical properties of thermodynamic and moisture variables at the base of the MBL inversion. Model behavior is tested under homogeneous static conditions and advective conditions over the northeastern subtropical and southeastern subtropical Pacific Oceans. Simulated variations of MBL and cloud properties are in good agreement with observations along selected trajectories. This model represents a kind of maximum simplification of the MBL in the sense that omission of any major component would cause it to fail badly. Park is trying to directly apply the new single column parameterization of MBL clouds discussed to a GCM. He is analyzing satellite cloud product using the Medium Resolution Imaging Spectroradiometer (MODIS) for realistic tuning of internal model parameters over the Californian and Peruvian stratocumulus deck during September-October, 2000.

Eugene Wahl (ESIG/CGD/MMM) focused on three areas of research in FY-03. The first area, reducing uncertainty in paleoenvironmental reconstruction with fossil pollen records, he focused on technical issues in the calibration of fossil pollen records for environmental reconstruction, specifically how modern pollen assemblages can be used to reconstruct modern and paleoenvironmental conditions and vegetation. The results of this work will help to establish new methods and calibration benchmarks in this field, and are applicable to other micro-fossil paleoenvironmental archives, such as foraminifera and diatoms.

The second area, use of millennium-length climate model hindcasts to examine fundamental uncertainty in proxy-based measures of ENSO activity, was done in conjunction with Caspar Ammann (CGD), and involved using fully-forced global climate simulations over the period AD 1000-1999 to compare model-derived proxy measures of ENSO activity with the model's actual ENSO activity over very long time scales.

The final area was the ethics of generation and use of climate and short-term weather forecasts, which was worked on with Rebecca Morss (ESIG/MMM), to systematically analyze assumptions, methods, limitations, and uses/abuses of weather and climate forecasts from an ethical perspective. Criteria and methods were drawn from modern applied ethics (in particular, the "Georgetown School" of analytical criteria and the method of iterative "reflective specification" developed by John Rawls) and developed into a framework specifically oriented to quandaries that arise in the preparation and provision of forecasts.

Turbulence

Jai Sukhatme (GTP) started his postdoctoral fellowship in the last part of FY-2003 and is in the process of identifying suitable research projects. One of the problems he worked on concerns the advection-diffusion of passive scalars. He aims to develop a statistical characterization of a passive scalar field under the action of both advection via smooth velocity fields and molecular diffusion. Sukhatme is now investigating the effect of boundary conditions.

Mark Miesch (HAO) studies turbulence, shear, and instabilities in the solar convection zone and tachocline using high-resolution numerical simulations. A major step forward in the past year has been the incorporation of magnetic fields into these models. Simulations of turbulent MHD convection in rotating spherical shells have yielded sustained dynamo action and are providing new insight into the generation and transport of magnetic fields in the solar envelope. Collaborators in this work include J. Toomre (University of Colorado, Boulder) and A.S. Brun (Saclay). Studies of convective penetration and sub-grid scale modeling techniques are also being pursued with collaborators at NASA Ames including N. Mansour, M. Rogers, and Y.-N. Young. A separate set of models has focused on the solar tachocline, which is a stably-stratified shear layer located below the convective envelope. Miesch and P. Gilman (HAO) have derived a new system of equations to study the tachocline which are based on a thin-shell limit of the 3D MHD equations. In one of the first applications of this thin-shell system, they have demonstrated that the meridional circulation in the convective envelope cannot penetrate as deep into the stable interior as required by some current solar dynamo models.

Geophysical Turbulence Program

Scientific highlights of specific GTP members are included below.

Aimé Fournier continued research on the use of the Spectral Element Method (SEM) for high-order, adaptive computation of atmospheric fluid dynamics. Part of this work, including a priori adaptive SE refinement for shallow-water flow on the sphere, is reported in an article co-authored with M. A. Taylor at LANL and J. J. Tribbia in CGD/CDP, in press at Mon. Wea. Rev. (2003). In collaboration with G. Beylkin and V. Cheruvu at CU, Fournier also helped develop dynamically adaptive SEM for the 2D Burgers dynamics, a model of atmospheric front formation, reported in Proc. Chicago Workshop on Adaptive Mesh Refinement Methods (2003). Fournier also developed a new form of turbulent energetics, describing cascades across scales, localized in space, reported in J. Atmos. Sci. (2003) and J. Climate (2003).

In collaboration with Miroslaw Andrejczuk and Szymon Malinowski (Warsaw University, Poland), Wojciech Grabowski and Piotr Smolarkiewicz (MMM) extended a modeling study of decaying moist turbulence reported previously. This problem is important, beyond fundamental understanding, for applications such as radiative transfer through clouds, initiation of precipitation in warm (i.e., ice-free) clouds, and parameterization of small-scale and microscale processes in models resolving larger scales. In the moist case, kinetic energy of small-scale motions originates not only from the classical downscale energy cascade, but it can be also generated/enhanced internally by the phase change processes and droplet sedimentation. The new set of simulations showed that results obtained in a pilot low-spatial resolution simulations reported previously are confirmed by high-resolution (direct numerical simulation-type) simulations with improved representation of cloud microphysics. In addition, a range of diverse initial conditions was considered to further extend previous findings. This work validated the generality of the conclusions derived from the pilot study and improved the accuracy of quantitative predictions.

Hanli Liu, Paul Charbonneau, Annick Pouquet, Tom Bogdan, and Scott McIntosh investigated the continuum limit of a class of self-organized-critical lattice models for solar flares. Such models differ from the classical numerical sandpile model in their formulation of stability criteria in terms of the curvature of the nodal field and are known to belong to a different universality class. A fourth-order nonlinear hyperdiffusion equation is reverse-engineered from the discrete model's redistribution rule. A dynamical renormalization-group analysis of the equation yields scaling exponents which compare favorably with those measured in the discrete lattice model within the relevant spectral range dictated by the sizes of the domain and the lattice grid. These scientists argued that the fourth-order nonlinear diffusion equation that models the behavior of the discrete model in the continuum limit is in fact compatible with magnetohydrodynamics (MHD) of the flaring phenomenon in the regime of strong magnetic field and effective magnetic diffusivity characteristic of strong MHD turbulence.

Pouquet continued work on the transition from weak to strong turbulence in the MHD framework, applying the findings of the study of the magnetosphere of Jupiter. She also began an investigation of singularities in MHD and development of turbulent structures at moderate to high magnetic Reynolds numbers; modeling MHD flows as it applied to the geo-dynamo at low magnetic Prandtl numbers is in the planning stage, a Collaborations in Mathematical Geosciences NSF grant having been obtained on this topic.

Sébastien Galtier came from IAS (Orsay) to continue working with Paul Swarztrauber (SCD) and Annick Pouquet on developing, ab initio and at first for Burgers turbulence, a Lagrangian code using a pseudo-spectral method. The long term goal is to have a better numerical technique to handle the long-standing and all important problem of the heating of the solar corona, in a simplified modeling statistical framework. The results for a stationary shock are very encouraging: at a fixed resolution (of N = 128 points), the Reynolds number can be increased by two orders of magnitude with the new method when compared to a computation on a fixed grid. However, for a moving shock, problems occur in the coarsening/refining process and more work needs to be done, likely introducing a grid diffusion term.

The idea behind inviting Sébastien Galtier, Dimitri Lavender and Alain Noullez at the same time was to coordinate our efforts to build AMR (Adaptive Mesh Refinement) codes for turbulent flows. More precisely, they identified a problem with Pouquet, the Hwa-Kardar (or HK) equation, on which several codes will be inter-compared. The HK equation was written in the context of the dynamical evolution of overlapping avalanches, as a model of heating the solar corona. The equation was written for a scalar field u(x,y,t) representing either the height variation of a sand-pile or a two dimensional velocity field. The equation is that of Burgers (advection-diffusion in one space dimension) to which is added a transverse diffusion term. It is thus one of the simplest physical set-up one can think of to test the two-dimensionality of an algorithm, since there is no pressure term in that case. It is well known that the shocks that form in a standard Burgers equation in a finite time along the parallel direction leads to a catastrophic growth of the parallel dissipation. In the HK case, these shocks are also responsible for an exponential growth of gradients in the perpendicular direction, with moreover an e-folding factor that itself grows catastrophically in time. The four codes used in this study were: (I) a fixed grid pseudo-spectral algorithm taken as the fully resolved case; (II) a finite difference low-order AMR developed by Dimitri Lavender in Nice and using the PARAMESH library; (III) a spectral element high-order AMR code (GASPAR) developed by Duane Rosenberg at NCAR and finally (IV) a spectral-Lagrangian code, developed with P. Swarztrauber (SCD). GASPAR will be tested both with and without dynamical adaptation. Errors will be measured both with L₂ and L_∞ norms, examining the time and location of shocks, together with more sophisticated statistics of the flow such as energy spectra, relative amount of parallel and perpendicular dissipation, and self-similar decay of the energy.

The investigation of Ronald Adrian and B. J. Balakumar (both of the University of Illinois at Urbana-Champaign) focused on a particular structure consisting of hairpin vortices organized into a self-replicating packet and how it occurred as the dominant mechanism of turbulence stress creation in the planetary boundary layer (PBL) as well as in the laboratory boundary layers. They collaborated with Lenschow, Peter Sullivan and Tom Horst (MMM and ATD) to use the turbulent velocity data from the Horizontal Array Turbulence Study (HATS) field experiment as a test bed. They started to sift through the database seeking maxima in the correlations between the HATS time series and velocity signatures found in the laboratory. Demonstration studies with another experiment in the PBL indicated that a large fraction of the seemingly random PBL time series could be represented in detail by sequences of a small number of signatures from the lab flow, the tentative conclusion being that the deterministic mechanism underlying the signatures is ubiquitous. Adrian and Balakumara hope to add weight to this conclusion by applying a similar but more advanced analysis to the HATS data. During their visit an idea formed for extending this approach by developing a library of signature from different coherent structures in turbulence and applying it to the analysis of large data sets so as to determine the occurrence of different coherent forms at various points throughout the data set (i.e., plumes, vortex packets, etc.). This might prove useful in an automatic means of classifying segments of data sets and streamlining their analysis.

David Gurarie (Case Western Reserve University) collaborated with Don Lenschow (MMM) on atmospheric transport models of reactive species and tracer gases. They published a paper: "A simple model for relating concentrations and fluctuations of trace reactive species to their lifetimes in the atmosphere", JGR, 2002. Further plans to include higher level model with parameterized convection, meridional transport and seasonal variability was begun through an NSF proposal with Lenschow and Ian Faloona. Gurarie and Sergey Danilov (Alfred Wegener Institute for Polar and Marine Research) continued their study of quasigeostrophic turbulence (single layer and multi-layer), particularly the effect of rotation (beta-plane).

Besides an ongoing research with William Large (CGD) on double-diffusive convection, there are two projects with applications to the atmospheric boundary layer that Robert Kerr (University of Warwick) is developing in collaboration with NCAR.

One is on stochastic parameterizations for convection over rough terrain, taking into consideration the evidence for upscale energy transfer in the atmospheric boundary layer and its implications for new reduced models for the equatorial convection zone. The simulations are idealized thermal convection in a box and the observational data is from scatterometer data, a source of surface velocities over the oceans from remote satellites. It appears that new results on understanding and using structure functions could aid in data assimilation and decisions about the best reduced models for the equatorial convection zone.

The second project, with HAO, deals with connecting artificial data generated from small-scale simulations with large-scale observational data from the heliosphere. In particular, one can look for signatures for small-scale turbulent bursts in the solar wind based on results from simulations, with a possible collaboration with the Warwick group using their new compressible, nested mesh code. A collaboration with Tom Holzer, B-C Low and Keith MacGregor (all three with HAO) may occur along these lines.

Senior Research Associates Highlights

ASP's current Senior Research Associates (SRAs) are Guy Brasseur (Max Planck Institute), Hans Friedli (ACD), John Latham (University of Manchester), Jerry Mahlman (NOAA/GFDL) and Lawrence Radke(University of Rhode Island). In addition to providing an on-going resource for current postdocs and graduate fellows, these SRAs also represent NCAR in their community work.

Hans Friedli's investigations (with collaborators Lawrence Radke and the Canadian Forest Service) on the emission of mercury species from wildfires has led to the conclusion that the organic soil above the mineral layer is a major but highly variable source of mercury emission during wildfires. New fieldwork in a boreal forest in Canada provided a mercury inventory for live and dead vegetation and profiles of the mercury concentration in the organic soil layer. A commonly found carbon layer on top of the mineral layer indicated that the organic soil layer had been completely burnt in previous wildfires and thus must have caused large spikes in mercury emission. The new data suggest that the mercury temporarily sequestered in organic soil may be larger than the oceanic reservoir as had been previously assumed.

Friedli (with colleagues from the University of Iowa, Carmichael et al.) characterized the distribution of mercury in the air around Japan/Korea/China based on airborne measurements made during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) campaign. The mercury distribution is extremely heterogeneous, consisting of air attributable to anthropogenic, biomass and volcanic emissions, dust storms, as well as highly processed clean air high in mercury. The results support the use of gaseous elementary mercury as a conserved tracer for long-range transport as well as a tracer (with appropriate co-tracers) for the identification of individual sources. The measurements indicate that China is a very large exporter of atmospheric mercury.

John Latham has further extended his research into a novel idea for the amelioration of global warming by the advertent and controlled enhancement of the albedo A and longevity L of low-Level maritime clouds. More detailed calculations coupled with some limited computer modelling support the quantitative validity of the proposed technique, which involves increasing the droplet concentration in such clouds, with a corresponding increase in both A and L: and thus cooling. The idea involves the dissemination at the ocean surface of small seawater droplets in sufficient quantities to act as the dominant cloud condensation nucleus on which cloud droplets form. Satellite control of the overall dissemination rate is envisaged. Collaborators include Dr Keith Bower & Prof. Tom Choularton (UMIST, Manchester, UK), Dr Alan Blyth, Dr Alan Gadian & Prof. Mike Smith (University of Leeds, UK), Prof. Stephen Salter, (University of Edinburgh, UK) and Dr Tom Wigley (CGD). If this technique were to prove workable on the scales required, it could be of great societal importance.

Latham and collaborators are continuing their examination of the extent to which it is possible to determine thundercloud ice characteristics from satellite observations of lightning, which are now routinely made on a global scale, using NASA/MSFC devices. A specific goal is to ascertain whether measurements of lightning frequency f can yield estimates of precipitating and non-precipitating ice fluxes. Our computations - and particularly, recent data analysis - support our hypothesis that f is roughly proportional to the product of the downward flux fg of graupel through the body of the thundercloud and the upward flux fi of ice crystals into its anvil. This raises the possibility of determining, on a global basis, values of fg and/or fi from the lightning measurements. Such information could have considerable climatological and nowcasting importance. Collaborators include Dr Hugh Christian, Dr Walt Petersen & Ms Wiebke Deierling (NASA/MSFC), Dr Alan Gadian & Dr Alan Blyth, (University of Leeds, UK), Dr Rumjana Mitzeva (University of Sofia, Bulgaria), Mr Scott Ellis (ATD) and Dr Jim Dye (MMM).

Jerry Mahlman served as a part-time ambassador and free agent to represent NCAR's priorities around the world. His activities ranged from one-hour tutorials on global-warming science to the newly formed National Council on Energy Policy to all-day interactive sessions with ethicists and philosophers to chairing the National Research Council's Workshop on Estimating Climate Sensitivity. Mahlman's representation of NCAR is a different and much broader role than some other senior research associates owing to his background at Princeton/Geophysical Fluid Dynamics Laboratory.

NCAR Aerosol Program