Postdoctoral Fellows' Research:

In atmospheric chemistry, Stephen Ball (in collaboration with Fred Eisele of ACD) has completed a set of laboratory measurements of nucleation rates for clusters of sulfuric acid and water, and has conducted some preliminary extensions of those measurements to the case where ammonia is also present. The power-law dependence of the nucleation rate on concentration reveals the number of molecules involved in the formation of critical clusters, and so reveals molecular-level details of gas-to-particle conversion processes that may occur in the atmosphere. Laura Iraci collaborated with scientists in ACD to measure the Henry's Law coefficients of some volatile organic compounds, finding them to be too small for significant update into cloud water under equilibrium conditions. Therefore update by cloudwater is not a significant sink for those compounds and therefore oxidation by OH is likely the dominant removal mechanism for these compounds.

Several observational or instrumentation projects also contributed to progress in atmospheric chemistry. Barry Lefer, working with Richard Shetter of ACD, helped develop and test new airborne actinometers that measure the actinic flux driving key photochemical reactions in the troposphere. At the Research Aviation Facility, Susan Durlak collaborated with Bruce Gandrud to design an instrument for the measurement of the sulfate content of aerosols and for discrimination by size in such measurements. Also concentrating on observations, Karsten Baumann (in collaboration with others from ACD and MMM) helped collect and analyze measurements from thunderstorms that added to evidence that lightning is a significant source of atmospheric NO_x. Denise Mauzerall and Larry Horowitz both used the MOZART model of atmospheric chemistry to study tropospheric ozone. Horowitz focussed on the effects of convection on tropospheric chemistry and especially on ozone.   Mauzerall has adapted the model for the study of Asian sources of pollution on tropospheric ozone, and has developed methods of using observational meteorological data in order to be able to compare results to specific cases where there are comprehensive observations.

In studies of climate and global dynamics, Charles Zender incorporated sources and transport of mineral dust into the Community Climate Model (CCM) and obtained good comparison between the resulting global distributions of dust and those observed by satellite. Joel Norris continued his studies of the characteristics of low cloud cover over the oceans by evaluating the relationship between observed clouds and synoptic-scale meteorological conditions. A significant correspondence was found between the low-level vertical motion (inferred from convergence) and the type of cloud that formed. Chris Torrence and Greg Duane also contributed to climate studies, Torrence in studies of decadal variability (esp. ENSO) using wavelet analysis tools and Duane in applications of chaos theory to dynamical systems.

In studies that bridge interests in global-scale and mesoscale dynamics, Wendell Welch used mesoscale simulations of airflow over topography to study potential effects of such airflow on global-scale dynamics. She is using the results in an effort to represent the effects of mesoscale topography in GCM results. (See elaboration)

In mesoscale meteorology, Greg Hakim has extended his earlier documentation of the links between tropopause features and weather systems by using models of idealized jet-stream flow to show that vortical upper-level disturbances trigger surface cyclogenesis. (See elaboration). In collaboration with MMM investigators and others, Bjorn Stevens used model simulations to study the process of entrainment at the top of the planetary boundary layer and to develop representations of the entrainment rate that include radiative effects, interfacial cooling, and the roles of small-scale mixing. Eileen Saiki, with Robert Kerr and William Holland, developed realistic simulations of the salt-finger structures produced by "double diffusion" in the ocean, where salinity and heat diffuse at different rates at stratified interfaces.

Rajul Pandya, in collaboration with Richard Rotunno and William Skamarock, used a two-layer shallow water model to show that coastally trapped disturbances resemble a hybrid disturbance that arises in such a model from coupling between Rossby and Kelvin waves at different levels. (See elaboration) Kevin Petty collaborated with Jordan Powers in the development of a coupled ocean-atmosphere mesoscale model. Thomas Hamill (in collaboration with Chris Snyder) developed new methods for treating the situation-dependent forecast error in data assimilation schemes by using ensemble forecasts from a quasigeostrophic model to provide statistical descriptions of the error field.

In studies of solar physics, Hardi Peter has used observations of the Doppler shifts of various spectral lines (representing different altitudes above the Sun) to deduce characteristics of the outflow from the Sun. These studies provide clear documentation of outflow from the polar coronal holes and provide measurements of the outflow velocity that leads to the fast solar wind. Another result, not yet understood, was the measurement of inward flow at lower levels in the disk center but outflow at upper and hotter levels. Mausumi Dikpati, in collaboration with Peter Gilman and Peter Fox, has analyzed potential instabilities that arise from combined hydrodynamic and magnetic effects at the base of the convection zone. They have shown that, in the presence of differential rotation and a concentrated toroidal field, the joint system is unstable to horizontal disturbances that extract energy from the toroidal field. In collaboration with Paul Charbonneau, she has also developed a dynamo model that reproduces many features of the magnetic cycle. (See elaboration)

In an extension of her earlier studies of particle fluxes in the upper atmosphere of the Earth, Marina Galand collaborated with investigators in Alaska to obtain improved observational documentation of a small outgoing component in such fluxes. Her model studies indicate that magnetic reflections do not account for this outgoing flux, but processes involving collisions with neutral atoms are the likely cause.

ASP postdoctoral fellows also contributed to studies of numerical and computational methods. Beth Wingate, in collaboration with Mark Taylor of SCD, developed a new numerical approach for use with triangular geometry that has attractive features competitive with or superior to those conventionally used with quadrilateral geometry. (See elaboration) William Spotz worked with Taylor and Paul Swartztrauber to develop new methods for computation in latitude-longitude coordinates that rely on efficient filtering. They showed that the accuracy and stability of their approach is the same as for the spectral transform method, but their method is more efficient.

In the area of societal impacts, ASP postdoctoral fellow John Magistro has focused on Senegal as an example of a society where climate variability and climate change have a large impact on agriculture and on the society. His studies have investigated the means by which this society receives and responds to information on weather and climate. A particular focus of his work is the construction of dams, which he argues is a response to climate change that can have deletorious as well as beneficial results on the society.

To assist such studies of surface hydrology, David Yates has worked with others in RAP to acquire a hydrological model for use at NCAR and to make this model available to researchers within and outside NCAR. Also in RAP, Rong-Shyang Sheu collaborated with V. Vivekanandan in development of a new algorithm for using microwave measurements from satellites to estimate precipitation, and tested those measurements by comparing to measurements deduced using radar.

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