The development of wind profiler technology over the past decade or so is a direct outgrowth of several decades of radar studies of the optically clear atmosphere (Hardy and Gage, 1990). It has also benefitted greatly from a transfer of technology from the radio ionospheric research community. Many of the researchers who participated in the early development of wind profiler technology were ionospheric physicists redirecting their efforts to studies of the lower neutral atmosphere. Early results which helped to stimulate the development of wind profiling in the lower atmosphere were reported by Woodman and Gúillen (1974) and Ecklund et al (1979).
In the past five years wind profilers have increasingly been used in support of large field programs and for operational wind monitoring. Field programs that have utilized wind profilers in the last five years include HARP, TCM-90, STORMFEST, ROSE, TOGA and TOGA COARE. A demonstration network of 30 wind profilers has been operated by NOAA for the past few years over the Central United States. In addition wind profilers have been used extensively for atmospheric research over the past decade.
It is important to recognize that wind profilers do much more than simply measure the wind. Since they provide almost continuous measurement of wind simultaneously and nearly continuously over a range of altitudes, they provide detailed information on vertical structure and wind variability. Vertically directed beams provide for direct measurement of vertical motions. In addition to wind and wind variability, the Doppler wind profiler provides measurement of signal strength and Doppler spectral width. Signal strength is influenced by the intensity of inhomogeneities in the radio refractive index, which depends upon the gradients of atmospheric temperature and humidity as well as the intensity of turbulence.
Wind profilers have been operated in several frequency bands. Much of the early work with wind profilers was accomplished at VHF primarily in the 40 - 50 MHz frequency band. The VHF wind profilers are characterized by their large phased array antennas that typically cover 100m X 100m. The wavelength of these VHF wind profilers (6 to 7.4 m) is long enough for these profilers to be relatively insensitive to hydrometeors. At these lower VHF frequencies backscattered power is often very anisotropic with large enhancements from quasi-specular reflection at vertical incidence from the hydrostatically stable atmosphere. Thus VHF profilers observe over an extended height range looking vertically compared to looking obliquely. Typically VHF profilers are capable of observing to higher altitudes than UHF wind profilers of comparable sensitivity.
A recent development that is already revolutionizing the way wind profilers are used operationally is the addition of Radio Acoustic Sounding System (RASS). RASS utilizes an acoustic source that is matched in frequency so that the wavelength of the acoustic wave is matched to half the wavelength of the radar transmitted electromagnetic wave. RASS measures the speed of the acoustic wave which is dependent upon temperature. In this way RASS provides a remote measurement of the atmospheric virtual temperature (Tsuda et al, 1988; May et al, 1990; Angevine et al, 1993). The height to which RASS observations can be made depends largely on the absorption of the acoustic wave in the atmosphere (May et al, 1988). At the UHF frequencies soundings are limited to the lower troposphere. At VHF soundings up to 10 km are possible. The use of RASS with wind profiling radars is discussed in Moran et al, (1991). The accuracy of virtual temperature measurements by RASS are reported in May et al, (1989) and Angevine and Ecklund, (1993).
A network of wind profilers has been deployed in the central U. S. in the past few years to evaluate the utility of wind profilers for operational wind measurement and forecasting (see for example Van de Kamp, 1993). These wind profilers operate in the 405 MHz frequency band that is reserved for meteorological aids. Profilers that operate at this frequency do not observe the quasi-specular echoes seen at vertical incidence at VHF. These profilers are substantially more sensitive to hydrometeors than the VHF profilers (Law, 1990). Data from the demonstration network are being assimilated into operational analyses and forecasts in order to supplement wind measurement from routine balloon soundings to improve nowcasting and short-range forecasting capabilities.
During the past five years small, relatively inexpensive, low-powered wind profilers have been developed to operate near 915 MHz (Ecklund et al, 1988, 1990a). These small profilers are quite adequate for observing winds in the lower troposphere typically through the height of the planetary boundary layer. In the tropics they have been capable of observing to about 5 kilometers because of the humidity. These profilers are finding many commercial applications since they observe winds with good height resolution. Thus they are good tools for air quality monitoring (Neff et al, 1991). At this frequency the UHF lower tropospheric profiler is very sensitive to precipitation and can be used to diagnose precipitating cloud systems in the tropics and elsewhere (Gage et al, 1992a).
This chapter will serve as an introduction to wind profiling. Much of the background needed for an in depth understanding of wind profiler technology is contained in Chapter 21 (Röttger and Larsen, 1990) and Chapter 28 (Gage, 1990) of Radar in Meteorology (Atlas, 1990). In the next section the technology of wind profiling radars will be discussed. In Section 3 the measurement capabilities of wind profilers will be summarized. In Section 4 applications of wind profilers will be considered. References at the end of this chapter will guide the interested reader to relevant papers that will provide further details omitted here.
Illustrations of three Doppler wind profilers are shown in Fig. 4.1. Panel 'A' shows the 50 MHz VHF wind profiler at Christmas Island (Gage et al, 1993b). The Christmas Island is located at (2 N, 157 W) in the equatorial dry zone south of Hawaii. The Christmas Island wind profiler has been in nearly continuous operation since early 1986. It provides wind information from a data sparse region of the equatorial central Pacific and is the first wind profiler data to be used operationally by NMC and ECMWF for analyses and for routine forecasts (Gage et al, 1988). Shown in the photograph is the 100m X 100m phased array antenna and the laboratory building that is used for the transmitter, receiver, radar controller and data processing subsystems.
Figure 4.1b contains a picture of one of the demonstration network profilers (Beran, 1991) that operates near 405 MHz . Shown in the corners of the photograph of the network profiler are acoustic sources which are used with the Radio Acoustic Sounding System (RASS).
Figure 4.1c contains a picture of a 915 Mhz wind profiler. The 915 MHz wind profiler was designed primarily to provide high vertical resolution wind profiles from the lower troposphere. Specifically, it was developed for use with a VHF wind profiler to 'fill in' the lowest heights that cannot be observed using the 50 MHz wind profiler (Ecklund et al, 1990b). The 915 MHz wind profiler is relatively inexpensive and portable. As a consequence it is well suited for field programs of limited duration or for installation at sites that may be unsuitable for larger wind profilers that use more power and occupy more space.
A schematic block diagram showing how a Doppler wind profiler is configured is reproduced in Fig. 4.2. To the side of the figure is a sketch of the fixed beam configuration commonly used for Doppler wind profilers. In a typical configuration the antenna is phased to steer the beam in several fixed directions. A minimum of three beam directions is typical. One direction is vertical and at least two beams are directed obliquely in orthogonal planes. Since the Doppler radar measures the radial component of motion, the oblique beams provide a measurement of components of the horizontal velocity and the vertical beam provides a direct measurement of vertical velocity.
Switches are used to cycle the profiler through a pre-selected sequence of beam directions. The TR switch is used to alternate between transmitting and receiving. The Doppler radar is inherently phase coherent. Comparison of the received echo with the transmitted pulse permits the determination of the Doppler frequency. The Doppler shift is proportional to the velocity of the scatterer along the transmitted radar beam. Data are coherently averaged before the FFT is used to determine the Doppler spectrum. The Doppler spectrum is the primary information provided by the wind profiler. Almost all measurements are related to the moments of the Doppler spectrum. Doppler wind profilers are pulsed Doppler radars with typical duty cycles of about 1%. Thus a profiler with a 100 kW peak transmitted power will have an averaged transmitter power close to 1 KW.
For some applications, symmetric beams are employed in the oblique directions. This provides a redundant measurement of horizontal velocity that can be used to check consistency. It also provides an independent measurement of vertical velocity and is useful for momentum flux measurement as discussed in the next section.
Doppler wind profilers are being used principally for atmospheric sounding in three frequency bands. In the lower VHF close to 50 MHz, profilers are used primarily for observation of the free troposphere and the lower stratosphere. The less powerful radars in this category are referred to as Stratosphere-Troposphere (ST) radars. The more powerful radars in this category are often referred to as Mesosphere, Stratosphere, Troposphere (MST) radars. These long wavelength radars are especially well suited for observation of the middle atmosphere because they are not limited by the inner scale of turbulence (Balsley and Gage, 1980).
In the early 1980's when it became clear that routine operational wind profiling was possible throughout the troposphere, NOAA's Wave Propagation Laboratory (WPL) constructed a regional network of wind profilers in Colorado (Strauch et al, 1984). The concept of a larger interstate network of demonstration wind profilers was advanced by Gordon Little of WPL. These demonstration network profilers were designed to operate in the 405 MHz band because this frequency band is designated for the use of meteorological aids (Beran, 1991). However, because of the Search and Rescue Satellites that operate in this band, the frequencies of operational wind profilers of the future will most likely be operated at a frequency close to 449 MHz. The demonstration network has recently been completed and experience shows that the wind profilers provide a wealth of new information for the local forecaster. They also provide a continuous stream of wind observations for numerical weather forecast products.
During the past five years a smaller less powerful but highly portable wind profiler was developed by NOAA's Aeronomy Laboratory (Ecklund et al, 1988). This lower tropospheric wind profiler was designed initially to operate at 915 MHz. The antenna for this unit occupies an area of only a few meters on a side. With a phased array antenna the wind profiler can be steered sequentially to look in several pre-selected directions.
At 915 MHz the lower tropospheric wind profiler is very sensitive to precipitation. In rain the clear air echo is often overwhelmed by the precipitation echo. Wind measurement is still possible in rain but a correction must be made for the hydrometeor fall speed that can be determined from the vertical beam. This technique works reasonably well provided the rainfall is uniform so that the fall speed is not highly variable. If the fall speed is highly variable, wind measurement will be compromised. Under these circumstances, such as during a convective shower, it would be extremely difficult to measure a representative wind by any means.
There are several other wind profiling techniques that have been used experimentally that do not utilize the Doppler method. These techniques typically employ spaced multiple receivers as is shown schematically in Fig. 4.3. There are several related methods that use multiple spaced receivers: spaced antenna (SA), Radar Interferometry (RI) or Spatial Interferometry (SI) and Imaging Doppler Interferometry (IDI). In its simplest application the spaced antenna method utilizes a vertically directed transmitter and several spaced receivers. In its simplest form the technique involves cross correlation of the received signal between receiver pairs. The wind is determined from the lag time for maximum correlation between receiver pairs. When phase as well as amplitude information is kept in the analysis the angle of arrival of the echo can be deduced from the phase difference between spaced receivers. This technique is referred to as Radar Interferometry (RI) or spatial interferometry (SI). The Imaging Doppler Interferometry (IDI) method uses the phase difference for different frequencies in the Doppler spectrum (Adams et al, 1986).
Spaced antenna techniques in their rudimentary form go back several decades as they were originally used for wind measurement in the ionosphere. With the recent widespread interest in wind profiling technology in the lower atmosphere, considerable attention has been devoted within the research community to developing multiple receiver techniques (Vincent and Röttger, 1980; Vincent et al, 1987; Van Baelen et al, 1990 and Larsen et al, 1992). The method offers some advantages at VHF where anisotropic scatter prevails. There is little advantage at UHF, however. Recent developments in multiple receiver techniques are summarized in Larsen and Röttger (1989) and Van Baelen and Richmond (1991). In the following sections of this chapter we will be concerned almost exclusively with Doppler wind measurement from fixed beam wind profilers.
At lower VHF, aspect-sensitive quasi-specular echoes are often observed on vertically-directed radar beams. These echoes have been studied extensively and are strongest from hydrostatically stable portions of the atmosphere. It is the existence of these echoes that enables vertically directed VHF radars to observe well into the stratosphere and to detect and monitor the tropopause (Gage and Green, 1979).
For the UHF wind profilers, and to a more limited extent the VHF wind profilers, scattering from precipitation particles when present provides another source of atmospheric returns (Balsley and Gage, 1982). For the 915 MHz profilers the precipitation echoes are dominant and special techniques must be implemented to determine wind during rain. As is discussed later in this section and the next, the ability of UHF wind profilers to detect precipitation has important applications (Rogers et al, 1993).
The Doppler spectrum provides the meteorological information content from which almost all measurements are made by Doppler wind profilers. Doppler spectra are produced typically with about a one minute sample of data. Spectra are produced at each range gate. A sample Doppler spectrum is reproduced in Fig. 4.4. The Doppler spectrum is determined from the output of a fast Fourier transform (FFT). The FFT, which is calculated on a special digital signal processing (DSP) card that is part of the data processing subsystem of the profiler, is characterized by the number of spectral points in the FFT, the maximum unambiguous velocity of the spectrum and the resolution of the spectrum. The Doppler spectrum is characterized by its three spectral moments. The lowest moment yields the area under the spectrum and determines the signal power of the spectrum. The first moment gives the mean radial velocity. The third moment gives the spectral width. These parameters along with their temporal and spatial variation yield the measurement capabilities discussed below.
Wind profiles are typically deduced from a consensus of many samples of individual wind measurements taken over about 30 minutes. Individual samples are typically obtained on each beam in about one minute. When three beams are used for wind profiling, about ten values will be consensed to form the profile for the zonal and meridional components of the velocity. The consensus process compares velocities and discards outliers that do not agree with other values. Other more sophisticated algorithms have been developed to enforce temporal and spatial continuity in the wind field.
The precision of wind measurement by Doppler profilers has been examined extensively (Strauch et al, 1987; Weber and Wuertz, 1990). Intercomparisons are typically made against balloon soundings. When intercomparing winds measured by diverse systems the effect of spatial and temporal wind variability must be considered in evaluating the results. Taking this into account several studies have concluded that the intrinsic precision of horizontal velocities measured by profilers is close to 1 m s-1.
A sample comparison of radial velocities measured by two profilers operated side by side is shown in Fig. 4.5. This intercomparison is for a 405 MHz wind profiler and a 915 MHz wind profiler. The radial velocities in this figure show high vertical resolution profiles of the lower tropospheric wind that is clearly more accurate than 0.25 m s-1 (which would be consistent with the 1 m s-1 quoted above for the accuracy of horizontal velocity measurement).
One of the major advantages of wind profilers to other wind measurement systems is the ability to continuously monitor the wind field. Winds are typically summarized as time-height sections as shown in Fig. 4.6. This example is taken from the Christmas Island VHF wind profiler.
While direct measurement of the organized vertical motion field is entirely feasible using wind profilers, there is still some question of how well profilers can measure the much smaller long-term mean vertical motions associated with synoptic weather regimes. Some success in measuring synoptic vertical motion fields has been reported by Nastrom et al (1985). Any measurement of long-term vertical motion must be made by averaging the much larger fluctuating velocities associated with gravity waves that dominate the vertical velocity field and the uncertainties of the measurement depend on the sampling strategy (Nastrom et al, 1990a). The velocities are often small enough that problems are encountered with the FFT around zero Doppler. Great care must be taken to avoid the contaminating signals near zero Doppler that arise from ground and sea clutter.
In the past few years some authors have questioned whether VHF wind profilers can reliably measure vertical velocities because of the dominance of quasi-specular echoes that are thought to arise from layered structure in the refractive index field. If the layers are tilted and the vertical beam is effectively directed off vertical because of the anisotropic backscatter, it has been argued that the profiler will measure a component of the horizontal motion. This would not happen at UHF. To address this issue McAfee et al (1993) have compared vertical velocities observed simultaneously using a 404 MHz wind profiler and a 50 MHz wind profiler at Platteville, Colorado. Figure 4.8 shows a sample vertical velocity profile comparison taken from their work.
and 
.
However, it is possible to measure dw/dz and to infer the horizontal divergence
from the vertical beam as has been discussed by Clark et al (1986) using
the relationship
 |
(4.1) |
More recently, Cohn et al (1993) have used a 5-beam profiler measurement of horizontal divergence using the oblique beams to compare with the horizontal divergence inferred from using the vertical beam. These authors found reasonable agreement between the two methods.
While it is not possible in principle to measure vorticity with the radial velocities determined by a Doppler wind profiler, it is possible to make the measurement of vertical vorticity in principle using spaced antenna techniques as discussed by Larsen et al (1991) and Kudeki and Rastogi (1992).
Vincent and Reid (1983) developed a technique for the measurement of momentum flux which is well suited for use with fixed-beam Doppler wind profilers. The technique has been used to determine momentum flux associated with internal gravity waves. Momentum flux is measured by two beams symmetric with respect to the vertical direction. If beam 1 measures the radial component v1 (positive toward the radar),
 |
(4.2) |
and beam 2 measures the radial component v2,
 |
(4.3) |
then with a few assumptions concerning the horizontal homogeneity of the velocity field:
 |
(4.4) |
Studies of momentum flux in the middle atmosphere have been reported by Fukao et al (1988) and Fritts et al (1990).
Wind profiling systems equipped with RASS can be used to measure virtual heat flux as well as momentum flux. Recent measurements have been reported in Angevine et al (1993b). Measurement of heat flux and momentum flux are made difficult because of the uncertainties in the measurements. Considerable averaging is required to obtain results with a fair degree of precision.
Gage and Green (1979) and Gage et al (1986) showed that the VHF profiler was able to determine tropopause height in a straightforward way with a vertical resolution of only a fraction of the pulse resolution. Nastrom et al (1989) used the Flatland VHF radar in central Illinois to study tropopause height variability within upper level frontal zones. A classic illustration of the ability of the VHF radar to observe stability of the atmosphere is shown in Fig. 4.10.
The diverse applications of wind profilers to dynamics research are summarized in Gage (1990). While not yet complete, the development of wind profiling radars has led to an entirely new understanding of the nature of mesoscale wind variability in the atmosphere. The role of internal gravity waves in the atmosphere, for example, has been clarified through many studies using wind-profiling radars (Ecklund et al, 1981, 1986; VanZandt, 1985; Sato, 1990; Fritts and Nastrom, 1992; Nastrom et al, 1990b). All of these advances have been made possible by the rapid sampling capabilities of the wind profilers. Recently, Williams et al (1992) have used wind profiler observations at Christmas Island (2N, 157 W) to examine the diurnal tidal oscillations in the lower atmosphere.
In the past few years a network of VHF wind profiling radars has been established across the Pacific ocean (Gage et al, 1990, 1991a; Balsley et al, 1991). These profilers have been used to study tropical convection and large scale circulation across the Pacific and its relation to the El Niño and Southern Oscillation (ENSO). The wind profiler at Christmas Island has provided a long-term record of winds in a data sparse region of the central Pacific. Figure 4.12 shows a multi-height record of tropospheric zonal winds that shows a clear relationship with ENSO (Gage et al, 1993a). El Niño occur during the negative phase of the Southern Oscillation Index (SOI). The SOI is derived from the pressure difference between Darwin and Tahiti. During non-El Niño years the pressure is relatively low at Darwin and convection is very active over the western Pacific. During El Niño years the active centers of convection move eastward toward and across the International Dateline. This is accompanied by an increase in pressure at Darwin and a decrease in pressure at Tahiti. The El Niño years of 1986-1987 and 1991-1992 stand out in Figure 4.12 by the lack of an annual cycle in the upper tropospheric zonal winds and the weaker than normal trades. In non El-Niño years the upper tropospheric winds over the central Pacific strengthen during the northern Winter. This is due to a seasonal strengthening of the zonal Walker Circulation at the time of most active convection over the Indonesian Maritime Continent. During El Niño years the Walker Circulation is weaker than normal and the upper tropospheric westerlies are greatly reduced over the central Pacific.
The VHF wind profilers in the Pacific have also been used to study the long-term mean vertical motions. Long-term mean vertical motions associated with convective systems at Pohnpei, Federated States of Micronesia were observed by Balsley et al (1988) and for Darwin, Australia by Cifelli and Rutledge (1993). Long-term mean vertical motions under relatively undisturbed conditions at Christmas Island were reported by Gage et al (1991b). The diurnal cycle of the vertical velocities at Christmas island was investigated by Gage et al (1992b). Generally the observations show strong updrafts during active convection, sustained vertical motion in the tropospheric anvils of mesoscale convective systems and descent of order 1 cms-1 in relatively clear regions. The descending motion is needed to balance radiative cooling to space (Gage et al, 1991b).
The VHF wind profilers have been used to support several field campaigns. Augustine and Zipser (1987) report on their use in the pre-Storm experiment. The Pohnpei, FSM and a specially-constructed Saipan wind profiler were utilized in support of the TCM-90 Experiment. Recently 50 MHz wind profilers have provided valuable data for TOGA COARE.
As mentioned in Section 2, the 915 MHz wind profilers were developed to provide detailed information on lower tropospheric winds. In addition to the wind finding capability, the 915 MHz profiler is quite sensitive to precipitation. Rogers et al (1993) report on the use of the 915 MHz profiler in HARP. The profiler was able to monitor the height of the inversion at the top of the marine boundary layer. It showed clearly the nature of the warm-rain precipitation that was confined well below the melting layer at Hilo. Ecklund et al (1992) and Gage et al (1992a) have reported on the use of the 915 MHz profiler to classify precipitating cloud systems in the tropics.
Figure 4.13 shows the stacked Doppler spectra observed by a 915 MHz wind profiler during stratiform rain at Christmas Island. Note that the precipitation echoes are observed to the highest altitudes shown. Above the melting layer precipitation fall speeds for frozen hydrometeors is in the range of 1-2 ms-1. In the melting layer precipitation fall speeds increase rapidly to 8-10 ms-1 for rain.
Figure 4.14 shows the time height section of precipitation fall speed observed by the 915 MHz wind profiler at Christmas Island for a 48-hour period on March 13 and 14, 1990. This figure shows clearly the periods of stratiform rain. A short period of low-level rain is visible near 10 hr on March 13 and a deep convective shower associated with heavy rain can be seen at just after 0 hr on March 14. Observations such as these can be used to classify rainfall by precipitation type as has been discussed in Gage et al (1992a).
The 915 MHz profilers have been utilized in several field programs. In addition to HARP they have been used in TCM-90, STORMFEST, TOGA and TOGA COARE. In addition the 915 MHz profilers have been combined with other instruments to yield facilities such as the Integrated Sounding System (ISS) reported by Dabberdt et al (1991). The ISS have been utilized in an ad hoc fashion for DOE ARM (Martner et al, 1993) and for the first time as integrated systems in TOGA COARE ( Webster and Lukas, 1992 and Parsons et al, 1993).
The ultimate utility of the data from remote sounding systems including wind profilers are realized fully only when they are assimilated with other sources of data in a numerical model (Kuo and Guo, 1989). Recent research indicates that the usefulness of the profiler data extends well beyond the direct measurements of winds and other parameters that they provide. For example, Kuo et al (1987b) have shown the potential for retrieving temperature and geopotential fields using a network of profilers.
In addition to the weather forecasting application, there are numerous aviation, aeronautical and commercial applications of profilers. Several rocket launching sites have acquired one or more profilers or have plans to do so. Profilers can be used at airports to monitor low-level winds in the vicinity of runways used for takeoffs and landings. Wind profilers are very useful in air quality monitoring systems where it is crucial to know the wind field around sources of air pollution or simply to perform trajectory analysis to determine where air-borne are coming from or going.
In addition profilers are being adapted to a variety of platforms so that soundings can be made where they are needed most. For example, there is a need for soundings over the vast portions of the globe that are covered by ocean. As reported in Carter et al (1992) wind profilers have now been successfully operated aboard ship. Furthermore, profilers are being combined with other instruments to form Integrated Sounding Systems. The ISS can be designed for specific research programs as was done for TOGA COARE or they may be used for monitoring programs especially where infrastructure is lacking to support the long-term operation of other less-automated weather instruments. While wind-profiling systems continue to evolve and profilers are being used increasingly for diverse observations, there is little doubt that profilers will be an important part of any observational program of the future.
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List of Figures
Figure 4.1. Profiler Sites: A) 50 MHz VHF wind profiler at Christmas Island; B) 405 MHz UHF wind profiler in the 'Demonstration Network'; C) 915 MHz UHF wind profiler.
Figure 4.2. Block Diagram of Doppler wind profiler.
Figure 4.3. Schematic of spaced multiple receiver radar wind profiler.
Figure 4.4. Illustration of Doppler spectrum
Figure 4.5. Intercomparison of radial velocities measured simultaneously at 915 MHz and 405 MHz near Platteville, Colorado (After Ecklund et al, 1990).
Figure 4.6. Time-height section of horizontal wind velocities observed on Christmas Island (After Gage et al, 1993a).
Figure 4.7. Multiple height time series of vertical velocities observed at Liberal, Kansas (After Gage et al, 1989)
Figure 4.8. Intercomparison of vertical velocities measured simultaneously at 50 MHz and 405 MHz near Platteville, Colorado (After McAfee et al, 1993).
Figure 4.9. Vertical profiles of the normalized received signal observed by the Sunset VHF radar near 0000 UTC 26 March 1977 compared to the vertical profiles of temperature and potential temperature gradient from the 0000 UTC 26 March 1977, Denver NWS sounding. (After Gage and Green, 1978).
Figure 4.10. Time-Height section of atmospheric stability illustrated by means of reflectivity contours observed by the SOUSY VHF radar. Difference between contour intervals is 2 dB. (After Larsen and Röttger, 1982).
Figure 4.11. Doppler spectrum showing clear-air and precipitation echoes during light rain, the data were observed by the Penn State 400 MHz profiler during the MIST program. (G. Forbes, peronal Communication).
Figure 4.12. Time Series of tropospheric zonal winds observed at Christmas Island. Also shown is the Southern Oscillation Index (SOI). (After Gage et al, 1993).
Figure 4.13. Doppler spectra of precipitation echoes observed by the 915 MHz profiler at Christmas Island on April 21, 1990. (After Ecklund et al, 1992).
Figure 4.14. Contours of vertical velocity/precipitation fall speed observed at Christmas Island on 13/14 March 1990. (After Gage et al, 1992a).
Figure 4.15. Map showing locations of wind profilers that comprise
the NOAA Demonstration Network.