Radio Science, Volume 25, Number 5, Pages 917-931, September/October 1990

Simultaneous observations of the troposphere and lower stratosphere by the Flatland and Urbana ST radars

G. D. Dester, C. H. Liu, and S. J. Franke

Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign

J. L. Green

Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado

(Received May 31, 1989; revised September 18, 1989; accepted October 11, 1989.)

Simultaneous VHF radar observations of the troposphere and lower stratosphere were carried out on August 17-19, 1988, near Urbana, Illinois, using the Flailand radar and the Urbana stratosphere-troposphere (ST) radar, which are operated by the Aeronomy Laboratory of the National Oceanic and Atmospheric Administration and the University of Illinois Department of Electrical and Computer Engineering, respectively. The two radar sites are located on the vast plain area of central Illinois, separated by approximately 25 km. The geographical and observational configuration is considered most suitable for investigating mesoscale structures in the troposphere-stratosphere region. One horizontally stratified layer at about 15.5 km related to the tropopause, and another in the troposphere at about 8 km were observed at both radar sites for a long period of time. Velocity fluctuations with periods of 100 min < T < 200 min were observed to be well correlated during a period of quiet horizontal wind. A convective thunderstorm on the evening of August 18 coincided with the dissipation of the tropospheric layer observed at 8 km. We present here a qualitative comparison of low-pass and band-pa.ss filtered echo power and radial velocities, and a comparison of vertical power profiles from the two radars.

1. INTRODUCTION

The Doppler radar technique has been used in the past two decades or so to measure a variety of atmospheric parameters and phenomena. Doppler radars have proved to be quite valuable instruments in the study of atmospheric gravity waves, vertical velocity fluctuations, backscattered power, and other atmospheric properties. Different radar systems have been employed to study the atmospheric characteristics in a wide variety of climates and terrains, including those arranged in special network configurations, such as the Colorado

Copyright 1990 by the American Geophysical Union

Paper number 89RS03244. 0048-6604/90/89RS-03244508.00

profiler network [Strauch et al., 1984] and the Penn State Doppler Network [Williams and Peters, 1986] for compilation of simultaneous data at several related sites. In the near future

the planned wind profiler demonstration system will utilize 30 radar stations on a grid with spacings of-400 km in an attempt to assess the impact of such a profiling system on meteorological forecasting [Chadwick, 1988]. However, each of these networks mentioned, and most other clear-air Doppler radars, or wind profilers, currently in use are separated by spacings of the order of 100 km or more. It would therefore be advantageous to have some radars separated by distances of less than 100 km to aid in the understanding of certain atmospheric parameters and phenomena on scales less than 100 km.

Ecklund et al. [1985] and later, Carter et al. [1989] have made simultaneous comparisons of stratospheric-tropospheric vertical velocities in the Alpine Experiment (ALPEX) campaign in France, which utilized three closely spaced (-4-6 km) 50-MHz radars. Carter et al. [1989] were able to observe

917

918 DESTER ET AL.: OBSERVATIONS BY ST RADARS

several specific internal gravity wave events and determined detailed information on their wave parameters. Now with the recent completion of the Flatland radar [Green et al., 1988] by the Aeronomy Laboratory of the National Oceanic and Atmospheric Administration (NOAA) and with the recent modifications for stratospheric-tropospheric (ST) observations made to the Urbana VHF radar, operated by the De•ent of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign, it has now become possible to study some of the atmospheric characteristics on an ~25-km scale in the vast plain area of central Illinois.

When originally proposed, it was hoped that measurements made by the Flatland radar could be compared with those made by the Urbana radar in a joint scientific experiment [Green et al., 1986]. It is of particular interest to carry out such a study in the plain area where the orographic effects are at a minimum [Green et al., 1988]. Hence a study of this type might help in providing answers for questions such as: without the dominant common source of an orographic nature, how are the velocity fluctuations observed at distances of 25 km apart related to each other? Under what conditions and in what frequency bands are these velocity fluctuations correlated? Can such a combination of radars aid in the study of the wavelength and speed of gravity waves? It is also well known that horizontally layered structures in the atmosphere contribute to the enhanced echo power of vertically pointing VHF radars [Gage and Green, 1978; ROttger and Liu, 1978]. With a 25-km spacing between radars it might also be possible to determine if certain layers observed in the troposphere and stratosphere exist on such a scale.

Thus the study of the spatial and temporal structures of those horizontal layers using the echo powers and measurements of vertical velocities observed in the troposphere and stratosphere at the Flatland and Urbana sites will provide us with important information in the effort to understand mesoscale structures and vertical velocity activity in the absence of orographic influences. In this paper we report the initial results from the analysis of the data recorded by the Flatland and Urbana ST radars during a 47-hour period on August 17-19, 1988. In sections 2 and 3, the experiment and experimental conditions will be described, followed by a description of the procedures used to analyze the data in specific frequency bands in section 4. General observations are presented in section 5, with a more detailed discussion of specific events and layers following in section 6. Finally, some conclusions and suggestions are presented in section 7.

2. DESCRIFI'ION OF THE EXPERIMENT

The Urbana ST radar and the Flatland radar are located

approximately 25 km from each other in the very fiat terrain of east central Illinois near Champaign-Urbana, Illinois, as shown in Figure 1. System parameters for these two radars are given in Table 1. As mentioned previously, the Flatland radar

41'N

40'

91' 90' 89' W 88' 87'

Fig. 1. View showing the Flatland and Urbana radar sites with respect to the Peoria and Salem radiosonde locations in Illinois. Insert shows location of radars with respect to Champaign-Urbana, Illinois.

is maintained by the Aeronomy Laboratory of NOAA at the Bondville Field Site of the Department of Electrical and Computer Engineering at the University of Illinois. It became operational in February of 1987 to study the stratosphere and troposphere in the fiat tenain where the influence of orographic effects would be minimized. It presently has the capability to operate in five different beam-pointing directions, but at the time of this campaign, the Flatland radar was alternating between its vertical beam and an oblique beam pointed 20' off- vertical toward east. This resulted in Doppler spec• moment estimates of power and velocity alternating every 1.7 min between vertical and oblique beams, giving 3A-min temporal resolution in each pointing direction. The Flatland radar operates at a frequency of 49.8 MHz and transmitted during this campaign with an average power of 1 kW, peak power of 80 kW, pulse length of 5 Bs, and a corresponding height resolution of 750 m. Additional details concerning the Flatland radar and its signal processing aspects are descri• by Green et al. [1988].

The Urbana radar is located at the University of Illinois Aeronomy Laboratory Field Station approximately 25 km northeast of the Flatland radar (Figure 1). The present configuration of the Urbana radar in ST mode utilizes the antenna previously used in MST applications [Miller et al., 1978; Goss and Bowhill, 1983; Gibbs and Bowhill, 1983]. For ST applications we have modified the system to use a transmitter identical to the transmitter used at the Flatland

radar. However, the power supply used for the Urbana radar is smaller than the power supply used for the Flatland radar, resulting in less transmitted power. The Urbana radar experiences tinging from the antenna, hence increasing the recovery time of the system and causing some difficulties in observing accurate velocities in the troposphere below 6.5 km. The Urbana radar operates at a frequency of 40.92 MHz with an average transmitted power of 100 W, peak transmitted power

DESTER ET AL.: OBSERVATIONS BY ST RADARS 919

TABLE 1. System parameters for Flatland and Urbana ST radars

Parameter Flafiand Urbana

Frequency Antenna type Antenna dimensions

Average transmitted power Peak power Pulse width

Height resolution Time resolution

Beam-pointing direction

Beam width (two way) Location

49.8 MHz

coaxial-colinear

60x 60m 2 1 kW

80 kW

5 Ps 750 m

3.4 min

vertical, 20' off-vertical toward E

3.2'

40.1'N, 88.4'W

40.92 MHz

dipole 90 x 120 m 2 100 W

15 kW

10 Bs 1500 m

2.2 min

-3' off-vertical toward E

in E-W plane* _6 ø ß

40.15'N, 88.15'W

* Estimated from radio star calibration

of 15 kW, and transmits a 10-Bs pulse resulting in range resolution of 1.5 km. Data collection and coherent integration were carried out by an Apple II computer. It should be noted that the Urbana radar operates at a lower transmitted power and has coarser height resolution than the Flailand radar.

The Urbana VHF radar antenna array, which has been used previously in MST applications, consists of 1008 dipole elements in a 90 x 120 m 2 array which were constructed on ground which slopes -1.5' in a direction 36' south of east [Allman and Bowhill, 1976]. It has been previously accepted that the antenna beam pointed 1.5' off-vertical toward the southeast. But since it had been several years since a calibration was performed on the antenna array, we decided to verify that this was indeed the pointing angle. In an attempt to estimate the present pointing direction of the beam, a radio star calibration with Cygnus-A was computed for this data set. Since the radio star passes overhead from east to west, only the projection of the beam onto the east-west plane could be calculated. The calibration indicated that the beam pointed- 3' off vertical in that plane toward the east with a related beam width of-5'-6'. The southward component of the tilt could not be determined. As will be discussed later, we believe that this off-vertical tilt of the beam causes some contamination of

the Urbana velocity data by the horizontal wind. Upon examination of the array after the campaign, some deteriorated portions were discovered and repaired, thus reducing the off- vertical tilt of the array for future experiments.

The Urbana radar operates with an interpulse period of 1.5 ms, and 166 samples are coherenfly integrated to give complex data samples every 0.25 s. These samples are collected for 8.5 min and dumped to tape. A 128-point Doppler spectrum is then computed, and four spectra are incoherently averaged together to obtain spectral moment estimates every 2.13 min by using the method described by Woodman [1985]. This resolution is slightly better than the 3.4-min resolution for the Flailand data. Airplanes from a nearby airport sometimes

contaminate the spectra, and the spectra are therefore checked for the presence of airplanes and flagged if contamination has occurred. The Doppler spectra have a velocity resolution of 11.5 cm/s and can measure velocities of + 7.36 m/s without aliasing in the spectra.

The data set used in these comparisons was taken over a 2- day period which began at approximately 0830 on August 17, 1988, and ended at 0730 on August 19, 1988, local time. The data have been separated into two segments, each covering 23.5-hour periods, which will be referred to henceforth as the August 17 and August 18 data sets, denoting their starting dates.

3. EXPERIMENTAL CONDITIONS

The Urbana radar began operation during this period on August 15, 1988, and the weather condition at this time was very hot and humid. Data recorded by the Illinois State Water Survey at the Bondville site, where the Flailand radar is situated, showed that a high temperature of 97' F and daytime humidity readings greater than 50% occurred on August 15. This hot and humid weather continued for the rest of the

campaign, including August 17 and 18, with daytime highs near 100' F each day. Radiosonde data taken at Peoria (-140 km northwest of Champaign-Urbana) and at Salem (-180 km southwest of Champaign-Urbana -- see Figure 1) indicated that the tropopause was quite high, near 15 km for each day of this campaign. These same data indicated that in our region of main interest (-6-9 km), the horizontal winds were generally from the west to northwest direction.

The surface weather conditions were relatively calm during this period, with the exception being on the evening of August 18, 1988. At 1700 local time the University of Chicago- Illinois State Water Survey (CHILL) radar observed one

920 DESTER ET AL.: OBSERVATIONS BY ST RADARS

thunderstorm approximately 70 km south and another about 90 km north of Champaign-Urbana, but CHILL radar data were not available after this time. Later in the evening, at approximately 2300 LT, on-site observations at the Urbana radar observed a thunderstorm approaching the site, and intermittent periods of high winds and rain followed for approximately the next 2 hours. On-site observations at the Urbana radar recorded one particular period of heavy rain from approximately 0000 to 0015 LT on August 18. Using Illinois Sate Water Survey data at the Flatland radar site, one particular period of heavy rain was recorded at approximately 0035 to 0055 LT, roughly the same time period. It is not known if the precipitation observed is from the same storm or not. The following day, the weather resumed to the pattern of hot and humid conditions as it was during the preceding days. In this paper, one of the topics we will be discussing is the effect this storm had on the horizontal layers observed by both radars.

4. DATA ANALYSIS

In addition to low-pass filtering, the difference of the two low-pass filtered data sequences was obtained to give an effective band-pass filtered data sequence with a transfer function of

IHBp(z) l 2-- IH•oo(z) l 2 - IH200(z) l 2

This filter allows us to observe the velocity and power oscillations present with periods of 100 min < T < 200 min. An additional high-pass version of the data was obtained by subtracting the data set filtered with H 100(z) from the original data set to obtain a high-pass filter with transfer function

I HHp(z) 1 2 _ 1 -I Hx00(z) 1 2

These filters will then allow us to observe the simultaneous behavior of the fluctuations at both radar sites within the

respective frequency bands, T < 100 min, 100 min < T < 200 min, and T > 200 min. Such observations will give us an indication of how well correlated the Doppler velocities and echo powers are over a 25-km baseline in these bands.

The data from the Flatland and Urbana radars consisted of the

spectral moments computed from the respective Doppler spectra. In this paper we concentrate on the vertical velocity and vertical beam echo power derived from the radar signal and have made an attempt to determine how these velocity and echo power fluctuations are related to one another. More specifically, we have processed the data so that it may be better determined how vertical velocity and echo power fluctuations in three specie frequency bands are correlmed.

In order to accomplish these goals, the data were filtered with two different types of six-pole Butterworth infinite impulse response ClIR) low-pass filters. Two separate cutoff frequencies were chosen for these filters, at f = 1/100 min' 1 and at f = 1/200 min' 1. The subsequent transfer functions for the Butterworth filters are of the form

a(0) + a(1)z '1 + a(2)z '2 + a(3)z '3 H(z) =

1 - b(1)z- 1. b(2)z-2. b(3)z-3 where the coefficients are determined according to the data sampling rate and cutoff frequency. Due to the different temporal resolution of the two radars, the filter with a cutoff frequency of, for example, f = 1/100 min' 1, was not identical for both the Flatland and Urbana radars, but the transfer function is reasonably close for both filters. We shall denote the low-pass filters with cutoff periods of T=100 min and T= 200 min as H 100(z) and H200(z), respectively. To remove the phase distortion of the IIR filter, the data were passed through the filter in both the forward and the reverse direction, effectively obtaining a magnitude-squared transfer function I H(z) 1 2. The filters do exhibit a transient effect, but in the presentation of the data in the following figures, the first and last 50 min of each filtered data segment have been removed to eliminate the transient contamination of the radial velocities

and echo power.

5. OBSERVATIONS

In an attempt to investigate the existence of horizontally layered structures, vertical beam echo power profiles were plotted as a function of time and height and visually inspected for horizontal layers of enhanced echo power which existed for an extended period of time at both radar sites. Although the range resolution at the Urbana radar is only half that available from the Flatland data, both observed extended layers of enhanced echo power in the troposphere at about 8 km and in the stratosphere at about 15.5 km, as shown in Figure 2. The tropospheric layer appeared to move both above and below 8 km during the campaign, but for the majority of the time it remained at about 8 km. From Figure 2, we see the layer existed about 8 km at both sites for the entire day of August 17 and extended to the afternoon of August 18. As was noted in section 3, a convective thunderstorm moved into the area on the evening of August 18. The layer disappeared during that period. This layer showed no apparent relation to the temperature profries obtained from the radiosonde data at either Peoria or Salem. Another striking layer is at about 15.5 km at both sites. From the information obtained from radiosonde

data, this layer most probably corresponds to the layer in the stratosphere just above the tropepause. That layer remained for the entire data set, with some distortion during the thunderstorm. Also from Figure 2, we note that other layers existed at different heights. However, they moved in and out and usually disappeared in a few hours. In what follows, we will be concentrating on the two long-lasting layers at about 8 and 15.5 km and investigate the short-term and long-term temporal behavior of the layers at the two sites.

An attempt was also made to investigate the relationship between radial velocities at the Urbana and Flatland sites

DESTER ET AL.: OBSERVATIONS BY ST RADARS 921

(a• Eatland ß 0830 08/17/88 - 0'/30 05/19/88

Power averaged over 40 minutes

,

ß

Fig. 2. Vertical beam echo power profiles as a function of time and height for (a) the Flatland radar and Co) the Urbana radar on August 17-19, 1988. Each profile has been averaged over 40 min.

during this experiment. Figure 3 shows the time series for the radial velocities at the two sites for the entire duration of the

campaign. The arrow at 2300 LT on August 18, 1988, in this figure and subsequent figures, denotes the approximate time the thunderstorm activity was observed approaching the Urbana radar site. A striking correlation is apparent from the figure, i.e., enhanced velocity fluctuations were seen at both sites at all heights during the time of the convective storm. It should be noted that the three lower range gates at the Urbana radar (2 - 5 km) were partially contaminated by ringing from the transmitter and antenna, consequently biasing the velocity

estimates toward zero. During the quieter times these three range gates exhibit smaller velocity fluctuations than those observed by the Flatland radar.

Figure 4 displays 3-hour averages of the eastward component of the horizontal wind as measured by the 20' off-vertical beam of the Flatland radar. From the figure we See that the eastward component of the horizontal wind was between 6 and 10 m/s on August 17 for both heights displayed. During the first 9 hours of the August 18 data set the horizontal wind was the quietest at 6.5 km, while the wind at 8 km began to slowly increase. Then, later in the data set the wind at 6.5 km also increased to about the same level. This rapid increase in the horizontal wind on August 18 at 6.5 km is related to the observed increase in vertical velocity fluctuations at the same height in Figure 3.

6. DISCUSSION

6.1. Layer near the tropopause

As was noted from Figure 2, one significant layer exists at approximately 15.5 km for the entire duration of the simultaneous operation of the Flatland and Urbana radars. From radiosonde data, this layer appears to be most likely related to the tropopause, which was observed to be near 15 km during both August 17 and 18. This layer was observed when the Urbana radar came on-line (August 15, 1988; see Figure 5), during the 2 days of simultaneous observation with the Flatland radar (see Figure 2), and it also persisted in the days following, August 19-22 (not shown).

To better investigate the correlation of power fluctuations at this layer, we have plotted in Figure 6 the filtered time series of echo power at 15.5 km for both the Urbana and Flafiand radars. Because the two radar systems are not joinfly calibrated for power comparisons, the magnitudes of the echo power have been self-normalized so that power comparisons shown in this paper are on a relative basis.

In Figure 6a we have plotted the low-pass filtered data for the August 17 data set, for power fluctuations of T > 200 min. Especially during the first half of the data set, we see fairly good correlation in the long-period oscillations. Figure 6b shows the band-pass filtered version of the same data set, with fluctuations of 100 rain < T < 200 rain being within the passband. Within this frequency range we also see that the data seem to exhibit some degree of correlation, especially over the first 900 or 1000 min. First, it appears that Flatland leads Urbana, and then apparently Urbana and Flafiand observe the simultaneous fluctuations within this frequency band. We have also plotted in Figure 6c the high-pass filtered version of this data set using the filter HHp(z ). The Urbana data have been offset by -1.0 on the scale of relative power to allow for clearer viewing. From this plot it is difficult to say if there is any observable correlation at these higher frequencies. With this present method of data analysis, high-frequency correlation is not readily apparent.

922 DESTER ET AL.: OBSERVATIONS BY ST RADARS

(a)

14

13

12

• 7

FLATLAND RADAR' (3.4 min res.

--

I " -, • '

'

snr > -2.00, 2.0 m/s between heights

0 t t t • I I I • • I • I t i I ! ! i I I t i t i I I t • • I • , ! i I I i i i I ! i i i { i t t t

• 5 10 15 20 25 30 35 •40 45 50 0830 LT time (hours) 2300 tI

08/17/88 08/18/88

Fig. 3. Radial velocity time series for August 17-19, 1988. (a) Flatland velocities are separated by 2.0 m/s between range gates (0.75 km = 2.0 m/s). Co) Urbana velocities are separated by 4.0 m/s between range gates (1.5 km = 4.0 m/s). Only velocity estimates with a corresponding signal-to-noise ratio of greater than -2.0 are shown.

DESTER ET AL.: OBSERVATIONS BY ST RADARS 923

14

13

12

11

10

.-.8

4• 7

(b) URBANA RADAR: (2.25 min res.)

i • • • i i i , • I I • • • I , , I , I , , • • I • I , I I • , , , I • • I I I ' ' ' ' I ' ' ' ' --

snr > -2.00, 4.0 m/s between heights

'11 t

i

3 -- --

--

1 -

--

0 ,,,,I,it, I,

• 5 !0 0830 LT

08/17/88

2 ß

Ill] IIII] IllIll III] III !] I I II [ !1 II] I I11

15 20 25 30 35 • 40 45 50 time (hours) 2300LT

08/18/88

Fig. 3. (continued)

In Figures 6(1 and 6e we have plotted the low-pass and band- pass filtered data for August 18. These data exhibit some longer-period correlation, but the band-pass f'dtered data appear less correlated than their counterpart from August 17. This could be due to the passage of the thunderstorm on August 18 whose approximate time of passage is denoted by the solid arrow at 2300 LT in Figures 6d and 6e. After the passage of the thunderstorm the band-pass fdtered data do begin to exhibit correlation.

6.2. Tropospheric velocity behavior during a quiet period

As was mentioned previously, Figure 4 displays the 3-hour averages of the eastward component to the horizontal wind as derived from the 20' off-vertical beam of the Flailand radar.

We have displayed the winds at 6.5 and 8 km because we were able to obtain good vertical velocity estimates at these heights,

924 DESTER ET AL.' OBSERVATIONS BY ST RADARS

2O

Flatland Horizontal Velocity (Eastward Component)

0830 LT 0800 LT 2300 LT 08/17•88 08/18/88 08/18/88

Time (hours)

6.5 km

- 8 km

Velocities averaged over 3 hours

50

Fig. 4. Eastward component of the horizontal wind at 6.5 and 8 km derived from the 20' off-vertical beam of the Flatland

radar, for August 17-19, 1988. Velocities are averaged over 3 hours.

and the horizontal layer seemed to exist most of the time at 8 km, although for part of the time it appears to have dropped to 6.5 km. Throughout the August 17 data, the east-west component of the horizontal wind at 6.5 and 8 km did not vary significantly, and the winds at both heights are quite similar in their longer-period fluctuations. The magnitude of the horizontal wind was between 6 and 10 m/s or so during this

The velocity data were filtered using the low-pass, band- pass, and high-pass filters previously mentioned and displayed in a manner similar to that used to compare echo power in section 6.1. Although not shown in this paper, there did not appear to be a significant amount of correlation in any of the frequency bands at 8 km on August 17. Some similar features between Flatland and Urbana data were observed at 6.5 km, but it is difficult to say if one frequency band exhibited more correlation than another for the August 17 data set.

On August 18, however, the horizontal wind velocity exhibited the quietest activity at 6.5 km during the first 9 hours (540 min) or so as seen in Figure 4. Figure 7 shows the band-pass and low-pass filtered versions of the radial velocities. Again, as was the case for echo power, the high- pass filtered data are not very helpful in determining the amount of correlation and have been omitted from this

discussion. However, during the first 600 min or so, when the horizontal wind was the quietest at 6.5 kin, we see extremely

Urbana 0925 08/15/88 - 0850 08/16/88 Power averaged over 20 minutes

Urbana

0900 08/16/88 - 0825 08/17/88 Power averaged over 20 miunutes

Fig. 5. Vertical beam echo power profiles as a function of time and height for Urbana radar averaged over 20 min on August 15-17, 1988. The first segment is the first 23.5 hours of data, and the second segment is the second 23.5 hours of data. Height scales are the same as displayed in Figure 2b.

DESTER ET AL.: OBSERVATIONS BY ST RADARS 925

good correlation in the band-pass filtered version, Figure 7b. Then, although the fluctuations are not as well correlated, we see a similar increase in activity at both radars from 600 to 1000 min, roughly the time period at which the horizontal winds began to increase. Figure 7a, which shows oscillations with T > 200 min at 6.5 km, shows much less correlation in the corresponding time frames than was evident in the band- pass filtered data. This indicates that during this particular quiet period, vertical velocity fluctuations were highly correlated over a distance of 25 km in the band with periods of 100 min < T < 200 min. Even during active periods, as the horizontal wind velocity increased after the first 600 min, the radial velocities at the 6.5-kin height show much less correlation in specific events, but show a similar increase in overall activity in Figure 7b which is not as apparent in Figure 7a.

Figures 7c and 7d show similar plots for radial velocities at 8 km. As was noted above, the horizontal wind at 8 km did not decrease during the first part of August 18; in fact, it increased during the time the 6.5-km horizontal wind was decreasing. The plots of filtered velocities show much less noticeable correlation in the low-pass and band-pass frequency bands at 8 km than was present in the 6.5-km data. These observations indicate that in this particular experiment we were able to see higher degrees of correlation in vertical velocities in a specific frequency band at a height where the horizontal winds were quietest. In particular, oscillations with periods between 100 min < T < 200 min exhibited the highest degree of correlation at 6.5 km during this particular time period.

One other matter which should be addressed here is the

noticeable offset in the measurements of "vertical" velocities

for the Urbana data, as shown in Figures 7a and 7c. As mentioned before, by using a radio star calibration, we were able to estimate that the beam was approximately off-vertical by 3' in the eastward direction. Visually, it appears that the offset trend of the Urbana data seen in the low-pass filtered velocity plots in Figure 7 is closely related to the horizontal wind recorded by the Flatland radar in Figure 4. As a check in the estimate of the Urbana beam-pointing direction, we shall estimate the pointing angle of the beam, 0, in the eastward direction. If we estimate the contribution to the vertical

velocity (w) by the eastward horizontal velocity (u) we have /Jw =/Ju * sin0 for the vertical velocity offset. We estimate that from the beginning to the middle of the August 18 data set,/Jw _-- 0.2 m/s and/Ju _= 3 m/s at 8 km from Figure 7c and Figure 4, respectively. Then

0= sin- 1 (/J•u) sin-1 (0'•.•) = -= 3.8'

This is relatively consistent with the radio star estimate for the projection of the beam-pointing direction in the east-west plane, -3'. It should be noted that if any southward component of the horizontal wind, v, contaminated the beam, it was not taken into consideration here.

6.3. Active period

Figure 8 presents the low-pass and band-pass filtered echo power at 6.5 km and 8 km for the August 18 data set. From Figure 8a, we can see that the graph which includes the longer- period variations in power levels (T > 200 min) shows more correlation than is observed in the band-pass filtered version of the same data set (Figure 8c). The solid arrow in Figure 8 indicates the approximate time (- 2300 LT on August 18) at which a convective thunderstorm began to approach the Urbana site. In Figure 8a we see particularly good long-period correlation with very little time delay for the first 700 min or so on August 18. Since the data for this period were quite stationary, a correlation analysis was carried out. Figure 9 shows the correlation function between echo powers at the two radars after the time series were low-pass filtered at 200 min. We note the excellent correlation. In the period following the thunderstorm, however, this correlation is not quite so obvious. The overall power level of the peaks of the Haftand echo power increases in Figure 8a while the Urbana power level decreases after the passing of the thunderstorm. From this limited amount of data at this height it is difficult to say whether the structure exhibited a large time delay between sites, or whether the echo power was no longer correlated at that height. But it should be noted that the layer at about 8 km disappeared about 5 or 6 hours before the thunderstorm approached, shortly before we began to observe a decrease in echo power correlation.

However, if we observe similar plots of echo power at the next lowest height over the same time period, the data exhibit a fairly good correlation, especially after the passing of the storm. In Figure 8b we see that the lower-frequency waves exhibit very similar characteristics over the last 300 min of data. Figure 8d also shows better correlation during and after the storm than before. From Figures 8a and 8b and taking Figure 2 into consideration, we suggest the following scenario for the lower layer. During the first-250 min the layer was probably slightly lower than 8 km, hence the power peak at 6.5 km in Figure 8b during that period. Then the layer moved slightly higher near 8 km until -450 min. As was mentioned earlier, the layer began to disappear-5-6 hours before the arrival of the thunderstorm, and it was noted that this disappearance corresponded roughly to an increase in the horizontal winds. We then see that for the remainder of the

August 18 data segment, the peaks in longer-period echo powers occur at roughly the same time at both heights, with the exception of the last peak at 8 km in the Flailand data in Figure 8a. This suggests that the two heights are somewhat coupled together in their ability to scatter power after the specific layer dissipates, and is supported by the observations made from Figure 2.

One additional observation of interest made during August 18 was that at the time the thunderstorm passed over the Urbana radar (-2300 LT), an increase in reflected power was seen at several heights in the lower stratosphere. The increase

926 DESTER ET AL.: OBSERVATIONS BY ST RADARS

(a)

BB/]7/BB

Lowpass cutoff , 200 minutes +++ = URBANA. - = FLATLAND (b)

15.50 km

................ .... 1,i,,t i "

._

e -.2

,3 - .4

.E -.6

-.•

• ''''' •''''''''''' •' •' •'ee' •'• ' • 100 200 300 400 5•8 688 700 888 988 1888 1 1 0 1 88 1488 0830 LT 08/17/88

.7

ß .4 L

08/17/88

Bandpass cutoff , 100 •nd 20• minutes +++ = URBANA, - = FLATLAND

15.50 km

[ I ] I ' I • I I I ] I ] I ' I '•1 [ I [ I • I [ I

-1.0•, , , , , • , [ , • , ' , ' , ' , ' , • ' •[80' ' ' [ ' time (minutes) time (minutes) 0830 LT 08/17/88

(c) ! i [

.8

.6

.4

•-.•

• .4 _

-1 4

-1

-1 8

-2 • , I I I I I [ I I I I I ' I , I , I. , I i t [ I I I I

time (mlnutesi 08•0 LT 08/17/88

06/17/88

Highp•ss cutoff , 100 minutes

URBANA, - = FLATLAND (d) 15.50 km

i [ i , i i i i i i i !

.?

._

.3

08/18/88

Lowpass cutoff , 200 minutes +++ = URBANA, .. ; FLATLAND

15.50 km

++ +

_

k 108 280 380 400 580 600 700 800 1 80 I 1200 1388 1480 0800 LT time (mlnutesl 2300 LT 08/18/88 08/18/88

Fig. 6. Simultaneous low-pass and band-pass filtered echo power at 15.5 km for the Urbana and Flatland radars. The data were filtered with filters as described in section 4: filters (a) H200(z ), (b) HBp(z ), and (½) HHp(z) were applied to the

August 17 data. Filters (d) H200(z) and (e) HBp(z) were applied to the August 18 data. Solid lines and crosses indicate the filtered Flatland and Urbana data, r•dvely.

DESTER ET AL.: OBSERVATIONS BY ST RADARS 927

08/18/BB

Bandpass cutoff , 10• •nd 200 minutes

(½) +++ = URBANA. - = FLATLAND 15.50 k-m

.4

+ +

Engineering, respectively, are now in operation in the flat terrain over east central Illinois. On August 17-19, 1988, these radars were operated simultaneously to obtain a data set from the troposphere and stratosphere which is approximately 47 hours long. To the best knowledge of the authors, our results are the first attempt to study the stratosphere and troposphere using VHF radar data from two sites separated by about 25 km.

In our observations, two horizontally stratified layers which extended for a long period of time at both radar sites were observed, indicating that these layers are of a scale size greater than 25 km. The upper layer is most likely related to the tropopause, and we observed that longer-period echo power fluctuations were better correlated than those observed at higher frequencies at this layer. Vertical velocities at this height were unreliable due to low signal-to-noise ratio. The lower layer existed throughout most of the data set, but disappeared 5 or 6 hours before the approach of a convective thunderstorm, as the horizontal wind began to increase. Echo power comparisons at ++

-.6 • this height (8.5 km) indicate that good correlation can be observed in the lower-frequency band (T > 200 min), but that

_ ,. , , , , , , • , , , , , , , ,•, , , , , , , , it may also be possible to see good correlation in other 800 1100 200 1300 1400 6• 7ee frequency bands as well. It is difficult to say that one

0800 LT t i me (m i nut es 12300 LT frequency band of echo power is more correlated than another, 08/18/88 08/18/88

Fig. 6. (continued)

in echo power at heights > 11 km can be seen in the echo power levels shown in Figure 2. The Flatland radar did observe an increase in reflected power, but it was for a much shorter amount of time at similar heights. We believe the enhanced echo power could be most likely attributed to the influence of the thunderstorm.

In previous observations of the troposphere and stratosphere with the Urbana VI-IF radar, an increase in echo power at stratospheric heights was also observed during updrafts and downdrafts associated with thunderstorms. Goss and Bowhill

[1983] suggested that the increase in scattered power could be due to increased turbulence in those regions, rather than some horizontally stratified struc•e. Green et al. [ 1978] observed a similar increase in reflectivity above a thunderstorm in Colorado well into the lower stratosphere for a short time duration. If the increase we have seen at Urbana is due to the

thunderstorm, it could indicate that the enhanced echo power during the thunderstorm was rather localized, since the increase observed in the stratosphere at the Flatland radar did not occur for nearly the same time period

7. CONCLUSION

The Flatland and Urbana ST radars, which are operated by the National Oceanic and Atmospheric Administration and the University of Illinois Department of Electrical and Computer

as a whole, since the data set observed was for a short period of time and correlation of varying degrees was observed in each band.

Perhaps the item of most interest was the extremely good correlation observed in the vertical velocity fluctuations during the quiet period on August 18 at 6.5 kin. Here we observed that in the frequency band 100 rain < T < 200 rain the correlation was clearly much better than that observed at periods of T > 200 min during the first 600 min or so of data. During periods of higher winds the correlation in most bands was not quite as good as at this specific instance during low winds. Although the velocity fluctuations were not well correlated during the specific period of high winds corresponding to the thunderstorm, both radars did observe similar increases in the variance of the vertical velocities

during that period, especially at 6.5 km. Carter et al. [1989] observed that over a 6-week period the vertical velocity fluctuations at stations spaced -4-6 km apart tended to exhibit increased correlation with decreasing frequency. However, on several days they isolated some specific wave events during periods of quiet winds which had periods of-30-90 min. The observed correlated vertical velocity fluctuations here were with periods slightly longer than that. This may be due to the fact that the stations are spaced further apart. Waves with shorter horizontal wavelength and shorter periods may not be well correlated at the two stations.

As previously mentioned, Carter et al. [1989] have recently published a more detailed analysis of internal gravity waves observed during the ALPEX campaign using three wind profilers with a spacing of ~ 4-6 kin. They have shown that wind-profiling Doppler radars can be quite useful for studying

928 DESTER ET AL.: OBSERVATIONS BY ST RADARS

(a)

.4

.3

0800 LT t i ae { m i nutes I 2300 LT 08/18/88

0õ/1 8/BB 0B/1

Lowp•ss cutoff , 200 minutes B•tndp4ss cutoff , 100 and 2• minutes

+++ = URBANA. - = FL•TLAND (b) +++ = URBANA. - = FL•TLAND 6.5e km 6.5e km

.25

.le

.15

.25

,,,,,,,,,, ,,,,,, 0800 LT { i me ( m i nu{es I 2300 LT

08/10/86 08/18/88 08/18/88

(c)

-.1

Lowpass cutoff , 200 ml nutes +++ : URB/•NA. - : FL/•TLAND

B.00 km ! i • i , i ! i , i ! i ! i !

,Iflil,l,l,l,lflf•t; ill'Ill] 196 266 369 486 566 666 796 606 1 66 1109 1299 1366 1660 0800 LT 08/18/88

time (minutes] 2300LT 08/18/88

(d)

•B/1B/BB

Bandpass cutoff , 100 an.d 20B minutes +++ = URBANA. - = FLATLAND

8.00 km ! i ! i , i i i , i , i ! i i i ! i i i i i ! i i i

_

i i i i i i i i i I i i • i i i i i i , i i i , i lee 2ee 3ee ace see Bee 7• oB• • 19• 1lee 12ee 13•e 14ee

0800 LT • i me { mi nu•es 12300 LT 08/18/88 08/18/88

Fig. 7. Simultaneous low-pass and band-pass filtered radial HBp(z ) were applied to the August 18 radial velocity data for velocity at 6.5 and 8 km for the Urbana and Flatland radars. 8.5 km. Solid lines and crosses indicate the filtered Flatland Filters (a) H200(z) and (b) HBp(z ) were applied to the August and Urbana data, restively. 18 data from 6.5 km. Identical filters (c) H200(z ) and (d)

DESTER ET AL.: OBSERVATIONS BY ST RADARS 929

(a)

.9

.8

.7

L

.3

.2

.1

0

, •k 1 8 288 :388 408 588 688 7• 888 1888 11 100 1 88 1488 0800 LT 'E ime (minu'Ees 12300 LT 08/18/88

08/18/88 08/18/88 Lowpass cutoff , 200 minutes Lowpass cutoff , 200 minutes

+++ = URBANA. - = FLATLAND (b) +++ = URBANA. - = FLATLAND 8.00 km 6.50 km

_ .9 - •

.7

•.4

.3

.2

• 1• 2• 3• 4• 5• 6• 7• 8• 1• 1 • •2•e 13S• 0800 LT { i me ( m • mu{es ] •300 LT

08/18/88 08/• 8/88 08/18/88

0B/18/BB •8/18/BB Bandpass cutoff , 100 and 200 minutes Bandpass cutoff , 100 and 20B minutes

(C) +++ : URBANA. - = FLATLAND (d) "'+"' = URBANA. - = FLATLAND 8.00 km 6.50 km L' ' ' ' ' '"•' ' ' ' ' ' ' ' '/•' ' ' ' ' ' ' '

' :: ß .s ß

+++ + ++ + ß ; ß ß + • ++ + + + *t .4

L e • .t + + + ++ •

i.-.6

';_.. •.-. L' / ++

-1.2

-1 4 -.6

-1 6

-1 .•

-2. • I I • I i I • ! I I • I • I • I , I • I , I , I • I • -1.

0800 LT { i me { m I nu•es I 2300 LT 0800 LT { I me { m i nu•es I 2300 LT 08/18/88 08/18/88 08/! 8/88 08/! 8/88

Fig. 8. Simultaneous low-pass f'dtered echo power for Urbana and Flatland radars on August 18, 1988. The data have been filtered with H200(z) at (a) 8 km and (b) 6.5 km. The data

were also filtered with the band-pass filter HBp(z ) at (c) 8 km and (d) 6.5 km. Solid lines and crosses indicate the filtered Flatland and Urbana data, respectively.

930 DESTER ET AL.: OBSERVATIONS BY ST RADARS

.4 Z 0

-J-.•

0 •-J -.4

-.6

-.8

Correlation , 0-70• minutes 8/18/88 FLATLAND-URBANA Correlation Pwr. correl&•lon, LP cutoff : 2•0 mtn

I • I • I • I • I -20 -10 o 10 20

LAGS {1 LAG = 10 HINUTES}

km i i i • i , i , i E I

ß

I, I I I I -30 30 40

Fig. 9. Correlation function of the first 700 min of low-pass filtered radial velocity data from the August 18 data set at 8 km (data shown in Figure 8a). Negative lags indicate Flatland leads Urbana.

gravity waves in the lower atmosphere. Although it is beyond the scope of this paper, future experiments could be designed and conducted at the Flatland and Urbal• radars to supply more detailed information on wave observations on the 25-kin scale.

The wind profiler demonstration network currently planned for deployment in the central United States will provide stations with a separation of --400 km [Chadwick, 1988]. Additional information from Flatland-Urbana comparisons, coupled with comparisons of other radar stations could give a more complete description of wave parameters over a wide variety of scale sizes.

The particular method of analysis used here has been helpful in identifying the general frequency bands in which fluctuations in vertical velocity and echo power are correlated over a spacing of 25 km. However, this method does have problems in determining the specific frequency or frequencies of waves present in the data set. In the future, if a data set of longer length is available, additional methods such as the coherence method used by Ecklund et al. [1985] and Carter et al. [1989] would be helpful in detecting some of the weaker, underlying waves which are not visible with the present technique. This analysis would be especially helpful in determining the amount of correlation present for fluctuations with T < 100 min, which our present study was not able to do. Of additional interest to this experiment, another ST system is presently under construction at the University of Illinois Aeronomy Laboratory Field Station. This system will

improve range resolution and vertical stability and will enhance the observational capabilities for future comparisons with the Flatland radar.

Acknowledgments. Research at the Urbana radar is supported by the National Science Foundation under grant ATM 87-11938. The Flatland radar research is partially supported by the National Science Foundation under grant ATM-8512513. Necessary assistance in obtaining the Urbana data was graciously provided by Gary Stitt. The authors are also grateful to Pat Kennedy, Doug Jones, and Bob Scott from the Illinois State Water Survey for their helpful assistance in obtaining the CHILL radar data, pertinent weather conditions, and radiosonde data for this experiment. The authors would also like to gratefully acknowledge the constructive comments and observations of the reviewers which added greatly to the content of this paper.

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Carter, D. A., B. B. Balsley, W. L. Ecklund, K. S. Gage, A. C. Riddle, R. Garello, and M. Crochet, Investigations of internal gravity waves using three vertically directed closely spaced wind profilers, J. Geophys. Res., 94, 8633-8642, 1989.

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DESTER ET AL.: OBSERVATIONS BY ST RADARS 931

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