Ground based radiometric observations of atmospheric attenuation at 20.6, 31.65, and 90.0 GHz were made at Wallops Island, Virginia during April and May 1989. Early results from the analysis of the data set are compared with previous observations from California and Colorado. The relative attenuation ratios observed at each frequency during clear, cloudy, and rainy conditions are shown. Plans for complete analysis of the data are described

Beeler, R. H.

Jacobson, M. D.

Snider, J. B.

English

NASA Technical Reports Server

N90- 17 95 7
OBSERVATIONS OF ATTENUATION AT 20.6, 31.65 AND 90.0 GHz -
PRELIMINARY RESULTS FROM WALLOPS ISLAND, VA
J. B. Snider, M. D. Jacobson, and R. H. Beeler
NOAA/ERL/Wave Propagation Laboratory
Abstract - Ground-based radiometric observations of atmos-
pheric attenuation at 20.6, 31.65 and 90.0 GHz were made at Wallops
Island, VA during April and May 1989. Early results from the
analysis of the data set are compared with previous observations
from California and Colorado. The relative attenuation ratios
observed at each frequency during clear, cloudy, and rainy
conditions are shown. Plans for complete analysis of the data are
described.
i. Introduction
During 1987 and 1988 the NOAA/ERL/Wave Propagation Laboratory
(WPL) transportable three-channel ground-based radiometer measured
atmospheric attenuation at 20.6, 31.65, and 90.0 GHz at San Nicolas
Island, CA (July 1987) and Denver, CO (December 1987 and August
1988). From April ii to May 8, 1989, additional measurements were
made at Wallops Island, VA (hereafter referred to as Wallops for
brevity). Because of the relatively short time between
observations and the June 1989 NASA Propagation Experimenters
(NAPEX) meeting, and because all supporting data have not yet been
made available, the material presented in this paper is preliminary
and incomplete. However, since both the quality and the variety
of the data collected at Wallops were exceptional, it is desirable
to present some early results.
2. Description of Experiment
The observations at Wallops were made in conjunction with an
experiment designed to investigate the measurement of atmospheric
moisture by an array of ground-based remote sensors and in situ
balloon-borne humidity sensors. The experiment, ATmospheric
Moisture Intercomparison Study (ATMIS), was conducted primarily
during the nighttime hours from April ii to April 18. The
organizing agency for ATMIS was NASA/Goddard.
During ATMIS m joint observations of water vapor were made
simultaneously by radiometers operated by Jet Propulsion Laboratory
(JPL), NASA/Goddard, Pennsylvania State University (Penn State),
and WPL, and by an ultraviolet lidar operated by NASA/Goddard. In
addition, when clouds were present, simultaneous observations were
made by the SPANDAR radar operated by the Applied Physics
Laboratory of Johns Hopkins University. Supporting data consisted
of several radiosondes daily, each equipped with three different
types of humidity sensor, and a lidar ceilometer and multi-beam
138
https://ntrs.nasa.gov/search.jsp?R=19900008641 2020-03-20T00:06:51+00:00Z
Doppler acoustic sounder, both owned and operated by Penn State.
During the week of the ATMIS, and for the following three weeks,
the WPL radiometer operated mostly in the zenith direction
providing both water vapor data and atmospheric attenuation data.
However, on a few occasions the system was employed in an azimuthal
scan mode to make joint observations with other radiometers having
scan capability to determine the homogeneity of the water vapor
field. Other brief interruptions of the attenuation measurements
were necessary for instrument calibration. Approximately 570 hours
of attenuation data were collected by the WPL system.
Procedures employed by the WPL microwave radiometer in the
measurement of atmospheric attenuation have been described at
previous NAPEX meetings and in the literature (Westwater et al.,
1988; Westwater and Snider, 1989) and will not be presented here.
However, whereas previous observations have employed either 60- or
120-sec averaging times, we used a 30-sec interval at Wallops in
order to extend subsequent spectral analysis to higher temporal
frequencies.
3. Preliminary Results
Weather at Wallops varied from clear and dry to four events
in which the precipitation rate was perhaps the highest observed
to date (inferred from the attenuation values as quantitative
precipitation rates were not measured). Because of the high
rainfall rates, radiometer saturation at all frequencies occurred
on a few occasions. More typically, however, saturation was found
to occur at 90 GHz but not at the two lower frequencies.
Saturation events have been removed from the data set by limiting
attenuation values derived from the radiometer data to less than
12 dB. A case where saturation occurs is indicated in the time
series of Fig. i.
Cumulative distributions of zenith attenuation are presented
in Fig. 2, along with means and standard deviations. Note that at
a relatively wet location such as Wallops, the atmospheric
attenuation at 20.6 GHz generally exceeds that at 31.65 GHz. This
is in contrast to the situation at Denver in December where
attenuation at 31.65 GHz exceeds that at 20.6 GHz for about 65
percent of the time. The general shape of the distribution is
similar for Denver and Wallops since some precipitation occurred.
However, for SNI where no precipitation fell, the distributions are
dissimilar. Table I contains a statistical comparison of our
radiometrically-derived atmospheric attenuation at various
locations.
In another useful type of comparison that deserves increasing
attention, we examined the ratios of attenuation observed at the
three frequencies for both combined clear-cloudy conditions and for
139
clear conditions at Wallops. Results for the former are in Fig. 3a,
which shows how the ratios vary as conditions change from clear to
cloudy to precipitating. In general, the attenuation ratios remain
relatively constant on the right side of the distributions. This
portion of the distribution is associated with clear weather and
non-precipitating clouds. However, on the left side of the
distribution, where precipitation and high liquid water contents
are present, the ratios change rather dramatically, approaching a
factor of 9 for the 90/20 GHz attenuation ratio.
Similar data for the clear weather case (Fig. 3b) show a much
smaller variation, in general, although some non-linear effects
occur during conditions of higher atmospheric moisture.
Understanding these effects, of course, is critically important to
the eventual extension of the satellite communication bands to 90
GHz and higher. During the next year, we plan to investigate
attenuation ratios in greater detail and include the data sets
collected earlier in the NAPEX program. Fig. 4 contains
attenuation ratio data observed in Denver during August 1988 for
clear, cloudy, and rainy conditions.
4. Future Plans
Plans for future analysis of the Wallops data set include
studies similar to those previously reported as well as new
investigations:
Comparisons of measured and calculated values of atmospheric
absorption and attempts to improve present absorption models.
The simultaneous observations of atmospheric humidity by three
different types of sensor may help shed new light upon the
modelling problem.
Further study of regression relationships for prediction of
attenuation between various frequencies.
- Refinement of liquid absorption models.
Further studies of the attenuation ratios observed during the
Wallops observations and extension of the study to the entire
data set.
140
Table I - Comparison of 20.6/31.65/90.0 GHz Attenuation Statistics
For Three Locations at Various Times of the Year. Clear
and Cloudy Data Combined.
Location
20.6 GHz
Mean Std. Dev.
31.65 GHz
Mean Std, Dev.
90.0 GHz
Mean Std. D@v.
San Nicolas
(el 13 m)
July 1987
(All values are in dB)
0.398 0.113 0.321 0.096 1.128 0.498
Denver
(el 1611 m) 0.159
Dec. 1987
0.068 0.158 0.081 0.411 0.323
Denver
(el 1611 m) 0.497
August 1988
0.195 0.278 0.259 1.190 1.850
Wallops Is.
(el 2 m)
April/May
1989
0.428 0.291 0.398 0.407 1.239 1.323
References
Westwater, E. R., J. B. Snider, and M. J. Falls, 1988: Obser-
vations of Atmospheric Emission and Attenuation at 20.6,
31.65, and 90.0 GHz by a Ground-based Radiometer, NOAA
Technical Memorandum ERL WPL-156, November 1988, ppl6.
Westwater, E. R., and J. B. Snider, 1989: Ground-based Radiometric
Observations of Atmospheric Emission at 20.6, 31.65, and 90.0
GHz, Proc. Sixth International Conference on Antennas and
Propagation ICAP 89, Part i: Antennas April 1989, pp 229-233.
141
WALLOPS IS., VA.
37.51 N. LAT. , 75.29 W. LONG.
2.0 M MSL
20.8 GHz --
31.65 GHZ ...........
90.0 CHz
IIOUR OF DAY (Lrl'C) (89,5 2)
Figure 1. Time series of brightness temperatures at 20.6, 31.65,
and 90.0 GHz during clouds and precipitation. Radio-
meter saturation at all frequencies occurs near 1000
and 1210 UTC.
001 0.1 0.5 1 2 5 10 20 30405060 70 N 90 95 96 99995 99.9 9U9
Percent _ T'cme Ordinate is F.xcuded
Figure 2. Cumulative distributions of zenith attenuation at
Wallops Island, VA, for clear and cloudy conditions.
142
3.o
j2
90/20
[ i I I ! [ I 1 I I 1 I
Wallops Island, VA
April - May 1989
Clear and Cloudy
Ratio Mean StcLDev.
30/----20 0.928 -(3.195
90/20 2.673 0.764
90/30 2.929 0.627
0,01
1
0
0.01
I I I I
0,1 0,5 1 2
I 1 I I
I I ] I I I 1 ] I I 1 I I I I I
5 10 20 30 40 5060 70 80 90 95 98 9999.5 99.q
Percent of Time Ordinate is Exceeded
I I I I I I 11 I I I I I I I
-- Wallops Island, VA --
------.- April - May 1989 -
-----... _ 90/30 Clear _
_%% Ratio Mean StcLDev.90/20
-__ % 3---_00.---_-5-2
%_ 90/20 2.392 0.176 -
_-._... 90/30 2.580 0.327 -
- "qnb 'W_,
I I I I I I I I I I I L I I I
5 10 20 30 40 5060 70 80 90 95 98 9999.5 99.9 99.99
[ I [ I
0,1 0,5 1 2
Percent of Time Ordinate is Exceeded
Figure 3. Cumulative distributions of attenuation ratios
for (a) clear and cloudy conditions, and (b) clear
conditions. Wallops Island, VA, April-May 1989.
143
90/20
I
0.1
I I I I 1
0,5 1 2 5 10
I I I I I I I I I I I I
Denver, Colorado
August, 1988
Clear and Cloudy
Ratio Mea. __d_Dev.
30/20 0.525 0.136
90/20 1.g42 0.651
90/30 90/30 3.701 0.431
I I I ] ] I I
20 30 40 50 60 70 80
I I J I ] [
90 95 98 99 99.5 99.9
Percent of Time Ordinate is Exceeded
99.99
Figure 4. Cumulative distributions of attenuation ratios for
clear, cloudy, and rainy conditions at Denver,
August 1988.
CO,
144


Observations of attenuation at 20.6, 31.65 and 90.0 GHz: Preliminary results from Wallops Island, VA

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server