The quality of rainfall intensity estimates derived from passive microwave measurements by the electrically scanned microwave radiometer (ESMR-5) aboard the Nimbus 5 satellite are evaluated. The microwave measurements used are those coincident with the global atmospheric research program Atlantic tropical experiment (GATE). The ESMR 5 derived rainfall intensity estimates are compared with hourly averaged GATE radar rainfall measurements. It is determined from the transfer curves derived using the radar measurements as ground truth, that the ESMR 5 derived data consistently over estimates rainfall by a factor of approximately 1.4

Cottrell, R. R.

Knox, N.

English

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i	 +
ASSESSMENT OF THE QUALITY OF 'GATE' AREA RAINFALL
DATA FROM A NIMBUS-5 RADIOMETER
Final Report Under NASA Grant NAG 5-14
ASSESSMENT OF THE QUALITY OF GATE AREA	 N81-16685
RAINFALL DATA FROM A NIMBUS-5 RADIOMETER
Final Report, (Morgan State Univ., Baltimore,
Ma.) 18 p HC A02/MF A01	 CSCL 04B G3/47 411575
Dr. Nathaniel Knox - Principal Investigator
Rosalind R. Cottrell - Research Assistant
Department of Mathematics
Morgan State University
Baltimore, Maryland 21239
February 1981
The purpose of the present atudy is to evaluate the quality
of rainfall intensity estimates derived from passive microwave
Measurements by the Electrically Scanned Microwave Radiometer
(ESMR-5) aboard the Himbus-5 satellite. The microwave measurements
used are those coincident with the GARP (Global Atmospheric Research
Program) Atlantic Tropical Experiment (GATE). ESMR-5 derived rain-
fall intensity estimates are compared with hourly averaged GATE radar
rainfall measurements. Using the radar measurements as ground truth
it is determined that with the transfer curves derived herein the
OW-5 derived data consistently over estimates rainfall by a factor
of approximately 1.4.
RESEARCH MAN
The ESMR-5 data set used herein consists of computer printouts
of microwave brightness temperature measurements for seventy-nine
(79) Nimbus-5 overpasses coincident with GATE radar rainfall measure-
meats. The following tasks wore completed in assessing the quality
of these data:
1. Collect and verify computer printouts of ESMR-5 GATE coincident
data.
2. Convert ESMR-5 brightness temperatures to rain intensity esti-
mates for the tiro-degree square of the earth's surface centered
at ( 23.5o W, 8.5 0 N).
3.,, Compare ESMR-5 derived rain intensity estimates with coincident
GATE radar measurements of rainfall.
i
A
le Data' Collection and Preparation
The LSMR-5 data are available on computer tape at Goddard
Space Flight Center. A search was made of the LSMR-5 data
catalogues to locate GATE coincident data. Their references are
given in Table 1, computer printouts of these data were provided
by Dr. Paul W. Hwang of Godezrd. The printouts indicate bright-
ness temperatures with their latitude-longitude locations and
bean positions.
Those data points coincident with the GATE are located and
plotted on a grid representing the two degree square centered at
(23-e W, 8.50 N). Beam position, brightness temperature, and
scan angle are recorded for each. ;hen a correction of the bright-
noes temperature is carried out. This correction scheme is given
by Wilheit [3, Table 5-33 and is a function of bean position.
GATE radar rainfall data are available on both magnetic tape
and microfilm from the OAT,,$ World Archives. Also, a GATE Radar
Rainfall Atlas [23 is available. Among the data sets contained
in the Atlas are tables indicr";ng one hour, three hour, six hour,
twelve hour, and twenty-four hour mean area precipitation rate for
the fifteen geographic areas shown in Figure 1 D -i . GATE radar
derived hourly rainfall intensity estimates for the entire GATE
area on a grid of 0*25 0 x 0.250 were obtained on microfilm from
the GATE World Archives, See Figure 4.
002
I
x	 r
R
-3-
Figure 3 indicates the four on"agree square regions into
which the trio-degree square region centered at (23.5 0 W. 8.50 d)
was divided. Mean ESMR-5 derived rainfall intensity estimates
for the entire region and for AL,-,ach of the four subregions were
computed for GATE coincident Himbus-5 overpasses. These Mans
were also computed for corresponding GATE radar rain rate esti-
hates.
2. Conversion of ESMR-5 Brightness Temperatures to Rain Rates
Rain intensity for each data point is determined via
an appropriate brightness temperature/rain rate relation.
In the present study two such relations are used.
Using ESMR-5 data for September 2, 1974, GMT i2:57-
13:04, when Nimbus-5 was directly over the GATE area, arith-
metically averaged brightness temperatures for areas l through
12 in Figure 1 were determined. A brightness temperature/rain
rate relation was determined using a least squares linear fit
of these temperatures plotted against hourly precipitation
rates for corresponding areas [2J . The resulting relation
has the equation
R • 0.031T - 4.258.
Where R denotes rain rate and T denotes brightness temperature.
The second is a freezing level dependent relation derived
from the Wilheit curves [4] shown in Figure 2. This relation
is an interpolation of the 4k= and 5 ka freezing levvI curves
ORIGINAL PAGE I
OF POUR QUALITY
-4-
to correspond to a freezing level of 4.7 ka which amore closely
approximates GATE conditions. The formula for the relation is
0	 , 0 T.:r 185
0.101T	 x,8.643 , 1816 -4 T = 217
R•
0.116T - 21,962, 218,= T s 247
0.217T - 46.829. 248 :^! T
where R denotes rain rate and T denotes brightness temperature,.
RESULTS MD CONFUSIONS
The relation ( *)R • 0.031T - 4.258 was used to calculate rain
intensity for each ESMR-5 data point. Of the twelve areas in Figure
I t only area ll is entirely within the two-degree square centered at
(23.5 0 W, 8.5 0 N). Thus, in this instance, ESMR-5 rain intensity
estimates for area 11, deriv%^d using the relation (*) were compared
with GATE radar derived hourly mean precipitation rates [2] for this
area. Table 3 is a summary of the results and shown that the relation
(*) leads to an over estimation of rainfall. How!rer, consistency in
the estimations is evident.
Since the relation
0	 , 0<T-^-185
0.101T - 18.643, 166-:-- T 217
R•
0.116T - 21.962, 218=T1247
0.217T - 46.829, 248 t T
is derived from theoretically sound curves, a wre comprehensive
t CiJ
-5-
analysis was carried out on rain rate estimates derived via this
formula. Of the 78 available GATE coincident ESMR-5 sensings only
68 could be matched with coincident GATE radar derived hourly rain-
fall intensity measurements. Using these data, for each matched
overpass, mean rain intensity estimates (EM-5 and radar) were cook-
ported for each of the f ive regions ( I, II, 111 & IV, and G) shown in
figure 3.
Mean rain rate estimates ( ESMh-5 and radar) were computed for
the five regions for the entire GATE experiment and for each phase
of DATE. For Region I and Region G 14 day and 7 day means were
computed, In each instance the ratio ESMR-5 rainfall/radar rainfall
was computed. The following is a listing of the results. Rain rates
are in fir.
GATE
REGION ESMR-5 RADAR
I 0.73 0.42
II 0.74 0.41
III 0 . 72 0.67
IV 0.67 0.79
G 0.72 0.54
Mean ratio 1.36
Standard derivation of ratio 0.41
00 I^t
^ 
^A
RATIO
1.74
1.80
1.07
.85
1.33
_..._ 	 .. _._r_.......a .._...<.-ca...c._.....ar^+...^.e......,at_. 	e. e....... .. _.	 ..._^svum^NMr.	 n.m^....,.:v..	 aw^K,+^: . .................^..._ ^ 	'_.^	 ...
......._
....
	 ....
F
PHASE I
iIObT Bt^t-5 RADAR 2"10
F	 I 0.50 0.45 1.11
r
II 0.36 0.46 0.78
III 0.54 0.61 0.89
IV 0.56 0.54 1.03
G
1
0.49 0.64 0.77
Mean of ratio 0.91
Standard deviation of ratio 0.15
PHASE II
REGION ESMR-5 RADAR RATIO
I 0.87 0.24 3.63
II 0085 0.12 7.OS
III 0 . 90 0.76 1.18
IV 1.14 1000 1.14
E
G 0.94 0.53 1.77
f	
Mean of ratio 2.96
Standard deviation of ratio 2.52
PHASE III
REGION ESMR-5 RADAR RATIO
I 0.S2 0.52 1.58
II 0.94 0.53 1.77
III 0.75 0.66 1.14
IV 0.52 0 .94 0.96
G 0.7 6 0.56 1.36	 a
Mean of ratio 1.36
Standard deviation of ratio 0.33 -
-7-
FOURTEEN DAY MEM REGION I
RATIO
1.04
1.26
4.07
1.60
PERIOD	 WHR-5	 RAL"
1	 0.27	 0.26
E 2	 0.78	 0.62
3	 1.14	 0.28
4	 0.85	 0.53
R Mean of ratio 1.99
Standard deviation of ratio 1.40
FOURTEEN DAY MEANS REGION G
PERIOD	 ESM-5	 RADAR
1	 0.47	 0166
2	 0.72	 0.59
3	 1.16	 0.53
4	 0.68	 0.46
Mean of ratio 1.38
Standard deviation of ,ratio 0.61
RATIO
0.71
1.22
2.18
1.46
SEVEN DAY M BAN S REGION I
PERIOD ESKR-5 RADAR
1 0.07 0119
2 0.47 0.40
3 0.90 0.76
4 0.66 0.49
5 1.19 0.02
6 1109 0.54
7 1.21 0.35
8 0.50 0.70
9 0.39 0.39
Mean of ratio 7.89
Standard deviation of ratio 19.37
t When 59.5 is removed we have:
Mean -of ratio 1. 44
Standard deviation of ratio 0.93
RATIO
0.58
1.18
1.18
1.40
59.5
2.02
3.46
0.71
1.0
-a-
SEVEN DAY MEANS REGION G
P=RIOD ESMR-5 RAM RATIO
1 0.33 0.70 0.47
2 0160 0.63 0195
3 0.68 0.64 1.06
4 0.76 0.53 1.433
5 1015 0.43 2.67
6 1.16 0.63 1.84
7 1100 0.46 2,17
8 0.38 0.46 0.83
4 0.42 0.41 142
Mean of ratio 1.38
Standard deviation of ratio 0.71
observe that the mean of the ratio
ESMR-5 rain rate/radar rain rate
hovers consistently about 1.4. It is known that during Phase II
of the GATE, the operation of ESMR -5 was anomalous. For this
reason, data from only 15 GATE coincident overpasses were retrie-
vable, The results presented here indicates that these, too, may
not be true readings, When the ratio for the Phase II estimates
of rain rate are removed from the computations the mean ratio is
1.4 to the nearest hundredth.
These results indicate that ocean;tc rain rate estimates de-
rived from ESMR-5 data are very consistent %hem compared to radar
estimates.
s	 s
-9-
OTIMR
 
pERTINENT QUESTIONS
Several analyses suggested in the grant proposal, could not W
performed in the allotted time• They are as follows:
1. Compare the quality
 with that
 f
su^ gemeat
griater than 30 logo
than or equal
Z. Compare the quality of ESMR-5 measurements at scan angl8s less
than or equal to 400 with that of angle greater than 40 .
3o jkpproxim&te the fraction of ra in during the GATE that was such
that EM-5 saturation affected rain intensity measurement.
1^9 P00^I `YV^G^ ^r
I V^Lr,^^ 
1
r
REFERENCES
a
r Eli Austin, P, and S. Geotis 1978: 'Evaluation of the Quality
of Precipitation Data from a Satel,liLo—Borne Radiometer',
Final Report under NASA Grant NSG 6024*
L2] Hudlow, M. D. and V, L. Patterson; 1979: Gate Radar Rainfall
s Atlasp Center for Environmental Assessment Services, NOAA,
Washington, D. Co
[33 Wilheit T. T.; 1972: 'The Electrically Scanning Microwave
Radiometer (ESMR) Experiment's The Nimbus-5 Users Guide,
NASA Goddard Space Flight Center, Greenbelt, Maryland,
L 4] Wilheit, T. T., A. T. Co Chang, M. S• 17 6 Rao, E• Be Rogers,and J. S. Theon; 1977: 'A Satellite Technique for Quantitatively
Mapping Rainfall Rate Over Oceans O , Journal of applied
Meteorol:lcy, 16, 551-560.
4
GF
C\	 !t
D
G
F
C
D
6AUNDARY OF MASTER ARRAY_
	 B
C	 G•	 H.	 •C
8
A
D	 F'	 D
E	 E
15
B	 B
a
i
G•	 H 	 °C	 G 	 H• 	 'C
11	 13	 14
JA	 A	 ^j
F•	 12	 D	 Fe	 'D
E	 E
BOUNDARY OF MASTER ARRAY
• SHIP POSITIONS
Figure 1, Key giving geometric areas corresponding to
the area numbers appearing above the columns of the
daily rainfall tabulations.
 Letters designate ship
positions.
-
-12-
Table 1. ESMR-$ GATE DATA
Time Span Tape-File D Time Span Tape-File
179 0107-0117 L9697-17 211 0008-0017 L5349-6
179 1252-1304 L96?9-1 211 0157-o2o4 L5%9-4
180 1207-1215 L9679-9 211 1340=1349 L5349-10
181 1309-1315 L9679-17 212 0110-011.9 L5349-16
182 1223-1232 L5308-10 212 1255-1304 L5;49-21
183 0142-0153 12441-4 222 0047-0054 L5362 -4
183 1325-1332 L5308-18 222 1230-1237 L5362-15
184 0054-0105 L5308-24 223 0149-o156 L5362-7
186 olio-ong L5326-20 224 1245-1253 L5368-9
186 1256-1304 L5228-3 225 0021-0026 L5368-17
187 0027-0035 L5228-8 227 0035-0041 L5361-10
187 1210-1218 L5228-14 227 1216-1224 L55381-15
188 1313-1319 L5225-4 242 0143-0150 LS399-u
188 0130-0137 L5G28-22 242 1325-1333 L5399-16
169 1227-1238 L5225-7 243 0100-0108 L5399-22
190 0145-0157 L5225-13 243 1241-1248 1,6861-4
193 0117-0125 1,1599-20 244 0013-0021 L6861-12
193 1300-1307 1,5327-2 244 0200-0209 L6861-11
194 0033-OO4o L5327-14 244 1154-1202 L6861-17
194 1213-1223 L5327-11 244 1344-1350 L6861-18
195 1316-1324 L5360-4 245 1257-13o4 L6896-3
196 0048-0055 L5360-11 246 1210-1219 L6896-16
196 1230-1237 L5360-16 247 0133-0139 L9661-i
197 0150-0157 L5360-22 247 1313-1321 L9661-6
197 0005-0012 L5360-23 248 0043-0053 L9661-18
197 1332-1340 L5359-6 248 1228-1235 L9661-16
209 0140-0150 x5269-4 249 0147-0155 L5255-6
209 1325-1332 L5269-10 249 1330-1337 L525.5-4
210 0053-0103 L5269-16 250 1243-1252 L5255-18
210 1237-1245 L5269-21 251 0015-0023 L5276-1
i
-13-
Table 1 (continued)
Pa Time Spar. Tape_ —File D, air, T ime Span Tape-File
2y" 0203-0210 L5276-2 257 1250-1258 16872-6
251 1159-1205 L5276-6 258 1202-1210 L6872-13
252 0116-0125 L5276-14 259 0123-0129 L6872-21
252 1300-1308 L5276-20 259 1305-1311 L9649-3
253 0032-0040 L5343-9 260 0037-oo44 L9649-10
253 1215-1223 L5343-3 260 1219-1228 LA9-15
254 1316-1324 25343-15 261 0138-0145 L5266-17
2S5 1232-1240 16842-3 261 1321-1327 L5268-3
256 1333-1340 16842-14 262 00S3-0100 L5268-9
262 1235-1243 L5268-15
X 250
r	 •i
i
r
5 km
Q km l	 /	 ^r
3_km
2 km
1 km
	 rrl-'"	 .	
.1	 .	 . A .
w
D
aCr
w
a
N200
wZH
0
Q
to
150
0.1 10	 100
RAINFALL. RATE (mm/hr)
Fig. 2, Calculated brightness temperature at 1.55 cm as a
function of rain rate for melting levels of 1, 2, 3, 4 and 5 km
(from Wilheit et al, 1977).
	 ,
1000
j
TABLE 2; DERIVATION OF THE LINE R : 0.031 T - 4.258
0
T = Brightness Temperature
R = Rain Rate
Region, T R R estimate Res^l
1
1 171 0.64 0198 -.34
2 175 1108 1.10 -.02
3 1.81 1.09 1.28 -.19
4 165 0.43 0.79 -.36
5 172 1.04 1.01 +.03
6 174 1.86 1.07 +.79
7 171 0.82 0.98
-.16
8 175 0.98 1.10
-.12
9 173 1.02 1.04 -.02
10 173 1.05 1.04 +J
11 172 1051 1.01 +' L
12 191 1.46 1.59 -.13
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 R)	 (22e5 We 9.50m)
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-17-
Figure 4 The GATE Area
180
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11


Assessment of the quality of Gate area rainfall data from a Nimbus-5 radiometer

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