Andrew L. Reehorst

Glenn Research Center, Cleveland, Ohio

Comparison of Profiling Microwave

Radiometer, Aircraft, and Radiosonde

Measurements From the Alliance

Icing Research Study (AIRS)

NASA/TM—2001-211101

August 2001

https://ntrs.nasa.gov/search.jsp?R=20010098322 2018-05-14T14:45:32+00:00Z
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National Aeronautics and

Space Administration

Glenn Research Center

Andrew L. Reehorst

Glenn Research Center, Cleveland, Ohio

Comparison of Profiling Microwave

Radiometer, Aircraft, and Radiosonde

Measurements From the Alliance

Icing Research Study (AIRS)

NASA/TM—2001-211101
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NASA/TM2001-211101 1 

Comparison of Profiling Microwave Radiometer, Aircraft, 

and Radiosonde Measurements From the 

Alliance Icing Research Study (AIRS) 

Andrew Reehorst 

Icing Branch 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Measurements from a profiling microwave radiometer are compared to measurements 

from a research aircraft and radiosondes. Data compared is temperature, water vapor, 

and liquid water profiles. Data was gathered at the Alliance Icing Research Study 

(AIRS) at Mirabel Airport outside Montreal, Canada during December 1999 and January 

2000. All radiometer measurements were found to lose accuracy when the radome was 

wet. When the radome was not wetted, the radiometer was seen to indicate an inverted 

distribution of liquid water within a cloud. When the radiometer measurements were 

made at 15 degrees instead of the standard zenith, the measurements were less accurate. 

Introduction 

This report is a review of the comparison of the Radiometrics profiling microwave 

radiometer (MP3000) (Reference 1), the NASA Glenn Research Center Twin Otter, and 

radiosonde data from the Alliance Icing Research Study (AIRS) during December 1999 

and January 2000 (Reference 2). 

The Radiometrics MP3000 radiometer was operated at Mirabel Airport outside of 

Montreal, Canada as part of the AIRS field test. The MP3000 is capable of producing 

vertical profiles of temperature, humidity, and liquid water. Also as part of AIRS, the 

NASA Glenn Research Center Twin Otter Icing Research Aircraft overflew the ground 

test site. The Twin Otter is equipped to measure the icing cloud environment with in-situ 

instrumentation that records latitude, longitude, altitude, temperature, cloud liquid water 

content, and cloud droplet size information, in addition to many other parameters. Data 

was acquired during spiraling descents centered near the radiometer ground location and 

missed approaches to a nearby runway. These maneuvers were designed to keep the 

aircraft as close to the airspace that the radiometer was sampling as possible. Also, 

radiosondes were released regularly during the active test periods of AIRS. The 

radiosondes provided profiles of wind direction and velocity, temperature, humidity, and 

pressure. The radiosondes were released from the same building that supported the 

radiometer operations. The radiosondes were released at predefined times and do not 

coincide with the aircraft and radiometer measurements presented. However, to allow 

comparisons, the profiles of temperature and humidity are assumed to not change 

dramatically during the test period. The following discussions relate to the comparison of 
NASA/TM2001-211101 2 

data from these three sources. The accuracy of both vertical (zenith) profiles and those at 

15 degrees above the horizon will be examined. 

Vertical profiles 

Temperature

In all cases, when the radiometer was not indicating rain, the MP3000 temperature 

profiles agreed well with both the radiosonde and the Twin Otter. As has been noted in 

earlier studies (Reference 1), the radiometer will smooth through temperature inversions. 

Some level of this kind of smoothing is visible in Figures 1 through 7. Temperature 

differences at the peak inversion location reached a maximum of around 5 K. In Figures 

1-3 and 5-7, differences at higher altitudes are also seen. These could be caused by a 

smoothed inversion (Figures 1-3), or a gradual decay of accuracy with increasing altitude 

(Figures 5-7). These higher altitude errors typically rose to around 3K at 10km (Figures 

1-3), but could be as great at 10K (Figure 5). Below 6km, the temperature profiles were 

typically better. Two cases are included where the radiometer was indicating rain. One 

shows no degradation in the temperature profile (Figure 3) while another shows 

considerable error above 1km (Figure 4). The radiometer had been indicating rain for a 

considerable time before the case shown in Figure 4, so the radome was likely very wet 

in that case; while the case shown in Figure 3 occurred at the very beginning of the "rain" 

indication, so the radome probably did not have a great deal of water contamination at the 

time of the scans. 

Humidity

As with temperature, the radiometer generally agreed well with the aircraft and 

radiosonde humidity measurements except where the humidity changed quickly with 

altitude. In Figure 1 the radiometer does very well in identifying the area of maximum 

humidity (saturation), but misses the dry band between 300 and1500 meters. It also 

misidentifies an area of much lower humidity above 8km. However, the air is so dry at 

these altitudes, that the error would have little consequence for remote detection of icing. 

Other than the error with the dry zone at low altitudes, the radiometer agrees within the 

allowable error margins of the radiosonde (Reference 3). In Figure 2 the radiometer does 

not agree well with the radiosonde (10 to 20% errors). However, based upon the aircraft 

measurements, the conditions may have been changing fairly rapidly, so the error levels 

are not conclusive. However, it does appear that the radiometer again misses the dry 

bands at 300-1500 and above 8km. The case shown by Figure 3 indicates results similar 

to Figure 1 and 2. The radiometer captures the saturation region, but is 0.5km high for 

both the lower and upper boundaries. It should be noted that again the radiometer 

seemed to do a good job with this case although it was indicating rain. However, Figure 

4 shows significant error along with an indication of rain on the radome. Again, these 

two cases demonstrated the same trend regarding rain indication and error as did the 

temperature profiles, so the amount and time of moisture on the radome is likely the key 

to rain induced error. Figure 5 shows the best agreement for humidity. The largest errors 

in this case were caused by the rapid humidity drop at 5.5 km. Figure 6 demonstrates a 

similar trend, but with the addition of a l.2-2km dry zone that the radiometer identified, 

but did not capture fully. Figure 7 shows the radiometer capturing the proper trends with 

reasonable accuracy, but as in all cases, smoothing through the areas of rapid change. 
NASA/TM2001-211101 3 

Liquid Water

The typical profile for liquid water in an icing cloud has the greatest amounts of water 

near the cloud tops (Reference 4). However, the LWC profiles exhibited by the neural 

net output for the cases examined were predominantly the opposite of this. This is most 

easily seen in Figures 2 and 6, where they are compared to the Twin Otter measured 

profiles (which follow the typical cloud liquid water profile). However, the general trend 

of the radiometer profiles can also be seen in Figures 5,7,8,9,10, and 11. 

As discussed earlier, the radiometer looses accuracy when the radome is wet. The 

relationship between the "rain" indication and liquid water profile inaccuracy appears to 

be even stronger than the relationship for temperature and humidity. Similar to the 

temperature and humidity measurements, the liquid water profile is significantly 

inaccurate in Figure 4. However, the liquid water profile seen in Figure 3 (which had 

fairly accurate temperature and humidity profiles) is also significantly inaccurate. 

The radiometer output liquid water profiles all appear to include signature errors at higher 

altitudes. The liquid water profile above 5 km seen in Figure 1 is present in all profiles 

(although it may be hidden with x-axis scale changes in some cases). Although they 

represent a very low LWC, these measurements are obviously an artifact of the neural net 

processing since they are present in all cases. 

15 degree profiles 

Figures 8 through 11 show the comparison between vertical profiles and profiles made at 

15 degrees above the horizon. These figures show the profile data for times when the 

radiometer was switched from one mode to the other fairly quickly. The third line on 

these figures is the radiosonde trace from the balloon launch closest in time to the two 

radiometer profiles. For all four figures the accuracy of the vertical temperature and 

humidity profiles can be seen to be much better than those of the 15 degree profile. 

Interestingly, the liquid water profiles do not show similar differences. In fact the 

profiles in Figures 8 and 11 are remarkably similar. When the liquid water profiles 

showed a difference (Figures 9 and 10), the vertical pointing profile showed higher levels 

of liquid water. The accuracy of these cases cannot be judged, because no liquid water 

measurement is available for comparison to the radiometer data. However, since the 

neural net is trained to liquid water profiles derived from temperature and humidity 

profiles, it would be surprising to find the less accurate temperature and humidity profiles 

associated with better liquid water profiles. 

Conclusions 

Liquid water profiles

Liquid water profiles from the radiometer appear inverted within an indicated cloud. The 

radiometer typically shows higher liquid water at the cloud base with decreasing LWC 

with altitude, while experience and Twin Otter profiles indicate that the LWC should 

peak at or near cloud tops, decreasing with decreasing altitude. Since the neural net 

consistently produces profiles with this shape, it must be assumed that the neural net 

training is the source of the error. An effort should be made to verify the training method 
NASA/TM2001-211101 4 

for liquid water profiles, and if found to be inadequate, corrective action should be 

undertaken. 

15 degree elevation profiles

Profiles taken at an elevation angle of 15 degrees above the horizon typically show 

significantly lower accuracy than the vertical profiles. Analysis should be conducted to 

determine if non-vertical profiles can produce the same levels of accuracy as vertical 

profiles, and if so, if there are limits to the angle or effective range (or profile height) that 

are imposed on the results by this method. 

References 

1.) Solheim, F., Godwin, J., Westwater, E., Han, Y., Keihm, S., Marsh, K., and Ware R., 

"Radiometric Profiling of Temperature, Water Vapor, and Cloud Liquid Water using 

Various Inversion Methods," Radio Science 33, 393–404, March–April 1998. 

2.) Isaac, G.A., Cober, S.G., Strapp, J.W., Hudak, D., Ratvasky, T.P., and Marcotte, D.L., 

"Preliminary Results from the Alliance Icing Research Study (AIRS)," AIAA–2001–

0393, January, 2001. 

3.) Pratt, Robert W.,"Review of Radiosonde Humidity and Temperature Errors," Journal 

of Atmospheric and Oceanic Technology, Volume 2, 1985, pages 404–407. 

4.) Heinrich, A., Ross, R., Zumwalt, G., Provorse, J., Padmanabhan, V., Thompson, J., 

and Riley, J., "Aircraft Icing Handbook," DOT/FAA/CT–88/8–1, March, 1991. 
NASA/TM2001-211101 5 

Figures 

Figure 1 

Figure 2 
NASA/TM2001-211101 6 

Figure 3 

Figure 4 
NASA/TM2001-211101 7 

Figure 5 

Figure 6 
NASA/TM2001-211101 8 

Figure 7 

Figure 8 
NASA/TM2001-211101 9 

Figure 9 

Figure 10 

Figure 11 
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15

Comparison of Profiling Microwave Radiometer, Aircraft, and Radiosonde

Measurements From the Alliance Icing Research Study (AIRS)

Andrew L. Reehorst

Aircraft icing; Remote sensing; Liquid water content; Temperatures; Water vapor

Unclassified -Unlimited

Subject Category: 03 Distribution: Nonstandard

Responsible person, Andrew L. Reehorst, organization code 5840, 216–433–3938.

Measurements from a profiling microwave radiometer are compared to measurements from a research aircraft and

radiosondes. Data compared is temperature, water vapor, and liquid water profiles. Data was gathered at the Alliance

Icing Research Study (AIRS) at Mirabel Airport outside Montreal, Canada during December 1999 and January 2000.

All radiometer measurements were found to lose accuracy when the radome was wet. When the radome was not wetted,

the radiometer was seen to indicate an inverted distribution of liquid water within a cloud. When the radiometer

measurements were made at 15° instead of the standard zenith, the measurements were less accurate.