| Start Date | Start Time | End Date | End Time | Data Quality Metric |
|---|---|---|---|---|
| 05/01/1998 | 0000 | 05/05/1998 | 0000 | Suspect |
| Subject: | SGP/MWR/B6/C1 - Instrument moved, tip field of view not clear |
| DataStreams: | sgpmwrtipB6.a0, sgpmwrlosB6.a0, sgpmwrlosB6.a1, sgpmwrtipC1.a0 |
| Description: | During the water vapor IOP in September 1997 it was observed that the ARM MWR reported about 2 mm more water vapor than other sensors. Unlike the 1996 WVIOP, only one ARM MWR was used. To assess the performance of the MWR used during the 1997 WVIOP, it was recomended that another ARM MWR should be operated in close proximity to the unit at the SGP central facility. Last week SGP operations staff set up MWR 18, normally located at BF-6 (Purcell), in proximity to MWR 10 at the SGP central facility near Lamont, OK. I travelled to Oklahoma to carry out the comparison of the radiometers. Although it rained or was cloudy until Thursday evening, I was able to make a good comparison. The results are summarized below and in the attached GIF plots. On or a about 20 November 1996 (after the 1996 WVIOP) a 10-m tower was erected approximately 200 feet south of the MWR and directly in the plane of the elevation scan used during tip mode. The top of the tower has an elevation angle of about 10 degrees relative to the radiometer position, whereas the lowest tip angle is 19.5 degrees. However, the first sidelobe in the antenna pattern is 11 degrees from center, so it is possible that the tips were influenced by the tower. Looking back at the calibration data it appears that the quality of the tips deteriorated in 1997. I had to lower the acceptance criterion from R=0.995 to R=0.990 in order to get a reasonable number of tips. (R is the correlation coefficient of the Langley regression of opacity vs airmass; 1.00=perfect co-linearity.) The situation worsened as the ambient temperature increased, which supports the notion of tower contamination. Much to my surprise, rotating the radiometers 90 degrees so that they could not view the tower did not greatly affect the derived brightness temperatures or PWV and LWP estimates, although the quality of the tip curves did dramatically increase (e.g. the R values were in the range 0.995-0.999). This is revealed in plot1.gif which presents the PWV and LWP dervied from both radiometers and the PWV from the radiosondes. NOTE that the difference between the radiometers and the radiosondes depends strongly on the sonde calibration lots. Note that during the 1997 WVIOP sondes from lots 733 and 726 (weeks 33 and 26 of 1997) were launched. These were very close in time to lots 729, 730 and 735 launched last week which also showed a bias of about -2 mm relative to the MWR. The brightness temperatures derived from the tip curves are presented in plot2.gif. These reveal a slight offset (0.3-0.5 K) in both channels, with MWR 18 (Purcell) higher. I suspect that this may be due to the temperature sensors in the blackbody target. In MWR 18 the two sensors are offset by about 0.75 K, whereas in MWR 10 the offset is only about 0.25 K. I use the average of the two readings in calculating the brightness temperature, so an error of 0.3-0.5 K could result if one sensor is high. The sondes exhibit the same batch-dependent behavior as in plot1.gif. Brightness temperature differences at each pair of elevation angles are presented in plot3a.gif and plot3b.gif for MWR 10 and 18 respectively. The orientation of the radiometer and the direction of the differences are printed along the bottom of the plot. For example, E-W means that the scan is from east to west and the differences are east minus west. These reveal several things: 1. I located the radiometers too close to each other in the north-south plane. When both radiometers were scanning East-West, MWR 18, which was east of MWR 10, was in the sidelobe of the MWR 10 antenna pattern, as indicated by the large differences for 10 when the differences for 18 were small. Rotating MWR 10 180 degrees to scan west-east moved its antenna about 2 feet farther north and its differences dramatically decreased and were the mirror image of those from MWR 18. 2. Although the polycarbonate foam window on MWR 10 was significantly weathered and more so on one side than the other, replacing the window did not noticeably affect the measurements. (I initially thought that the window was the cause of the large differences and later discovered that the radiometers were too close together.) 3. Significant east-west or north-south differences resulted in only small (~0.5 K) offsets in the derived brightness temperatures. (Note the change in brightness temperature differences in plot3a.gif at 05:00 on day 121 when MWR 10 was rotated 180 degrees to scan west-east and then examine the corresponding changes in TB or PWV and LWP in plot1.gif and plot2.gif.) The results of the September 1997 and May 1998 calibrations for MWR 10 are presented below. Date R min N tips TND23 tc23 dev23 TND31 tc31 dev31 (K) (K/K) (K) (K) (K/K) (K) 26-30 Sep 97 0.990 408 205.45 -0.079 0.36 191.02 -0.031 0.20 01-04 May 98 0.995 2768 205.27 -0.074 0.39 189.41 -0.011 0.27 There appears to have been a significant change in the 31 GHz (liquid- sensing) channel calibration since the IOP. However, the 23 GHz (vapor-sensing) channel calibration does not appear to have changed. Moreover, reprocessing the September 1997 data using the May 1998 calibration reduces the PWV by only about 0.4-0.5 mm but introduces a significant bias (0.04 mm) in the LWP. Thus, the May 1998 calibration does not appear to be appropriate for September 1997. The bottom line is that there does NOT appear to be any problem with MWR 10 at the Central Facility. The agreement between the two MWRs is within the manufacturer's specification, although it does appear that one of the temperature sensors in the blackbody target on MWR 18 may need to be replaced or recalibrated. From now on tip curves at the CF should be done west-east rather than north-south to avoid any possible influence of the SIRS tower. |
| Suggestions: | |
| Measurements: | sgpmwrtipB6.a0: more
sgpmwrtipC1.a0: more sgpmwrlosB6.a0: more sgpmwrlosB6.a1: more |
