Start Date | Start Time | End Date | End Time | Data Quality Metric |
---|---|---|---|---|
02/12/1994 | 0000 | 04/04/1996 | 2359 | Suspect |
Subject: | SGP/MWR/B1/B4/B5 - Reprocess: MWR Tuning Functions |
DataStreams: | sgpmwrlosB1.a1, sgpmwrlosB4.a1, sgpmwrlosB5.a1, sgpmwrlosB5.a0, sgpmwrlosB1.a0, sgpmwrlosB4.a0 |
Description: | The 'tuning functions' used to adjust the equivalent brightness temperatures (TBs) measured by the ARM microwave radiometers (MWRs) are now believed to be both incorrect and unnecessary. They should no longer be used and the data (going back to 1992) that incorporated them should be reprocessed. By eliminating these tuning functions the radiometer retrievals would be independent of the soundings. BACKGROUND A recent comparison by Barry Lesht (ANL) of the precipitable water vapor (PWV) retrieved from the MWR-measured brightness temperatures against PWV derived by integrating along the trajectory of radiosonde ascents has revealed that the MWR values are about 90% of those derived from the soundings. This is directly attributable to the slope of the tuning function for the vapor-sensing channel (23.8 GHz) of 0.915 which is applied to the measured brightness temperatures prior to retrieval of PWV. The rationale behind the use of the tuning functions is that the radiation model (Liebe 87), on which the retrieval is based, is imperfect whereas the radiosondes represent 'ground truth.' Thus the observed brightness temperatures must be adjusted to match those calculated with the model using co-located soundings so that the retrieval yields precipitable vapor amounts that agree with the soundings. Tuning functions were developed for the present ARM MWRs using co-located soundings launched between October 1992 and December 1993. These were adjusted slightly in January 1995 to account for the effects of the 1-point calibration check performed prior to launch (see DQR P950110.1): 23.8 GHz: TB_model = 0.789 + 0.915 TB_measured (R2 = 0.998) 31.4 GHz: TB_model = 1.142 + 0.910 TB_measured (R2 = 0.984) However, repeating this exercise for soundings launched during 1994 and 1995 (excepting those that were mis-calibrated by the manufacturer; see D960229.1) it now appears that the model-calculated brightness temperatures are in much closer agreement with the measured values and that the tuning functions account more for variations in the radiosonde calibration than for any deficiencies in the radiation model. Consequently, it appears that the present tuning functions are incorrect and bias the retrieved PWV low by 10%. In addition, given the present agreement between measured and modeled brightness temperatures, the tuning functions are also unnecessary. METHODOLOGY Brightness temperatures measured with microwave radiometer (MWR) serial number 10, which was deployed at the central facility in December 1993, have been compared against calculations using measurements from the co-located Balloon-Borne Sounding System (BBSS). The results are summarized in two tables. In each table, the calibration dates of the sondes and MWR are listed as well as the time period and number of samples included in each regression. Each MWR sample is a 40-minute average, centered on the time of the sonde launch, of the microwave brightness temperature. In order to include only clear sky conditions, samples for which the standard deviation of the liquid-sensing (31.4 GHz) channel exceeded 0.3 K were eliminated. To assure that the water vapor was reasonably homogeneous horizontally, samples for which the standard deviation in the vapor-sensing (23.8 GHz) channel exceeded 0.4 K (in 1995) or 0.5 K (in 1994) were eliminated. The 1994 threshold is larger in order to increase the number of samples and reduce the standard error in the results. The microwave radiometer measurements used in this comparison have been reprocessed to account for calibration changes and other problems (see P940813.1) TB vs PWV The first table is a comparison of microwave brightness temperature (TB_mwr) regressed against the precipitable water vapor (PWV) computed by integrating along the trajectory of the radiosonde ascent. The sondes launched during May - December 1994 are compared against two sets of MWR data; the first uses the May 1994 calibration, and the second uses the calibration of July 1994. A comparison is also made of TB_model vs PWV ('Liebe87') for reference. The intercepts indicate the contribution due to molecular oxygen (i.e. the tail of the 60 GHz line) which is affected by temperature and pressure. Note that the 'Liebe87' intercepts vary seasonally as the temperature changes. Note also that the effect of MWR calibration changes is most evident in the intercept: offsets of 1-2 K are observed. Because the MWR calibration values represent the slope of the radiometer equation (see Appendix), the magnitude of the offset is largest at 0 K (i.e. the intercept) and declines to zero at ambient temperature (~290 K). The slope of the regression is essentially unaffected by the MWR calibration. Variations in the slope of the regression correlate with sonde calibration date. The sondes calibrated in May 1994 or later appear to yield much closer agreement between the measured brightness temperatures and those calculated with the Liebe 87 model than those calibrated in January 1994 or earlier, with which the present tuning functions were developed. TABLE 1. Microwave brightness temperature vs. precipitable water vapor Relationship: TB_mwr (K) = intercept (K) + slope (K/cm) * PW_sonde (cm) Standard Error of the intercepts and slopes are given in parentheses. Date of Date of Period ------ 23.8 GHz ----- ----- 31.4 GHz ----- Sonde Cal MWR tip Covered N intercept slope intercept slope 1991-93 92-93 Oct92-Dec93 91 6.7 14.7 8.1 5.3 1992,93 Dec 93 Jan-Feb 94 85 6.9(0.19) 15.8(0.26) 8.8(0.13) 5.6(0.17) 1992,93 Liebe87 Jan-Feb 94 85 6.5(0.02) 13.8(0.03) 8.9(0.07) 4.5(0.09) Jun 93 Dec 93 Apr 94 16 10.6(1.11) 14.8(0.55) 10.1(0.51) 5.6(0.25) Jun 93 Liebe87 Apr 94 16 6.9(0.05) 13.6(0.02) 8.1(0.09) 5.0(0.05) 1992,93 May 94 May-Jun 94 48 7.0(1.03) 14.9(0.45) 7.8(0.41) 5.7(0.17) 1992,93 Jul 94 May-Jun 94 48 5.1(1.03) 14.9(0.44) 6.6(0.39) 5.7(0.17) 1992,93 Liebe87 May-Jun 94 48 7.1(0.11) 13.5(0.05) 8.4(0.16) 4.9(0.07) Jan 94 Dec 93 Feb-May 94 95 7.6(0.27) 14.3(0.14) 8.5(0.15) 5.5(0.08) Jan 94 Liebe87 Feb-May 94 95 6.9(0.05) 13.6(0.02) 8.1(0.09) 5.0(0.05) May 94 May 94 Jun-Aug 94 78 12.3(1.04) 13.0(0.34) 11.0(0.39) 4.8(0.13) May 94 Jul 94 Jun-Aug 94 78 10.3(1.04) 13.1(0.34) 9.8(0.39) 4.8(0.13) May 94 Liebe87 Jun-Aug 94 78 7.8(0.22) 13.3(0.07) 8.6(0.29) 4.9(0.10) Jun 94 May 94 Jul-Dec 94 57 8.3(0.37) 13.6(0.21) 8.8(0.19) 5.2(0.11) Jun 94 Jul 94 Jul-Dec 94 57 6.4(0.37) 13.6(0.21) 7.7(0.18) 5.2(0.10) Jun 94 Liebe87 Jul-Dec 94 57 6.9(0.08) 13.5(0.04) 8.3(0.10) 4.9(0.06) Aug 94 May 94 Sep-Dec 94 90 7.4(0.15) 13.5(0.09) 8.8(0.11) 5.1(0.07) Aug 94 Jul 94 Sep-Dec 94 90 5.5(0.14) 13.6(0.09) 7.8(0.12) 5.0(0.07) Aug 94 Liebe87 Sep-Dec 94 90 6.8(0.05) 13.6(0.03) 8.6(0.09) 4.9(0.06) |
Suggestions: | |
Measurements: | sgpmwrlosB4.a1: more
sgpmwrlosB1.a1: more sgpmwrlosB1.a0: more sgpmwrlosB4.a0: more sgpmwrlosB5.a1: more sgpmwrlosB5.a0: more |