Start Date | Start Time | End Date | End Time | Data Quality Metric |
---|---|---|---|---|
02/12/1994 | 0000 | 01/05/1995 | 2359 | Suspect |
12/22/1993 | 0000 | 02/28/1994 | 2359 | Suspect |
Subject: | Reprocess: Effect of BBSS ground check on MWR tuning functions |
DataStreams: | DsgpmwrlosB1.a0, sgpmwrlosB1.a1, DsgpmwrlosB1.a1 |
Description: | Due to concern over the impact of the Ground Check (a one-point calibration applied to the Vaisala sonde prior to launch) on the MWR tuning functions, I have recalculated the tuning functions without the ground check. The change in the tuning functions is slight and has only a very small (maximum of 1%) impact on the precipitable water vapor and cloud liquid water path values provided by the ARM microwave radiometers. TUNING FUNCTIONS The tuning functions relate microwave brightness temperatures measured by the the radiometer to those computed from simultaneous radiosondes using a model of microwave radiation transfer. The tuning functions are necessary to permit the use of the retrieval functions for precipitable water vapor and cloud liquid water which are based on the model calculations. The tuning functions are dependent only on the model; they are independent of the location and of the instrument (assuming it is properly calibrated). METHODOLOGY Barry Lesht, the BBSS instrument mentor, provided me with 325 soundings from the SGP central facility covering the period October 1992 - December 1993 from which he had removed the effect of the ground check. For each of these soundings I computed the microwave brightness temperatures at the frequencies used by the ARM microwave radiometers. Of these, 91 were determined to have occured during periods when the sky was clear and horizontally homogeneous on the basis of the standard deviation of the measured brightness temperature of the liquid-sensitive channel of the radiometer being less than 0.5 K for the period 10 minutes prior until 30 minutes subsequent to the sonde launch. The mean of the brightness temperatures measured during this time period were then used in a linear regression to determine the intercept and slope of the tuning functions. RESULTS: The new tuning functions are: 23.8 GHz: TB_model = 0.789 + 0.915 TB_measured (R2 = 0.994) 31.4 GHz: TB_model = 1.142 + 0.910 TB_measured (R2 = 0.988) These were applied to the MWR data acquired during the period October 1992 through December 1993 and compared with previous values. Only for values of PWV > 3 cm was the effect of the new tuning functions as much as 1%. For the 91 clear-sky soundings, the RMS difference between the preciptable water vapor from the MWR and from the BBSS was 0.1 cm; the mean difference was 0.00 cm. For 59 soundings during June 1993 (clear and cloudy sky), the mean difference in the PWV computed from the radiosondes (No ground check - ground check) was 0.05 cm; the RMS of the difference was 0.08 cm. Other observations/measurements impacted by this problem: qmemwrlos Suggested Corrections of the Problem: (e.g. change calibration factor and recompute, flag data with this comment, etc.) 1. The new tuning functions were implemented on all SGP MWRs on 5 Jan 1995. 2. The MWR data from Oct 92 - Dec 93 have been recalculated by the mentor and are available from him (as ten-minute averages in ASCII format) upon request. 3. A computer code has been provided by the mentor to the ARM Experiment Center to duplicate this reprocessing, if the PRB so desires. |
Suggestions: | |
Measurements: | DsgpmwrlosB1.a1: more
DsgpmwrlosB1.a0: more sgpmwrlosB1.a1: more |