The influence of radiosonde "age" on TRMM field campaign soundings humidity correction

Abstract

Hundreds of Vaisala sondes with an RS80-H Humicap thin-film capacitor humidity sensor were launched during the Tropical Rainfall Measuring Mission (TRMM) field campaigns (1999) Large Scale Biosphere-Atmosphere (LBA) experiment held in Brazil and the Kwajalein Experiment (KWAJEX) held in the Republic of the Marshall Islands. Six humidity error correction algorithms were used primarily for applying system-bias correction to RS80-H humidity data. All TRMM field campaign Vaisala humidity soundings were corrected for dry bias using this algorithm. An overall improvement of 3% RH for daytime and 5% RH for nighttime soundings was achieved, Sonde age was ascertained using respective serial numbers (in this case the range is 0.06-2.07 yr) and used in the algorithm for calculation of sensor aging error and chemical contamination errors. Chemical contamination error is also found to be a dominant error source. Daytime sensor-arm heating for the first 50 s of the sonde launch is found to bear a cosine variation with sonde age. Surface reference temperature and sonde registered surface temperature are both used for calculating surface saturation vapor pressure, which in turn is used for sensor-arm-heating error estimation during the first 50 s. Site-mean CAPE values are found to increase significantly after correction. It is suggested that sonde surface temperature error must also be corrected for sonde age while using the present RS80-H correction algorithm. An age-height plot of the differences between the uncorrected and corrected specific humidity value for all Vaisala soundings shows an age-dependent increase (approximately 3.4 g kg-1 for 2-yr-old sondes), Variation of specific humidity difference was not found to be very significant for the upper levels when the sensor is less than 1.25 yr old. © 2004 American Meteorological Society.