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.