Correction method for RS80-A humicap humidity profiles and their validation by lidar backscattering profiles in tropical cirrus clouds

Routine radiosonde relative humidity ( RH) measurements are not reliable as they are presently used in the global upper-air network. The new Lindenberg measuring and evaluation method, which provides RH profile measurements with an accuracy of +/-1% RH in the temperature range from 35degrees to -70degreesC near the tropical tropopause is described. This Standardized Frequencies (FN) method uses a thin-film capacitive polymer sensor of a modified RS90-H Humicap radiosonde. These research humidity reference radiosondes (FN sondes) are used to develop a correction method for operational RS80-A Humicap humidity profiles. All steps of correction and quality control for RS80-A radiosondes are shown: ground-check correction, time-lag and temperature-dependent correction, and the recognition of icing during the ascent. The results of a statistical comparison between FN sondes and RS80-A sondes are presented. Corrected humidity data of operational RS80-A sondes used in Lindenberg ( 4 times daily) show no bias when compared to FN radiosondes and have an uncertainty of about +/-3% RH at the 1 sigma or 68% confidence level from 1000 to about 150 hPa. Only a small dry bias of at most -2% RH remains in the lowest part of the boundary layer (up to 500-m height). Finally, some examples of corrected RS80-A RH profiles in cirrus clouds validated by lidar backscattering profiles in a region of the intertropical convergence (Maldive Islands) are demonstrated. The soundings indicate that ice-saturated and ice-supersaturated air above 10-km height were connected with cirrus clouds in all 47 investigated cases, and, second, that the corrected RS80-A RH profiles also provide good quality information on water vapor in the upper troposphere.