L-band Radiosonde System Relative Humidity Observation Error And Its Feature

The relative humidity sounding by L-band radiosonde system often occur an abnormal low phenomenon.In this paper L-band radiosonde data and Global Positioning System(GPS) radio occultation inversion data from December2008to November2009are used to studying the phenomenon by using statistical and comparative method.Moreover radio occultation inversion data and radiosonde data are compared,and the diural and seasonal variation are analysed,the main conclusions are as follows.The radiosonde relative humidity may be effected by cloud. Outside cloud the humidity is dryer, and the relative humidity observation data will decrease quickly.duing to the quality problem of the humidity sensor,it has lag effect and big inertia,when it reach the environmental value,the relative humidity will continue to drop,even decrease to the minimum cut-off valve2%,in following relative humidity observation the relative humidity value maintain2%,and the abnormal low phenomenon happen.The phenomenon can happen over all the provinces in China,the proportion is highest in southwest and east,and the proportion of the phenomenon account for20-40%in all radiosonde soundings.There is a obvious seasonal change for the phenomenon,and the number is highest in Guizhou,Guangxi and Guangdong province in winiter,and the region is gradually northward from spring to autumn.The movement is in accord with the rainbelt shift over eastern China.With the increase of the height the phenomenon often occur in the middle troposphere during650hPa to500hPa,and the number is few below900hP,and the high frequency in the high troposphere having a relationship with humidity sensor properties.The phenomenon can also occur in other type radiosonde system in other country.The view of the world,the phenomenon is more easily to appear in low latitude and mid latitude,and mainly along the30N or30S distribution,in the northern and southern hemispheres.The number of abnormal low is highest in northern and southern hemisphere winter,and least in summer.The quality control scheme is provided depending on the feature of the phenomenon. The comparison between GPS radio occultation inversion data and radiosonde data show that the main feature are similar in each season:the fractional in refractivity and pressure,and temperature bias are small from surface to30km,while the bias are big from30to40km;relative to radio occultation data,the radiosonde relative humidity data is dryer about5%in low and middle troposphere,while there is a large bias in relative humidity from upper troposphere to lower stratosphere between radio occultation data and radiosonde data. The radiosonde temperature is warmer in daytime due to the effect of radiation error,the seasonal difference in refractivity and relative humidity comparison relate to the variation of water vapour in atmosphere.