Calibration of Temperature in the Lower Stratosphere from Microwave Measurements Using COSMIC Radio Occultation Data: Preliminary Results

AB STRACTAc cu rate, con sis tent, and sta ble ob ser va tions from dif fer ent sat el lite mis sions are cru cial for cli mate change de tec tion.In this study, we use Global Po si tion ing Sys tem (GPS) Ra dio Occultation (RO) data from the early phase of the FORMOSAT-3/Con stel la tion Ob serv ing Sys tem for Me te o rol ogy, Ion o sphere, and Cli mate (COS MIC) mis sion, which was suc cess fully launched on 15 April 2006, to inter-cal i brate Tem per a ture in the Lower Strato sphere (TLS) taken from Ad vanced Mi cro wave Sound ing Unit (AMSU) mi cro wave mea sure ments from dif fer ent sat el lites for po ten tial im prove ments of strato spheric tem per a ture trend anal y sis.Be cause of the lim ited num ber of COS MIC soundings in the early phase of the mis sion, these re sults are con sid ered pre lim i nary.In this study, we use COS MIC RO data to sim u late mi cro wave bright ness tem per a tures for com par i son with AMSU Ch9 mea sure ments (e.g., TLS) on board NOAA15, 16, and 18. Ex cel lent cor re la tion was found be tween syn thetic COS MIC bright ness tem per a tures (Tbs) and Tbs from NOAA15, NOAA16, and NOAA18, re spec tively.How ever, sys tem atic dif fer ences on the or der of 0.7 to 2 K were found be tween COS MIC and AMSU ob ser va tions over Antarctica.Our re sults dem on strate that syn thetic COS MIC Tbs are very use ful in iden ti fy ing inter-sat el lite off sets among AMSU mea sure ments from dif fer ent sat el lites.To dem on strate the long-term sta bil ity of GPS RO data, we com pare COS MIC dry tem per a ture pro files to those from col lo cated CHAMP pro files, where CHAMP was launched in 2001.The fact that the CHAMP and COS MIC dry tem per a ture dif fer ence be tween 500 and 10 hPa ranges from -0.35 K (at 10 hPa) to 0.25 K (at 30 hPa) and their mean dif fer ence is about -0.034 K dem on strates the long-term sta bil ity of GPS RO sig nals.In or der to dem on strate the po ten tial us age of the GPS RO cal i brated AMSU Tbs to inter-cal i brate other over lap ping AMSU Tbs, we ex am ine the un cer tainty of the cal i bra tion co ef fi cients de rived from AMSU-GPS RO pairs.We found the dif fer ence be tween COS MIC cal i brated AMSU Tbs and those from CHAMP to be in the range of ±0.07 K with a 0.1 K stan dard de vi a tion.This dem on strates the ro bust ness of the cal i bra tion co ef fi cients found from AMSU-GPS RO pairs and shows the po ten tial to use the cal i brated AMSU Tbs to calibrate other overlapping AMSU Tbs where no coincident GPS RO data are available.