Profiling of atmospheric water vapor with MIR and LASE

This paperpresents the first andthe only simultaneous measurements watervaporby MIR (Millimeter-waveImagingRadiometer) andLASE (Lidar AtmosphericSounding Experiment)on boardthesameER-2aircraft. Watervaporis oneof the mostimportant constituents in the earth'satmosphere, asits spatialandtemporalvariationsaffecta wide spectrumof meteorological phenomena rangingfrom the formationof cloudsto the development of severestorms. Its concentration, asmeasured in termsof relative humidity, determinestheextinctioncoefficientof atmosphericaerosol particlesand thereforevisibility. Theseconsiderations point to the needfor effective andfrequent measurements of the atmospheric watervapor. The MIR andLASE instrumentsprovide measurements of watervaporprofileswith two markedlydifferenttechniques.LASE cangivewatervaporprofileswith excellentverticalresolutionunderclearcondition, while MIR canretrievewatervaporprofileswith a crudevertical resolutionevenundera moderate cloud cover. Additionally, millimeter-wavemeasurements arerelatively simple andprovidebetterspatialcoverage. Abstract. Profiling of ,,\lmosphcric Water Vapor with MIR and LASE Concurrent measurements of atmospheric water vapor profiles were conducted over the Atlantic Ocean on September 25, 1995 with both Millimeter-wave Imaging Radiometer (MIR) and Lidar Atmospheric Sounding Experiment (LASE) on board the NASA ER-2 aircraft. LASE provides high precision measurements of profiles of both aerosol backscatter and water vapor with high vertical resolution of 60 m for aerosol backscatter and 330 m in the low-to-mid troposphere and 550 m in the upper troposphere for water vapor. LASE measurements are, therefore, an excellent resource to calibrate the capability and limitation of MIR as a water vapor profiler. Previously, the water vapor profiles retrieved from the MIR measurements have been compared with those of rawinsonde and Raman iidar observations at point locations. The frequency and extent of the comparisons made in that fashion were largely constrained by the requirement of near coincidence in time and space. The data acquired concurrently by MIR and LASE from this ER-2 aircraft flight enable the comparison of MIR-retrieved and LASE-measured moisture profiles over a long stretch of time and space. In addition, the LASE-measured profiles of aerosol backscatter provide the means to assess the impact of clouds on retrieval of water vapor profiles from the MIR measurements. It is shown that profiles of water vapor mixing ratio retrieved from the MIR data generally conform to those measured by the EASE. Differences in the values of mixing ratio at individual altitude levels are quite often not small, however. A substantial portion of these differences is shown to be due to the poor vertical resolution inherent in the profile retrieval from the radiometric measurements. MIR water vapor profiling under cloudy conditions is also demonstrated, andthe location and height of the low-altitude clouds estimated from the retrieval process were generally consistent with those observed by the LASE. In the region of cirrus clouds …