Calibration and Evaluation of Precipitable Water Vapor From MODIS Infrared Observations at Night

Water vapor is one of the most variable atmospheric constituents. Knowledge of both the spatial and temporal variations of atmospheric water vapor is very important in forecasting regional weather and understanding the global climate system. The Moderate Resolution Imaging Spectroradiometer (MODIS) is the first space instrument to obtain precipitable water vapor (PWV) with near-infrared (nIR) bands and the traditional IR bands, which provides an opportunity to monitor PWV with wide coverage during both daytime and nighttime. However, the accuracy of PWV measurements obtained with IR bands is much lower than that with nIR bands. Moreover, seldom have studies been devoted to the calibrations of MODIS IR PWV. In this paper, the accuracy of MODIS IR water vapor product during the nighttime is assessed by ERA-Interim data, Global Positioning System, and radiosonde observations. Results reveal that the performance of MODIS IR water vapor product is much poorer than that from the other observations, and the MODIS IR PWV needs to be calibrated. As such, we propose a differential linear calibration model (DLCM) to calibrate the MODIS IR water vapor product during the nighttime. Case studies under both dry and moist atmosphere in midlatitude and equatorial regions are used to test and assess the performance of the DLCM. Results show that the DLCM can effectively enhance the accuracy of MODIS IR retrievals at nighttime. Furthermore, while the traditional least square model may over calibrate the MODIS IR PWV measurements occasionally, the DLCM can avoid that defect successfully.