Thursday, 13 February 2003
MODIS near-infrared column water vapor over land and comparisons with NCEP analyses
Since the August of 2000, MODIS (Moderate Resolution Imaging SpectroRadiometer) on the Terra spacecraft has been routinely providing atmospheric column or precipitable water vapor (PWV). The near-IR algorithm uses water vapor absorption channels near 0.94-micron and nearby atmospheric window channels. It provides reliable PWV estimates over land (and over water under sun-glint conditions), in contrast with satellite-based microwave and infrared PWV retrievals, which often have large uncertainties over land due to variations of surface emissivity. In order to gain more confidence in the MODIS PWV, we validate the near-infrared retrievals with ground-based upward-looking microwave radiometers at two observational sites. We also compare PWV estimates between MODIS and the National Center for Environment Prediction's (NCEP) data assimilation and analysis system. Specifically, we inter-compare the two sets of monthly averaged data between November 2000 and June 2002. The monthly averages have a 1-degree by 1-degree longitude and latitude grid. Cloudy data points are removed from the original MODIS data stream before the averaging since the retrievals under cloudy conditions only represent column water vapor above clouds.