Several VIS/NIR based rain detection and rain rate retrievals have been developed in last decades, which relate cloud optical properties to precipitation at daytime. Since there is no real sensitivity of the measurements in this spectral range on precipitation, it is assumed that the probability of precipitation is a function of cloud particle size and cloud liquid water path, which can in turn computed from cloud optical thickness (COD) and cloud effective radius (REF).
Furthermore, a recently developed retrieval for optical properties at night (NLCOMP) using observations of backscattered moonlight in the VIIRS/DNB channel during night can be transformed to precipitation rate also when sunlight is not present. Special focus lies on the performance for Alaska region, where daytime observations are rarely available during winter, and thus will be particularly valuable for weather forecast and climate research.
More physically based rain rate algorithms from satellites use passive microwave radiometer observations, which penetrate clouds. Their principle is based on the physical relationship between observed microwave brightness temperatures and column water vapor and liquid water path and works equally well at day and night. The main shortcomings are the low spatial resolution (up to 2000 times coarser) and the higher uncertainty over land surfaces.
The Suomi-NPP constellation with ATMS and VIIRS on the same satellite gives us ideal opportunity to make synergistic use of both precipitation retrieval techniques with merging the advantages of the high spatial resolution of VIS channels on S-NPP/VIIRS combined with quantitative adjustments of the more physically based rain rates from ATMS.
The potential advantage of this approach is obvious. However, combining observations with highly different spatial resolutions is challenging. We will elaborate and present the feasibility to solve this problem.