NESDIS "GMSRA" Estimates

Grid Resolution: ~4 km
Domain: North America

Experimental GOES Multispectral Rainfall Algorithm (GMSRA) The GOES Multispectral Rainfall Algorithm (GMSRA) uses combined information from visible (0.65 m), near-infrared (3.9 m) and infrared (6.7 m, 11 m, and 12 m) GOES measurements. For daytime rainfall, the first step consists of identifying optically thick clouds having a visible reflectance greater than 0.40. Non-precipitating cirrus is screened empirically using a gradient temperature based on the 11 m channel and the effective radius of cloud particles near their tops is derived from the reflected solar irradiance at 3.9 m. Negative Brightness Temperature Difference (BTD) IR-WV(11 m - 6.7 m), which corresponds well with rainfall areas for very deep convective cores (Inoue, 1997), is also used for the identification of rain for cloud tops colder than 230K. The algorithm uses the effective radius to separate raining from non-raining warm clouds during daytime. The algorithm relies on IR and WV only during nighttime and rainfall is estimated for clouds having brightness temperatures colder than 240K. For each pixel classified as containing raining clouds, the associated instantaneous rain rate is computed using a pre-calibrated probability of rain and mean rain rate for cloud top brightness temperature (11 m) groups of 10K. The rain rate is obtained by the product of the probability of rain and the mean rain rate. A cloud growth rate, defined as the difference between the current and the previous images, and a correction factor accounting for the available moisture are used to adjust the rainfall estimates.

REFERENCE: Ba, M., and A. Gruber, 20001: GOES Multispectral Rainfall Algorithm (GMSRA). J. Appl. Meteor., 40, 1500-1514.