JISAO data

Legates / MSU Precipitation Climatology

This document contains analyses and data for a global precipitation climatology that has been produced at JISAO. The spatial resolution of this climatology is 2.5 degrees latitude-longitude. The land data for this climatology is taken from the Legates and Willmott climatology (1990, Int. J. Climatology, 10, 111-127), which is based on the historical record of rain gauge measurements. The ocean precipitation estimates are from the Microwave Sounding Unit (MSU) produced by Roy Spencer of NASA. The original MSU algorithm is documented in Spencer (1993, J. Climate, 6, 1301-1326), and was revised by Spencer several years ago ("Limb93"). The estimates provided by the new algorithm are about 20% smaller in the tropics than the Spencer (1993) estimates.

There are several annoying features of the blended data set that I have removed or am in the process of removing. The details of tinkering with the climatology are discussed at the bottom of this page. Erroneous high rainfall values over the Sea of Okhotsk, Hudson Bay, Labrador Sea, and the coast of Antarctica have been largely removed in the climatology. The procedure that I used was not perfect, and I am not happy with the estimates off the coast of Antarctica. I do not plot south of 55 degrees S on the following plots for this reason.


The netCDF file (0.25 Mbytes)
Each map contains complete fields of data: there is no missing value flag.


Annual Total (mm)

Individual Calendar Months (mm month-1)













Figures produced with Freud.

Details of the calculation
The new climatology has this annoying feature of depicting excessive rainfall at high latitudes (Sea of Okhotsk, Hudson Bay, Labrador Sea, and the coast of Antarctica). This problem is due to the inability of the MSU estimates to differentiate between ice and very heavy rainfall. So far I have compared the Legates/MSU with the Legates and Willmott (hereafter LW) values at each grid point, and replaced the Legates/MSU values with LW values at each gridpoint where the Legates/MSU value exceeds the LW value by at least 50cm/mon. This cutoff is arbitrary. The data does not suggest a better cutoff. This substitution process has removed the great majority of the spurious values associated with MSU estimates over regions of sea ice. The ice line is still evident off of the coast of Antartica and there are point problems at other coastlines. I still have to fix these problems.

August 1998
Todd Mitchell (mitchell@atmos.washington.edu)
JISAO data