2004 Precipitation

Precipitation from a couple cases has been posted previously. Below are the monthly mean differences from GPCP for the GEOS 5 2004 January and July validation experiment, compared with JRA25 and NCEP. Qualitatively, the patterns of difference maps seem quite similar among the reanalyses. Though, the GEOS5 US precipitation seems low where the others are high. The GEOS5 tropics are also lower than the other reanalyses, a positive result. It is interesting to note that all the reanalyses underestimate GPCP in Europe in January. GPCP applies a snow undercatch correction, especially in Europe, which increases the precipitation. The GEOS5 southern hemisphere and North Atlantic biases appear quite low compared to the other reanalyses.

To expand on the comparisons, we are also producing Taylor Diagrams (Taylor 2001) for precipitation. The preliminary figures follow the mean difference maps. These show the spatial correlation of the reanalysis compared to the normalized standard deviation. The reference data set is GPCP, so the reanalyses are correlated to GPCP, and the standard deviation is normalized to GPCP. The closer the point is to 1:1, the closer a match to the reference data set. Here, CMAP is also included to show a portion of the uncertainty in the observations. By this metric, GEOS5 is improving the spatial pattern on the monthly precipitation. Spatial resolution of the analysis likely plays a role.

A more detailed examination of these statistics is underway, including more localized regions (such as the NA and SA continent). These relate to the monthly precipitation. The higher frequency precipitation is also being examined, compared to TRMM 3b42 3 hourly 1/4 degree. There is some uncertainty with those observations (satellite swaths are occasionally apparent in the spatial structure), but the comparisons so far have been promising where weather systems are reasonably reproduced.



Monthly mean precipitation differences from GPCP for NCEP R1, R2 , GEOS5 and JRA25. January 2004 (above) July 2004 (below).
Monthly Taylor diagrams for precipitation for the globe, NH, SH and Tropics using GPCP as a reference data set for NCEP R2 , GEOS5, JRA25 and CMAP. January 2004 (above) July 2004 (below). The normalized standard deviations increase with radial distance from the origin. All standard deviations are normalized to GPCP so that a value of 1.0 matches GPCP. Spatial correlation are plotted as the radial lines, so that the 1,1 point is identical to GPCP.