TRMM Monthly Rainfall (3A25 V6)
Parameters in Tovas:
This unit normalizes the number of days within each month so that the variations between months are comparable. Unconditional Rain Rate: TOVAS calculates the unconditional mean rain rate and weight the non-raining pixels by multiplying the conditional mean rain rate (I.e., the mean rate rate when it is raining) by the probability of rain (i.e., by the rainy counts/total counts) and by the number of hours during the month. For example, for the low resolution (5 degree) the unconditional mean rain rate is Ruc= e_surfRainMean1 x (e_surfRainpix1/tt1pix1) = P57 x (P89/P83) Parameter names and numbers are based on the list posted at http://disc.sci.gsfc.nasa.gov/precipitation/TRMM_README/3A25_quick.shtml. While e_surfRainMean for each pixel is based on averaging rainy counts (i.e., mean of all rainy counts within the pixel during a given month), the averaging for the Giovanni user selected area (e.g., plot type: time series) is based on an area weighting average. I.e., by averaging the Ruc calculated for each pixel that are within the user selected area rather than calculating first the overall conditional mean rate rate for user selected area following by conversion to unconditional rain rate. Therefore, the averages of 0.5 degree boxes over 5x5 degree, for example, might not agree with the 5x5 degree unconditional rain rates. For high resolution (0.5. degree) e_surfRainMean includes 148x720 entries. This includes 4x720 entries for the area between lat (+36)-(+37), and lat (-36)-(-37). In this region the PR sample is very limited and data from this region are not used to avoid underestimation of the total rain amounts. |
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