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(last update: December 6, 2015)

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The animated gif above consists of consecutive scans made by the NEXRAD "KDAX" radar near Sacramento, showing the evolution of the California blizzard on 4 January 2008 from 21:37Z to 22:11Z. Each panel shows the vertically-integrated dbZ observed reflectivity factors, which could be converted into water content values q using a power-law Z=aqb were it not for the fact that the coefficient a (and to a lesser extent b) depends in a nonlinear way on the unknown mean hydrometeor size. The variability of a is so large that it would induce an uncertainty on q that can exceed 100%, with an unknown portion of this uncertainty taken up by systematic bias. The topography is indicated by the navy-colored 250m contour line, and the red-colored 1000m contour line.


This animated gif shows the "derived observations" from the data illustrated in previous gif. Specifically, horizontal motion fields V were first derived from the dbZ measurements themselves, by locally correlating consecutive pairs of radar scans. Then these motion fields were used to calculate (using finite-difference approximations) the total derivatives D(dbZ)/Dt + V grad(dbZ), which is proportional to the relative sources-minus-sinks term (qcondensed - qevaporated - qsedimented ) / qtotal during the elapsed 4.6 minutes. Note how the sources (positive values, in red) and sinks (negative values, in blue) form wave fronts that appear to be systematically oriented almost perpendicular to the contours of the measured reflectivity factors themselves in the previous slide (aligned so that the reader can easily toggle between this page and the previous one), and appear to propagate systematically along the orthogonal, in the direction of the water-vapor flux (from the water-vapor atmospheric river not otherwise detectable by NEXRAD). Neither these patterns nor their evolution can be immediately inferred by looking at the direct observations without our derived processing. These images are from the preprint by O.O. Sy and Z.S. Haddad entitled "Derived observations from frequently-sampled microwave measurements of rain and their relations to cloud dynamics and thermo-dynamics".
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