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<br />The PPS does not perform as well with stratiform rain with no more than weak convection (Klazura <br />et aI., 1998). They show that stratiform rain underestimation by about a factor of two is common. This <br />finding is mentioned here because problems dealing with shallow stratiform rain are similar to those <br />encountered with snow-producing clouds, esp(~cially when they are shallow. Both stratiform rain and <br />snowfall tend to have maximum Ze values near the terrain (Joss and Lee, 1995). <br /> <br />While Z calculations are usually made with data obtained at the surface, radar Ze measurements are made <br />at varying heights above the surface, depending upon range, topography and the radar's view (e.g, terrain <br />blockage effects). Snowfall rates typically decrease with height above the surface (sometimes sharply) <br />resulting in an inequality of Z and Ze in addition to that shown by equation (4). The value of Ze becomes <br />increasingly smaller than Z with increasing beam height above the surface because Ze usually decreases <br />with altitude in snow-producing clouds with snow particle growth continuing to near-surface levels. <br /> <br />The vertical profile of Ze during snowfall, with maximum values near the surface, has been discussed by <br />several investigators, especially J. Joss and colleagues in Switzerland. Approaches to compensate for the <br />resulting range underestimation of precipitation are given in several publications over the years (e.g., Joss <br />and Waldvogel, 1970; Joss and Waldvogel, 1989; Joss and Lee, 1995). Wilson (1975) concluded that, <br />"Radar can be used to estimate water equivalents of snowfall with approximately the same accuracy as <br />for rainfall." However, he pointed out that range underestimation was a serious problem with lake effect <br />snowfall at even close range with a 1.7 deg wide beam tilted at 0.9 deg elevation angle. Only recently <br />has the vertical profile of Ze been recognized as the main source of error in estimating widespread <br />precipitation with radar as discussed by Joss and Waldvogel (1990) and Koistinen (1991). <br /> <br />A season average range correction, based on many storms, can provide a "seasonal" correction for all <br />storms. Joss and Waldvogel (1989) concluded that even a crude seasonal estimate can significantly <br />improve precipitation estimates at mid-to-far range. They showed that the general decrease in radar- <br />estimated precipitation in the lowest few kilometers in Switzerland averaged a pronounced 86 percent <br />km-l in winter. Moreover, the decrease in precipitation estimation started immediately above the ground. <br /> <br />While working on a related project, Super (1998) investigated the vertical profiles of S calculated from <br />Ze for several snow storms observed by the Minneapolis WSR-88D (KMPX). The approach used was to <br />"save" the Ze values from each range bin betw(:en 33 through 37 km range for all 360 deg of azimuth. <br />This was done for each volume scan and for each of the five lowest beam tilts over the entire course of <br />each storm. A single storm would often produee several hundred thousand samples (range bins) for each <br />of the 5 tilts which have nominal beam centers" for standard refraction, at 0.38,0.96, 1.54,2.12 and 2.70 <br />km above the radar. The Ze value for each range bin sample would then be processed to provide an <br />estimate of S and the many resulting estimates would be used to calculate a storm average S for each <br />height above the radar. <br /> <br />The SAA uses the lowest tilt (0.5 deg) beam wherever possible while avoiding ground clutter <br />contamination. The result for relatively flat terrain is that the 0.5 deg beam is used at all but the nearest <br />ranges. Heights of the 0.5 deg beam center for the noted heights above the radar (0.38 to 2.70 km) <br />correspond to ranges of35, 75, 104, 129, and 152 km, respectively. <br /> <br />It may be assumed to a first approximation that the many samples from the approximately 35 km radius <br />"circle" around the radar represent, on average, all ranges over a storm's duration. Use of this mean <br />vertical profile then provides a means of range-correcting SAA storm total S accumulations based on <br />radar data alone. For example, the profile from the 5th to the 1 st tilt near 35 km range can be used to <br />extrapolate from the center of the 1st tilt at 152 km range (2.70 km height) to near surface (0.38 kIn <br />height) levels. <br /> <br />17 <br />