Forecasting Precip. Algorithm

[Jason234]

"The thickness algorithm diagnoses precipitation type based upon the average virtual temperature, as determined by the hypsometric equation and the difference of the geopotential height of two pressure surfaces. This distance is commonly called the "thickness" of the layer. The bottom and top pressures of the three tropospheric layers used in this algorithm are:1000 and 850 mb, 850 and 700 mb, and 700 and 500 mb. These mandatory layers have traditionally been used by forecasters for the purpose of diagnosing (forecasting) precipitation type.

We assigned the critical thickness values after examining the studies of Keeter and Cline (1991), Younkin (1967), and Zerr (1997) and determining the thickness values that were consistent among the studies. Using the geopotential height data from the rawinsonde data, we determined the precipitation type near the ground. Snow was diagnosed if the 850—700 mb thickness was < or = 1540 m. Rain was diagnosed if the 1000—850 mb thickness was > 1310 m or if the 850—700 mb thickness > 1560 m and the surface Tw > 0°C; otherwise, if the surface Tw < or = 0°C, then the algorithm diagnoses freezing rain. If the 850—700 mb thickness > 1540 m and < or = 1560 m, then ice pellets are diagnosed. The algorithm only diagnoses the precipitation type if the geopotential height data at all four mandatory levels are available (i.e., no extrapolated data)."

FROM:http://www.cimms.ou.edu/~cortinas/prepri...nada6.html