Estimation of Orographic Precipitation Distribution Using Orographic Moisture Flux in the South-Eastern Alps

Orographic precipitation is often not well represented even by the high resolution numerical models because of lack of measurements at lower levels. Underestimation of radio sonde measured wind speed and incorrect humidity profile usually makes the model atmosphere potentially to stable. Even at realistic model grid point altitude, the model orography shape could be misleading making the air trajectory to be convex instead of a concave one. Also the parameterization of convective precipitation in numerical model could introduce big errors to the result. A method for evaluating and possibly improving model precipitation results using measured data and orographic moisture flux is discussed in the paper. The classical methods for diagnosing precipitation rates using convective available potential energy (CAPE) are not applicable in mountains because of their high spatial variability. In the previous studies a method for estimation of maximum precipitation rate in the South-Eastern Alps based on Lin (2002) estimation calculated from measured data from the Udine radiosounding was presented. For several heavy precipitation cases measured maximum hourly precipitation rates in the region were plot against the estimated precipitation with the good agreement of regression line. In the paper the method will be extended to two dimensions using numerical model fields. According to Lin the distribution of numerical model orographically induced vertical moisture flux represents the distribution and maximum precipitation even better than the model precipitation field. This quantity is used to compare for selected case the results of numerical simulation against the observed precipitation distribution, with the aim to improve the operational forecast of precipitation distribution and maximum rates, especially in the extreme cases.