Abstract
Abstract. The sensitivity of quantitative precipitation forecasts to various modifications of the Kain-Fritsch (KF) convective parameterization scheme (CPS) is examined for twenty selected cases characterized by intense convective activity and widespread precipitation over Greece, during the warm period of 2005–2007. The study is conducted using the MM5 model with a two nested domains strategy, with horizontal grid increments of 24 and 8 km, respectively. Five modifications to the KF CPS, each designed to test the sensitivity of the model to the convective scheme formulation, are discussed. The modifications include: (i) the maximization of the convective scheme precipitation efficiency, (ii) the change of the convective time step, (iii) the forcing of the convective scheme to produce more/less cloud material, (iv) changes to the trigger function and (v) the alteration of the vertical profile of updraft mass flux detrainment. The simulated precipitation from the 8-km grid is verified against raingauge measurements. Although skill scores vary widely among the cases and the precipitation thresholds, model results using the modifications of the convective scheme show improvements in 6-h precipitation totals compared to simulations generated using the unmodified convective scheme. In general, forcing the model to produce less cloud material improves the precipitation forecast for the moderate and high precipitation amounts, while the same modification and the change of the convective time step to 1 min has the same result for the high precipitation thresholds. The increase of convective time step to 15 min, the maximization of precipitation efficiency and the changes to the trigger function give similar results for medium and high precipitation. On the other hand, the forecast for the light precipitation is improved by forcing the model to produce more cloud material as well as by the alteration of the vertical profile of updraft mass flux detrainment.
Highlights
Greece is a topographically diverse country; there are many variations in the weather conditions especially during the warm period of the year
Convection cannot be explicitly resolved at the resolution used by operational mesoscale models when the resolution is greater than 4 km, and it is parameterized using one of the various convective parameterization schemes (CPS)
Many studies have shown that among the various CPS used in the numerical weather prediction (NWP), the KainFritsch (KF) scheme (Kain and Fritsch, 1990, 1992) performs well in forecasts of convective systems (Kuo et al, 1996; Wang and Seaman, 1997; Kotroni and Lagouvardos, 2001, 2004; Ferrier, 2004; Mazarakis et al, 2009)
Summary
Greece is a topographically diverse country; there are many variations in the weather conditions especially during the warm period of the year. Ferrier (2004) managed to alleviate the problem of high QPF bias, testing various modifications of the KF CPS and found some improvements in QPF In his modifications (i) all of the rain and snow calculated in the updrafts are detrained onto the grid, in which all subsequent cloud and precipitation processes are calculated by the grid-scale microphysics, (ii) fields of hydrometeors calculated by the model are used as input into the scheme and are modified by the convective processes, (iii) hydrometeors, except for rain, are converted back to cloud water, (iv) convective downdrafts are turned off and (v) the updraft radius is a function of cloud-base vertical motion. One hundred eighty simulations have been performed in total
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