Applicability of grid-size-dependent Convection Parameterization to meso-gamma-scale HIRLAM

The representation of convective phenomena is very difficult in meso-gamma-scale models. It could be possible to resolve organized convective structures explicitly with a smaller grid spacing than 4 km. However, the grid spacing range of 5-20 km is especially difficult for convection schemes to handle. The convection parameterization is highly dependent on the model resolution (in both time and space) in the range of meso-gamma scales. Traditionally, convection parameterizations are "tuned" for a fixed grid spacing. Therefore, the behaviour of non-resolution-dependent convection schemes may be undesirable, when these schemes are applied for different scales.

The objective of this study is to evaluate the applicability of the existing convection and condensation scheme of the HIgh Resolution Limited Area Model (HIRLAM) in meso-gamma-scale convective conditions. HIRLAM is a complete NWP system including an anelastic nonhydrostatic model with an extensive set of physical parameterizations and data assimilation. The default convection and condensation scheme of the model parameterizes both convective and stratiform condensation, clouds and precipitation. It also allows a gradual transition between both regimes. The convection scheme is a modified Kuo scheme that includes cloud water as a prognostic variable. The diagnostic precipitation release depends on the amount of cloud water. Improved applicability for meso-gamma scales is sought by introducing simple grid-size-dependent entrainment function in the triggering mechanism for convection scheme. The purpose of this function is to lower the convective entities and, consequently, gradually to switch the convection parameterization off as the grid size decreases. In this study, we concentrate on the issue of the grid-size-dependent convection scheme. The performance of the different model configurations is mainly validated using radar reflectivity data from the Finnish radar network.

6 different model simulations of a single cold air outbreak event with small scale convective precipitation are carried out. The event occurred over Southern Finland on 25 May 2001. Two types of experiments are made; with and without grid-size-dependent convection scheme. Both types utilize three different horizontal grid spacings: 11, 5.6 and 2.8 km.

Both experiments are able to form cellular structures similar to the observed ones. However, the grid-size-dependent scheme produces the precipitation area that most resembles the observed field. Moreover, the reflectivity distribution from the grid-size-dependent scheme is closer to the observed one, whereas the "traditional" scheme increasingly overestimates the amount of moderate reflectivities as the grid size reduces. Evidently, without any dependency on model resolution the convection scheme is too active. However, in this case the grid-size-dependent scheme does not have an effect on strong reflectivities, which are overestimated by both models.