Abstract
Abstract. A convective parameterization is described and evaluated that may be used in high resolution non-hydrostatic mesoscale models as well as in modeling system with unstructured varying grid resolutions and for convection aware simulations. This scheme is based on a stochastic approach originally implemented by Grell and Devenyi (2002). Two approaches are tested on resolutions ranging from 20 km to 5 km. One approach is based on spreading subsidence to neighboring grid points, the other one on a recently introduced method by Arakawa et al. (2011). Results from model intercomparisons, as well as verification with observations indicate that both the spreading of the subsidence and Arakawa's approach work well for the highest resolution runs. Because of its simplicity and its capability for an automatic smooth transition as the resolution is increased, Arakawa's approach may be preferred. Additionally, interactions with aerosols have been implemented through a cloud condensation nuclei (CCN) dependent autoconversion of cloud water to rain as well as an aerosol dependent evaporation of cloud drops. Initial tests with this newly implemented aerosol approach show plausible results with a decrease in predicted precipitation in some areas, caused by the changed autoconversion mechanism. This change also causes a significant increase of cloud water and ice detrainment near the cloud tops. Some areas also experience an increase of precipitation, most likely caused by strengthened downdrafts.
Highlights
There are many different parameterizations for deep and shallow convection that exploit the current understanding of the complicated physics and dynamics of convective clouds to express the interaction between the larger scale flow and the convective clouds in simple “parameterized" terms
The parameterization that we describe below has been released to users of the Weather Research and Forecasting (WRF, Skamarock et al, 2008) modeling system as well as the Brazilian version of the Regional Atmospheric Modeling system (BRAMS, Freitas et al, 2009)
A unified approach for convective parameterizations is introduced by A2011, which re-derive the vertical eddy fluxes by assuming that since the parameterization must converge to an explicit simulation of cloud processes as σ → 1, it follows that lim wc = w and lim ψc = ψ
Summary
There are many different parameterizations for deep and shallow convection that exploit the current understanding of the complicated physics and dynamics of convective clouds to express the interaction between the larger scale flow and the convective clouds in simple “parameterized" terms. With the increase in computer power, high resolution numerical modeling using horizontal grid scales of dx < 10 km is becoming widespread, even at operational centers. A convective parameterization should be scale dependent (see Arakawa et al, 2011) with assumptions that may vary with horizontal resolution Another complicating factor is the increased development of integrated models that combine weather and chemistry. It is recognized that chemical species will influence the weather by changing the atmospheric radiation budget as well as through cloud formation While many of these coupled modeling systems include sub-grid scale transport of chemical constituents and interaction of aerosols with radiation as well as interaction with microphysical schemes for explicit treatment of the aerosol indirect effect, little work has been done trying to couple aerosols with convective parameterizations. The parameterization that we describe below has been released to users of the Weather Research and Forecasting (WRF, Skamarock et al, 2008) modeling system as well as the Brazilian version of the Regional Atmospheric Modeling system (BRAMS, Freitas et al, 2009)
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