COST ES0905: Basic Concepts for Convection Parameterization in Weather Forecast and Climate Models WG 1 + 2 Meeting on convection parameterization closure Monday, 2. May 2011 -­‐MIM, LMU, Munich Meeting report Background: A core element of any convection parameterization is its closure as it defines (possibly using an additional triggering function) where, when and how strong convection is active. The closure assumption is a hypothesis linking the subgrid-­‐scale convective activity to prognostic resolved scale processes. Currently used closures can be roughly divided into two categories: closures based on moisture budget and closure based on CAPE amount. Discussion: An introduction given by M. Bister pointed out that convection depends on 1. Free tropospheric temperatures 2. Boundary layer (moist entropy) 3. Mid-­‐tropospheric humidity Over the tropical ocean free tropospheric temperatures as well as diurnal Boundary Layer (BL) variations are relatively small, thus mid-­‐tropospheric moisture can be a limiting factor. It is generally agreed that moisture closures in the tropics work quite well. In continental Mid-­‐latitudes (without wave influence) diurnal variations in the BL moist entropy are large which draws attention to the diurnal cycle and thus suggest the importance of convective triggering. In continental Mid-­‐latitudes under influence of waves, however, variations of tropospheric temperature is relatively large, thus convection will be well defined by CAPE. Therefore, the location on the globe will favour certain closure formulations. Mid-­‐tropospheric moisture has been identified as important influence on convection because it has strong effects on 1. the process of convective entrainment 2. the introduction of lower entropy air into BL (via unsaturated downdraughts) 3. size /strength of cold pools J.-­‐I. Yano showed that in context of convective quasi-­‐equilibrium (CQE) the assumption of a steady state plume contradicts the notion of convective triggering. In CQE the convective mass flux over the vertical column is completely depended on the large-­‐scale environment and converts to a new mass flux profile in correspondence with the large-­‐scale evolution. Thus there is no room for convective triggering. As G. Craig points out, however, an adjustment to CQE is not instantaneous but happens at the velocity of gravity wave spreading which can be regarded as the velocity that clouds “communicate” their existence. Depending on the time scale of environmental variation convection might not be in equilibrium. In Mid-­‐latitude continental convection with strong diurnal cycle CQE might not be reached. Thus, a time derivative of some variable connected to convection is needed. J.-­‐M. Piriou suggest that it might be more important to specify which clouds develop a given environment and in turn avoid to solve for a cloud spectrum in CQE. Importantly, clouds experience diluted CAPE and convective inhibition (CIN). This approach would in turn focus again on correct convective triggering. In a short presentation L. Gerard showed that in case of an NWP model in Mid-­‐
latitudes a CAPE closure with buoyancy triggering seems to produce better results than a moisture convergence closure. At high resolutions (few kilometres), however, a convection parameterization representing the subgrid-­‐ scale variability originating from moist convection does not represent a whole ensemble of clouds or a whole cloud life cycle. Moreover, the host model will capture a certain part of buoyancy driven motions leading to grid cell storms. In those circumstances the convection scheme needs a prognostic element. J.-­‐I. Yano argues that a sound diagnostic closure is based on either a prognostic version where time derivatives are explicitly and consciously dropped, or on an asymptotic higher order formulation. But generally accepted sets of equation do not exist yet. Observations show best correlations between convective activity and humidity rather than CAPE (e.g. Sherwood, 1999; Zhang and Klein, 2010). These measurements and correlations might be misleading, however, since occurrence of convection immediately reduces CAPE. J.-­‐I. Yano suggest that potential energy convertability (PEC) (Yano, 2005) instead of (undiluted) CAPE is the correct measure for the buoyancy an actual cloud experiences. PEC, similar to cloud work function, incorporates mid-­‐tropospheric moisture effects and possibly CIN. First results suggest that CIN in the BL is not an important factor for cloud occurrence. Results and conclusions: In general most participants agreed on the following three points. First, a closure should be physically based and therefore some form of grid column buoyancy measure should be used. Second, moisture and especially mid-­‐tropospheric moisture is playing an important role. This is why moisture closures work especially in the tropics and why any closure has to be sensitive to moisture. Third, future convection schemes should/will include some prognostic element. The discussions of this meeting as well as the preceding discussions will be gathered in a joint document with the following proposed outline: 1. Identification of controlling parameters 1.1 List of controlling parameters and physical basis 1.2 Statistical correlations obtained by observations 1.3 Activation control principle 1.4 Discussion 2. Perspective from large-­‐scale dynamics 3. Column-­‐integrated water-­‐vapour (saturation deficit) 4. Energy cycle of convective system Participants (COST member participants in bold): M. Bister, S. Brinkop , G. Craig, E. Fiori, J-­‐F. Geleyn, L. Gerard, M. Keane, M. Koehler. B. Plant, T. Wagner, J-­‐I. Yano References: Sherwood, Steven C., 1999: Convective Precursors and Predictability in the Tropical Western Pacific. Mon. Wea. Rev., 127, 2977–2991 Yano, J.I. and Chaboureau, J.P. and Guichard, F., 2005: A generalization of CAPE into potential-­‐energy convertibility, QJRMS, 607, 861-­‐876 Zhang, Y. and S. Klein, 2010: Mechanisms affecting the transition from shallow to deep convection over land: Inferences from observations of the diurnal cycle col-­‐ lected at the ARM southern great plains site. JAS, 67, 2943 – 2959