Organized convective systems in the tropical western pacific as a process in general circulation models: A toga coare case‐study

Abstract

We examine the large‐scale effects of organized convective systems in the tropical western Pacific observed during the Tropical‐Ocean Global‐Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). In a case‐study approach, we examine realizations of a supercluster, associated with the onset of the December 1992 westerly wind burst, in the T213 operational medium‐range weather forecasting model of the European Centre for Medium‐Range Weather Forecasts (ECMWF). We idealize a supercluster as a hierarchy of three interacting scales, namely organized cumulonimbu 𝒞1 mesoscale convective systems 𝒞2, and the supercluster component 𝒞3. It is shown that the ECMWF model represents this hierarchy as a 𝒞3‐like surrogate whose influence dominates the effect of parametrized convection. This causes over‐prediction of the model tendencies which, in the case of zonal momentum, is explained in elementary terms. The structure of the resolved‐scale momentum flux is explained by Moncrieff's (1992) archetypal theory of organized convection which has been verified against observations and cloud‐resolving model data‐sets. the parametrization of subgrid‐scale convective momentum‐flux in the ECMWF model, based on a momentum mixing concept, produces subgrid‐scale tendencies that are physically different from transports associated with cumulonimbus convection in a shear flow. We outline a strategy for parametrizing the momentum flux by the 𝒞1 component based on the archetypal model. the 𝒞2 component, which is part‐resolved and part‐parametrized, is at odds with the assumptions of scale separation underpinning parametrization. It is argued that this component should be represented as part of the prognostic treatment of convectively generated cirrus. Finally, we suggest cloud‐resolving modelling studies to further quantify the structure and large‐scale impact of superclusters in a westerly‐wind‐burst environment, ranging from idealized models to models having data assimilation capability.

Observed Scaling in Clouds and Precipitation and Scale Incognizance in Regional to Global Atmospheric Models

“…The resolution-dependence of the precipitation partitioning between the convective and stratiform components is consistent with results from Moncrieff and Klinker (1997) using the European Centre for Medium-Range Weather Forecasts model. They show that stratiform precipitation dominates over convective precipitation in T106 and T213 simulations of an organized convective cluster.…”

Rethinking convective quasi-equilibrium: observational constraints for stochastic convective schemes in climate models

“…Moncrieff & Klinker 1997;Moncrieff & Liu 2006) or to give subgrid-scale plumes more individual identity (Kuell et al 2007), directions that may be highly useful to future stochastic parametrization.…”

Critical roles of convective momentum transfer in sustaining the multi-scale Madden–Julian oscillation

“…The most unstable system usually occurs at π/4<+<π/2, where easterly/westerly CMT is located in the front/rear part of the MJO, and is almost in phase with the planetary-scale zonal wind (Fig. 1): the 2-day waves prevail in front part of the MJO convective complex and produce low tropospheric easterly CMT, while the superclusters overwhelm the 2-day waves in the rear part, i.e., the Rossby-gyre region of the MJO (Moncrieff and Klinker 1997), and produce low tropospheric westerly CMT. Such optimal structure is consistent with the multi-scale structure of observed MJO (Moncrieff and Klinker 1997;Kikuchi and Wang 2010).…”

“…The eastward propagating super cloud clusters have horizontal convective scale of ∼1,500 km, time scale of ∼10-15 days, and eastward phase speed of 15-20 ms −1 (Nakazawa 1988;Wheeler and Kiladis 1999;Wheeler et al 2000). They involve heavy precipitation and fully developed deep convection and stratiform clouds, and show a westward tilt in the Rossby-gyre region of the active MJO (Moncrieff and Klinker 1997;Houze et al 2000). The westward-propagating 2-day waves, identified as n=1 westward Inertio-gravity waves (Takayabu 1994;Haertel and Kiladis 2004;Haertel et al 2008;Liu and Wang 2011;, have zonal convective scale of ∼1,000 km, time scale of ∼2 days, and westward propagation speed of 10-30 ms −1 .…”

“…1): the 2-day waves prevail in front part of the MJO convective complex and produce low tropospheric easterly CMT, while the superclusters overwhelm the 2-day waves in the rear part, i.e., the Rossby-gyre region of the MJO (Moncrieff and Klinker 1997), and produce low tropospheric westerly CMT. Such optimal structure is consistent with the multi-scale structure of observed MJO (Moncrieff and Klinker 1997;Kikuchi and Wang 2010). Using this structure, Biello and Majda (2005) obtained the horizontal quadrupole vortex and vertical westerly windburst structures of the MJO, and Wang et al (2011) also successfully simulated the phase speed and structures of the unstable MJO.…”