Evaluation of the Diurnal Evolution of the Size of Tropical Convective Systems in Large Domain, High Resolution Simulations using Observations of Outgoing Longwave Radiation

A long-standing problem in climate models is the failure to capture either the correct diurnal cycle in convective clouds or the growth of individual cells into larger scale complexes. Cascade is a multi-institution project to study the formation and development of tropical convective systems using high-resolution numerical modeling (down to 1.5~km) run over large domains ( ∼2000×2000~km) and observations. As one element of this, we have developed a technique for visualizing and testing the diurnal cycle in the size of convective cloud systems using observations of outgoing longwave radiation. This has been applied to a 2006 test case over Africa using GERB observations and models run with differing configurations and resolutions. We are now applying this to a large domain simulation of the Maritime continent covering several weeks during April 2009 comparing with TRMM observations. The image shows a comparison of the Met Office Unified Model run at 4~km and 12~km resolution, with and without convective parametrization respectively, for the West Africa test case. The grayscale represents the anomaly in the number of systems falling into each lengthscale bin against time. The middle panel is derived from observations by the Geostationary Earth Radiation Budget (GERB) instrument. It shows a broad upward stripe reflecting growth from smaller to larger systems beginning in the late afternoon. The 12~km model shows the effect of the parametrization scheme with systems of all sizes peaking at similar times much earlier than the observations. The 4~km model bears much closer comparison to the observations with growth in the middle to large size range occurring at a similar time to the observations. The small scale behavior, however, is affected by an unrealistic "shattering" of the large systems into many fragments in the early morning rather than a gradual decay.