Modelling Solar Fluxes for an Evolving Tropical Cloud System
Barker, H.W., Atmospheric Environment Service of Canada; Fu, Q., Dalhousie University
Ninth Atmospheric Radiation Measurement (ARM) Science Team Meeting
Domain-averaged, broadband solar radiative budgets for an evolving tropical mesoscale convective cloud system are computed by two approximate one-dimensional (1-D) models, which make different assumptions about the structure of unresolved clouds. One model is the standard plane-parallel, homogeneous (PPH) two-stream approximation. The other assumes that fluctuations in cloud extinction can be described by a gamma distribution. A three-dimensional (3-D) Monte Carlo (MC) algorithm provides reference calculations. The cloud system was simulated by a two-dimensional (2-D) cloud-resolving model and the domain measures 514 km horizontally with a grid-spacing of 1 km. Five types of hydrometeors are accounted for. Snapshots of the domain were saved every 5 model-minutes for 10 hours. It is shown that the conventional PPH two-stream is inappropriate as it yields mean top-of-the-atmosphere (TOA) albedo and surface absorptance of 0.56 and 0.20 while corresponding 3-D MC values are 0.32 and 0.47. For the gamma-weighted two-stream approximation (GWTSA), the values are 0.32 and 0.49. Moreover, heating rate errors for the PPH model are about -0.5 K/day near the surface and almost +2 K/day at 10 km, but are diminished at both altitudes to 0.25 K/day for the GWTSA. Also, it is shown that while the best possible PPH model (i.e., perfect account of cloud overlap) is vastly superior to the regular PPH model, it is significantly inferior to the GWTSA.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
