Published Online 13 Oct. '09
Assessing the diurnal cycle of precipitation in a multi-scale climate model
Michael S Pritchard, Richard Somerville
Abstract
A promising result that has emerged from the new Multi-scale Modeling Framework (MMF) approach to atmospheric modeling is a global improvement in the daily timing of peak precipitation over the continents, which is suggestive of improved moist dynamics at diurnal timescales overall. We scrutinize the simulated seasonal composite diurnal cycle of precipitation in an MMF developed by the Center for Multiscale Modeling of Atmospheric Processes (CMMAP) using a comprehensive suite of diurnal cycle diagnostics including traditional harmonic analysis, and non-traditional diagnostics such as the broadness of the peak precipitation in the mean summer day, reduced dimension transect analysis, and animations of the full spatial and temporal variability of the composite mean summer day. Precipitation in the MMF is evaluated against multi-satellite merged satellite data and a control simulation with a climate model that employs conventional cloud and boundary layer parameterizations. Our analysis highlights several improved features of the diurnal cycle of precipitation in the multi-scale climate model: It is less sinusoidal over the most energetic diurnal rainfall regimes, more horizontally inhomogeneous within continents and oceans, and more faithful to observed structural transitions in the composite diurnal cycle chronology straddling coastlines than the conventional climate model. A regional focus on North America links a seasonal summer dry bias over the continental United States in the CMMAP MMF at T42 resolution to its inability to capture diurnally propagating precipitation signals associated with organized convection in the lee of the Rockies. The chronology of precipitation events elsewhere in the vicinity of North America is improved in the MMF, especially over sea breeze circulation regions along the eastern seaboard and the Gulf of Mexico, as well as over the entirety of the Gulf Stream. Comparison of the convective heating and moistening suggests that improvements in the MMF coastal ocean diurnal rainfall may be a result of a local moist dynamical response to the improved representation of energetic diurnal forcing over adjacent land.
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Cite this article in JAMES:
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Pritchard, M. S. and R. Somerville, (2009): Assessing the diurnal cycle of precipitation in a multi-scale climate model. J. Adv. Model. Earth Syst., 1, Art. #12, 16 pp., doi:10.3894/JAMES.2009.1.12
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Supplementary Files
1. Download VIEW (385KB) Video S2 CONVTEND.MOV - Height-longitude section contrasting the animated composite JJA diurnal chronology of convective heating and convective moistening in SP-CAM and CAM, along a zonal transect straddling the Gulf Stream and eastern United States (Transect 1 in Figure 4). The quantities shown are tendencies exerted by convection in the physics package, i.e. by nudging towards the nested CRM in the SP-CAM and as diagnosed by conventional parameterization in CAM. The vertical coordinate is normalized pressure, and the land component at the western edge of the transect is identifiable as a blanked out region at the base of the domain.2. Download VIEW (940KB) Video S1 PRECIP.MOV - Animation showing the climatological composite diurnal cycle of precipitation over North America (looped five times) in a multi-scale modeling framework (SP-CAM), a conventional climate model, the Community Atmosphere Model v3.0 (CAM). Also shown are observations (OBS) - a multi-satellite, multi-instrument best estimate of precipitation as observed from orbit (TRMM 3B42).
This work is licensed under a Creative Commons Attribution 3.0 License.
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