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Pritchard, MS, Moncrieff MW, Somerville RCJ.  2011.  Orogenic Propagating Precipitation Systems over the United States in a Global Climate Model with Embedded Explicit Convection. Journal of the Atmospheric Sciences. 68:1821-1840.   10.1175/2011jas3699.1   AbstractWebsite

In the lee of major mountain chains worldwide, diurnal physics of organized propagating convection project onto seasonal and climate time scales of the hydrologic cycle, but this phenomenon is not represented in conventional global climate models (GCMs). Analysis of an experimental version of the superparameterized (SP) Community Atmosphere Model (CAM) demonstrates that propagating orogenic nocturnal convection in the central U.S. warm season is, however, representable in GCMs that use the embedded explicit convection model approach [i.e., multiscale modeling frameworks (MMFs)]. SP-CAM admits propagating organized convective systems in the lee of the Rockies during synoptic conditions similar to those that generate mesoscale convective systems in nature. The simulated convective systems exhibit spatial scales, phase speeds, and propagation speeds comparable to radar observations, and the genesis mechanism in the model agrees qualitatively with established conceptual models. Convective heating and condensate structures are examined on both resolved scales in SP-CAM, and coherently propagating cloud "metastructures" are shown to transcend individual cloud-resolving model arrays. In reconciling how this new mode of diurnal convective variability is admitted in SP-CAM despite the severe idealizations in the cloud-resolving model configuration, an updated discussion is presented of what physics may transcend the re-engineered scale interface in MMFs. The authors suggest that the improved diurnal propagation physics in SP-CAM are mediated by large-scale first-baroclinic gravity wave interactions with a prognostic organization life cycle, emphasizing the physical importance of preserving "memory" at the inner resolved scale.

Pritchard, MS, Somerville RCJ.  2009.  Empirical orthogonal function analysis of the diurnal cycle of precipitation in a multi-scale climate model. Geophysical Research Letters. 36   10.1029/2008gl036964   AbstractWebsite

Long-term variability in the hydrologic cycle is poorly simulated by current generation global climate models (GCMs), partly due to known climatological biases at shorter timescales. We demonstrate that a prototype Multi-scale Modeling Framework (MMF) provides a superior representation of the spatial and temporal structure of precipitation at diurnal timescales than a GCM. Results from empirical orthogonal function (EOF) decomposition of the boreal summer climatological composite diurnal cycle of precipitation in an MMF are compared to a GCM and satellite data from the Tropical Rainfall Measuring Mission. The eigenspectrum, principal component time series, and the spatial structure of leading EOFs in an eigenmode decomposition of the MMF composite day are a much better match to observations than the GCM. Regional deficiencies in the MMF diurnal cycle are manifest as localized anomalies in the spatial structures of the first two leading EOFs. Citation: Pritchard, M. S., and R. C. J. Somerville (2009), Empirical orthogonal function analysis of the diurnal cycle of precipitation in a multi-scale climate model, Geophys. Res. Lett., 36, L05812, doi: 10.1029/2008GL036964.

Pritchard, MS, Somerville RCJ.  2009.  Assessing the Diurnal Cycle of Precipitation in a Multi-Scale Climate Model. Journal of Advances in Modeling Earth Systems. 1   10.3894/james.2009.1.12   AbstractWebsite

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.