Influence of Spatial Resolution on Diurnal Variability during the North American Monsoon

Abstract

Diurnal variability is an important yet poorly understood aspect of the warm-season precipitation regime over southwestern North America. In an effort to improve its understanding, diurnal variability is investigated numerically using the fifth-generation Pennsylvania State University (PSU)-NCAR Mesoscale Model (MM5). The goal herein is to determine the possible influence of spatial resolution on the diurnal cycle.The model is initialized every 48 h using the operational NCEP Eta Model 212 grid (40 km) model analysis. Model simulations are carried out at horizontal resolutions of both 9 and 3 km. Overall, the model reproduces the basic features of the diurnal cycle of rainfall over the core monsoon region of northwestern Mexico and the southwestern United States. In particular, the model captures the diurnal amplitude and phase, with heavier rainfall at high elevations along the Sierra Madre Occidental in the early afternoon that shifts to lower elevations along the west slopes in the evening. A comparison to observations (gauge and radar data) shows that the high-resolution (3 km) model generates better rainfall distributions on time scales from monthly to hourly than the coarse-resolution (9 km) model, especially along the west slopes of the Sierra Madre Occidental. The model has difficulty with nighttime rainfall along the slopes, over the Gulf of California, and over Arizona.A comparison of surface wind data from three NCAR Integrated Sounding System (ISS) stations and the Quick Scatterometer (QuikSCAT) to the model reveals a low bias in the strength of the Gulf of California low-level jet, even at high resolution. The model results indicate that outflow from convection over northwestern Mexico can modulate the low-level jet, though the extent to which these relationships occur in nature was not investigated.

Simulation of a North American Monsoon Gulf Surge Event and Comparison to Observations

“…A faster evolution rate is found in the northern GoC convection as well. Li et al (2008) show similar results for convective evolution in a 3-km simulation over the same region and Trier et al (2011) show an enhanced evolution rate and motion for a midlatitude squall system. Li et al (2008) show the model overestimates precipitation during the 1800-0600 UTC timeframe and underestimates it from 0600-1200 UTC, which indicates that the initiation and evolution of precipitation occurs too fast in the model.…”

“…The 2-km simulation (not shown) has slightly more stratiform precipitation, smaller convective cores, and stronger updrafts in agreement with Bryan and Morrison (2012). Li et al (2008) demonstrated convection-permitting grid spacing (3 km) to be capable of resolving NAM processes and Weisman et al (2011, manuscript submitted to Mon. Wea.…”

High-Resolution Downscaled Simulations of Warm-Season Extreme Precipitation Events in the Colorado Front Range under Past and Future Climates*

“…The convective motions that drive much warm-season midlatitude precipitation are not resolved in the current generation of global climate models (GCMs) or in most regional climate models (RCMs). The combination of coarse model resolution and the resulting requisite parameterization of processes such as convection and boundary layer mixing are known to have deficiencies including (but not limited to) (i) difficulty simulating the diurnal cycle and representing interactions with parameterized radiative effects (e.g., Baldwin et al 2002;Guichard et al 2004;Mahoney and Lackmann 2006), (ii) large sensitivity to soil conditions (e.g., Hohenegger et al 2009), (iii) unusual vertical heating profiles (e.g., Baldwin et al 2002;Chao 2012), and (iv) unrealistic convective system organization and motion (e.g., Bukovsky et al 2006;Li et al 2008;Moncrieff and Liu 2006;Mahoney and Lackmann 2007;Pritchard et al 2011;Newman and Johnson 2012).…”

Objective Climatological Analysis of Extreme Weather Events in Arizona during the North American Monsoon

“…How severe weather events may change in the future is a subject of rapidly growing interest and research that is beginning to be addressed in the context of highresolution regional atmospheric modeling with explicitly resolved convection (e.g., Trapp et al 2011). To properly represent organized mesoscale convective systems in the NAM region, it is necessary to utilize a convective-permitting model with a spatial scale on the order of 1 km (e.g., Li et al 2008;Newman and Johnson 2012). As a consequence, objective methods for identifying atmospheric conditions suitable for the generation of severe weather become necessary to efficiently simulate them in long-term simulations with regional climate models.…”