Using high-resolution numerical simulations to understand the role of the Rocky mountain cordillera on the initiation of long-lived precipitation episodes

Recent observational studies (e.g., Carbone et al. 2002) have documented the common occurrence of successive time-space coherent episodes of heavy rainfall that travel across the eastern 2/3 of the United States during the warm season (May-August). Such traveling episodes of heavy precipitation have mesoscale convective systems (MCSs) as their primary constituents and may account for a significant fraction of their total warm season precipitation (e.g., Fritsch et al. 1986). The current work examines the initiation of these episodes, which often occurs at or within several hundred km in the lee of the continental divide. Past work has examined the role of PBL circulations in convection initiation near terrain including drylines and solenoidal circulations related to terrain slope, land-surface contrasts, and antecedent convection. In the current work, we examine the role of these PBL-based features but also the impact of free-tropospheric features (including PV anomalies) that may be influenced by the large-scale terrain.

The investigative tool used in this study is the coupled Weather Research and Forecasting (WRF) – Noah Land-Surface Modeling System. The model is integrated over a single large subcontinental scale domain that extends from the Pacific Ocean to the Midwestern United States using a deep convection-permitting horizontal grid spacing of 3 km. A preliminary simulation based on a convectively active period (03-10 July 2003) uses a time-averaged lateral boundary condition that varies only with time of day. Here, afternoon convection develops all along the continental divide but organizes into evening MCSs only at latitudes where the time-averaged midtropospheric flow is strong, which apparently contributes to the vertical shear necessary for upscale growth of deep convection. The time-averaging of the boundary condition greatly reduces the amplitude of the traveling midtropospheric PV disturbances that were present in the vicinity MCS initiation in a previous simulation of the period (Trier et al. 2006). This suggests that while the midtropospheric disturbances may help focus convection initiation they are not necessary for the onset of the traveling episodes of long-lived convection. Additional simulations of convection initiation using a time-averaged boundary condition from a longer period more representative of midsummer climatological conditions are currently being analyzed and will be presented at the conference.

References:

Carbone, R. E., J. D. Tuttle, D. A. Ahijevych, and S. B. Trier, 2002: Inferences of predictability associated with warm-season precipitation episodes. J. Atmos. Sci., 59, 2033-2056.

Fritsch, J. M., R. J. Kane, and C. R. Chelius, 1986: The contribution of mesoscale convective weather systems to the warm-season precipitation in the United States. J. Clim. Appl. Meteor., 25, 1333-1345.

Trier, S. B., C. A. Davis, D. A. Ahijevych, M. L. Weisman, and G. H. Bryan, 2006: Mechanisms supporting long-lived episodes of propagating nocturnal convection within a 7-day WRF model simulation. J. Atmos. Sci., 63, 2437-2461.