A numerical study of the evolving convective boundary layer and orographic circulation around the Santa Catalina Mountains in Arizona. Part II: Interaction with deep convection

The Weather, Research, and Forecasting (WRF) modeling system is used for several IOPs during the Cumulus Photogrammetric, In situ and Doppler Observations (CuPIDO) campaign, conducted in summer 2006 around the Santa Catalina Mountains in southeast Arizona (Damiani et al. 2008), with the purpose to examine the interplay between boundary-layer convergence and orographic thunderstorms. This study builds on Part I (Demko et al. 2009b), which examines thermally-forced orographic boundary-layer circulations without deep convection in a weakly-sheared environment subject to strong surface heating. The CuPIDO dataset is used as a basic model validation sources, in terms of mountain-scale convergence (MSC) at the surface and cumulus evolution, but it is not used to initialize or nudge the WRF simulations.

The study is motivated by the fact that operational models poorly capture the timing and intensity of orographic convection. Yet this convection is essential to warm-season precipitation and to surface-troposphere exchange of water and heat in the Mountain West. The poor predictability of orographic deep convection is due to inaccurate coupling between boundary-layer processes and cumulus convection over complex terrain. CuPIDO data suggest that no enhanced MSC occurs during cumulus growth, and that only the consequence of deep convection (i.e., outflow spreading) is apparent at the surface (Geerts et al. 2008; Demko et al. 2009a). This motivates the present modeling study.

The WRF simulations indicate that enhanced MSC does develop 1-2 hours prior to orographic deep convection. In the two case studies presented herein, MSC tends to peak about the time that Cbs reach their equilibrium level. As Cbs mature, their outflow boundaries destroy the surface convergent flow around the mountain. Deeper and more widespread convection produces a more intense cold pool and stronger divergence at the surface. Even though the surface flow is divergent an anomalous low pressure may persist over the mountain, leading to the reestablishment of MSC and a new cycle of orographic deep convection. In both case studies MSC occurred ~1 km above the surface in the evening (1-2 hours before sunset), resulting in more widespread convection.

References:

Damiani, R., J. Zehnder, B. Geerts, J. Demko, S. Haimov, J. Petti, G.S. Poulos, A. Razdan, J. Hu, M. Leuthold, and J. French, 2008: Cumulus Photogrammetric, In-situ and Doppler Observations: the CuPIDO 2006 experiment. Bull. Amer. Meteor. Soc., 89, 57–73.

Demko, J. C., B. Geerts, Q. Miao, and J. Zehnder, 2009a: Boundary-layer energy transport and cumulus development over a heated mountain: an observational study. Mon. Wea. Rev. , 137, 447–468.

Demko, J. C., B. Geerts, Q. Miao, 2009b: A Numerical Study of the Evolving Convective Boundary Layer and Orographic Circulation around the Santa Catalina Mountains in Arizona. Part I: Circulation without Deep Convection. Abstract for the AMS 13th Conference on Mesoscale Processes, Salt Lake City, August 2009.

Geerts, B., Q. Miao, and J.C. Demko, 2008: Pressure perturbations and upslope flow over a heated, isolated mountain. Mon. Wea. Rev., 136, 4272–4288.