Climatology of summer midtropospheric perturbations in the US northern plains. Part II: large-scale effects of the Rocky Mountains on genesis Shih-Yu Wang • Tsing-Chang Chen • Eugene S. Takle Received: 1 July 2009 / Accepted: 8 February 2010 Ó Springer-Verlag 2010 Abstract Propagating convective storms across the US northern plains are often coupled with preexisting mid- tropospheric perturbations (MPs) initiated over the Rocky Mountains. A companion study (Part I) notes that such MPs occur most commonly at 12 UTC (early morning) and 00 UTC (late afternoon). Using a regional reanalysis and a general circulation model (GCM), this study investigates how such a bimodal distribution of the MP frequency is formed. The results point to two possible mechanisms working together while each has a different timing in terms of maximum effect. The diurnal evolutions between the midtropospheric flows over the Rockies and over the Great Plains are nearly out-of-phase due to inertial oscillation. During the nighttime, the westerly flows at 700–500 mb over the Rockies intensify while flows at the same level over the Great Plains turn easterly. These two flows converge over the eastern Rockies and induce cyclonic vorticity through vortex stretching. After sunrise, the convergence dissipates and the cyclonic vorticity is redis- tributed by horizontal vorticity advection, moving it downstream. This process creates a climatological zonally propagating vorticity signal which, in turn, facilitates the early-morning MP genesis at 12 UTC. The analysis also reveals marked dynamic instability conducive to subsyn- optic-scale disturbances in the midtroposphere over the Rockies. Strong meridional temperature gradients appear over the north-facing slopes of the Rockies due to terrain heating to the south and the presence of cooler air to the north. This feature, along with persistent vertical shear, creates a Charney–Stern type of instability (i.e. sign changes of the meridional potential vorticity gradient). Meanwhile, the development of terrain boundary layer reduces the Rossby deformation radius which, subse- quently, enhances the likelihood for baroclinic short waves. Such effects are most pronounced in the late afternoon and therefore are supportive to the MP genesis around 00 UTC. Examination of GCM experiments with and without orography further supports the critical role of the Rocky Mountains and its associated boundary layer impacts on the formation of MPs. Keywords Midtropospheric perturbation  Short wave  Baroclinic instability  MCS  Boundary layer 1 Introduction The role of the Rocky Mountains on cold-season weather disturbances has been extensively studied (e.g., Karyampudi et al. 1995; Poulos et al. 2000; Steenburgh and Blazek 2001), particularly the lee cyclogenesis as reviewed in Hobbs et al. (1996). In comparison, the effect of the Rockies on summer weather disturbances is not so well documented. A majority of summer convective storms develop near the eastern slopes of the Rockies in the afternoon and then move east- ward. Their initiation, growth and propagation across the northern plains are attributed to the consecutive processes of diurnal heating over terrain and moisture supply by the S.-Y. Wang  T.-C. Chen  E. S. Takle Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA, USA E. S. Takle Department of Agronomy, Iowa State University, Ames, IA, USA S.-Y. Wang (&) Utah Climate Center, Utah State University, 4825 Old Main Hill, Logan, UT 84322, USA e-mail: simon.wang@usu.edu 123 Clim Dyn DOI 10.1007/s00382-010-0765-7