18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001: 4:29 PM
Numerical simulation of a dryline-outflow boundary intersection
Christopher C. Weiss, University of Oklahoma, Norman, OK; and H. B. Bluestein
The dryline has long been identified as a relatively favorable location for the initiation of deep moist convection. This susceptibility to deep convection is primarily a result of enhanced surface convergence in the boundary layer along the length of the dryline. However, the exact processes responsible for convective initiation on the dryline are still not well understood; particularly why some environments fail to develop convection when conventional forecasting parameters (e.g., CIN) indicate a high probability of occurrence. Recent research has indicated the importance of the upward branch of the dryline secondary circulation in ascending boundary layer parcels to their level of free convection (LFC). The intensity of, tilt of and parcel source region for the upward branch of the dryline secondary circulation appear to have some impact on the propensity for convective development. Understanding the nature of this circulation is therefore critical.

Recent numerical work has resolved the sensitivity of dryline structure to variations in surface characteristics (e.g., terrain slope, land-use gradients). In this investigation we will attempt to expand on this work by creating a triple-point regime in which we simulate the influence of a cold pool on the transverse dryline secondary circulation. Recent observational work using airborne pseudo-dual Doppler analyses has revealed the presence of a “residual dryline secondary circulation” (RDSC) to the north of a pre-existing east-west outflow boundary as the individual thermodynamic and kinematic environments to either side of the dryline are advected up and over the cold pool. We seek to reproduce this RDSC in the Advanced Regional Prediction System (ARPS) and analyze any sensitivity of the feature to initial conditions and surface characteristics.

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