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

Thursday, 2 August 2001: 2:00 PM
Regimes for a Conditionally Unstable Flow over an Idealised Three-Dimensional Mesoscale Mountain (formerly paper P5.9)
Shu-hua Chen, NCAR, Boulder, CO; and Y. L. Lin
Poster PDF (335.8 kB)
Moist flow responses and regimes associated with the propagation of orographically induced mesoscale convective systems in a conditionally unstable airstream over a three-dimensional, elongated mesoscale mountain, such as the Alps, are studied using the Weather Research Forecast (WRF) model. The low-level dry flow response is similar to two-dimensional hydrostatic mountain waves, except that flow develops around both ends of the mountain; the upper-level flow is characterized by a weak U-shaped disturbance, similar to that predicted by Smith's linear theory (1980 Tellus). In moist flow simulations, both the Kain-Fritsch cumulus parameterization and Lin-Farley-Orville microphysics parameterization schemes are adopted. The flow responses for moist simulations are completely different from those for dry simulations. Due to the presence of convective systems, an arc-shaped disturbance, that is much stronger and covers a much larger area compared to that for a dry flow, is generated upstream of the mountain. The divergence upstream of the mountain is much stronger and confined in a narrower region compared to the dry flow. In addition, there exists a weaker region of flow convergence further upstream of the primary divergence region. Unlike the repeating upward hydrostatic propagating mountain waves in dry flow, the vertical motion in the moist flow is dominated by strong upward motion associated with deep convective systems, that extends to tropopause, and downward motion associated with the downslope wind. Based on the propagation of convective systems, three regimes are identified: (I) an upstream propagating system and a stationary downslope convective system, (II) an upslope quasi-stationary convective system, and (III) a quasi-stationary upslope convective system, a stationary downslope convective system, and a downstream propagating convective system. These three regimes are controlled by a non-dimensional moist Froude number, which is defined as , where U is the incoming flow speed, the moist Brunt-Vaisala frequency (~6x10^3 s^-1 for the lower layer of the idealized sounding adopted in this study), and h the mountain height (2 km in the present study). The is about 1.25 for regime II, while it is less (greater) than 1.25 for Regime I (III). Even though this result is similar to what was found in two-dimensional moist flow regimes (Chu and Lin JAS 2000), there exist several major differences. First, the strength of the upslope convective system is proportional to , which allows the low-level jet often observed in real cases to produce strong convection and heavy rainfall over the upslope. Second, the downslope convective system over the lee slope is produced by the convergence associated with downslope hydraulic jump and the merging of the upstream split flow. This merging of upstream split flow in the lee of the mountain also helps generate hydraulic jump in large Froude number flow. Third, the upstream convective system in the critical flow regime (regime II) is located over the upslope, instead of located near the mountain peak as found in the two-dimensional case (Chu and Lin 2000 JAS). This is more consistent with observed convective systems associated with orographic heavy rainfall. This classification of flow regimes is applied to help explain the differences of convective system propagations between IOP 2 and 8 of the Mesoscale Alpine Program.

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