Wen-Yih Sun

An intercomparison of model predicted wave breaking for the 11 January 1972 Boulder windstorm

Abstract Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave bre...

Numerical Simulation of the 1993 Midwestern Flood: Land–Atmosphere Interactions

Abstract During the summer of 1993, persistent and heavy precipitation caused a long-lived, catastrophic flood in the midwestern United States. In this paper, Midwest hydrology, atmospheric circulation of the 1993 summer, and feedback between the surface and precipitating systems were investigated using the Purdue Regional Model (PRM). The 30-day PRM control simulations reproduced the large-scale atmospheric features that characterized the summer of 1993. Specifically, the upper-level jet stream and trough over the northwestern United States are present in control cases, as well as the Great Plains low-level jet, general pattern of moisture transport, and heavy precipitation in the Midwest. The daily precipitation record (area averaged over the heaviest rainfall) indicates that the model also reproduces the evolution and periodicity of precipitation events comparable with the observations and correctly depicts the differences between June and July. The sensitivity of the low-level jet, planetary boundary ...

Modeling the Evolution of the Convective Planetary Boundary Layer

Abstract The level 3 turbulence closure model proposed by Mellor and Yamada (1974) is modified 1) to incorporate the formulations for the turbulence third-order moments and pressure terms proposed by Zeman and Lumley (1976) and 2) to introduce turbulence length scales which depend upon the stratification of the atmosphere. The vertical heat and moisture fluxes and the temperature-humidity covariance are determined from differential equations. The model includes two other differential equations, one for the turbulence kinetic energy and the other for virtual potential temperature variance. All other turbulence variables are determined from algebraic equations. The model is used to simulate the daytime evolution of the planetary boundary layer observed on day 33 of the Wangara boundary-layer experiment. The calculated vertical profiles of the mean wind, temperature and humidity are found to be in good agreement with the observations. The calculated vertical distributions of turbulence variables, including k...

Numerical Simulation of Mesoscale Circulation in Taiwan and Surrounding Area

Abstract Mesoscale circulation around Taiwan and the surrounding area has been investigated using the Purdue mesoscale model. The numerical results generated in an inviscid atmosphere show: (a) A cyclonic vortex forms in the southeast and a slightly weaker anticyclonic vortex forms in the northeast of Taiwan uner a westerly or southwesterly wind. Subsidence warming also generates a relative low pressure on the southeastern coast. (b) A low pressure associated with a cyclonic flow forms in the northwest and a slightly weaker anticyclonic flow forms in the southwest of Taiwan under an easterly mean flow. The easterly wind tends to turn northeasterly over the Taiwan Strait, with a stronger wind speed, due to the blocking effects of the mountains in Taiwan and along the Chinese coast. (c) Under the existence of an easterly surface wind with a reverse shear, the horizontal temperature advection is not important in the formation of low pressure on the leeside, due to the small length scale of the island of Taiw...

Numerical Experiments of Vortices in the Wakes of Large Idealized Mountains

Abstract The Purdue Mesoscale Model (PMM) is applied to study the flow past large idealized mountains under a low Froude number. Results show that for Reynolds numbers in the range of 4 100. Results also show that small perturbations in the oncoming wind, the inclination of the oncoming wind and major axis of the mountain, the mountain shape, and the Coriolis force all can contribute to atmospheric vortex shedding. The Reynolds number is not a good indicator of whether a vortex will stay or break away from the mountain in the atmosphere. When the earths rotation is included, the simulated pressure field and wind increase considerably on the left-hand side (facing downstream) of the mountain, whic...

Planetary boundary layer and surface layer sensitivity to land surface parameters

The sensitivity of the development of the convective planetary boundary layer (PBL) and the surface layer are examined using a coupled surface parameterization and detailed PBL model. First, the coupling is verified against observations from the First ISLSCP (‘International Satellite Land Surface Climatology Project’) Field Experiment (FIFE). Results of the sensitivity experiments indicate that the PBL is most sensitive to the amount of soil water content, and the proximity of the soil water content to critical soil texture values (field capacity and wilting point). While vegetation cover is not the most sensitive parameter at the surface, its influence on the surface energy and hydrologic balance is crucial. Model sensitivity to minimum stomatal resistance, type of soil parameterization and canopy height (surface roughness and displacement depth) is also discussed.

Diurnal variation of lee vortices in Taiwan and the surrounding area

Abstract Lee vortices have been frequently observed in the wake of mesoscale mountains under a low Froude number flow regime. During the Taiwan Area Mesoscale Experiment (TAMEX), a cyclonic vortex was observed to the Ice of Taiwan by a P-3 aircraft. In this paper a numerical simulation is carried out to study this event. It is shown that the numerical results are capable of recapturing the detailed features as observed by airplane and surface analysis. The simulated surface pressure, wind field, and Ice vortex are in good agreement with observations. The diurnal oscillation of cloudiness and precipitation in Taiwan is also consistent with the observations under undisturbed conditions during the TAMEX period. Under a prevailing southwesterly-to-westerly summer monsson flow, numerical results demonstrate that the observed cyclonic vortex initially develops to the southeast of Taiwan after sunset, then drifts northeastward. The diurnal forcing not only generates land/sea breezes but also controls the vortex ...

Formulation and verification of a land surface parameterization for atmospheric models

The need for a well-defined lower boundary condition for atmospheric numerical models is well documented. This paper describes the formulation of a land surface parameterization, which will be used in atmospheric boundary-layer and mesoscale numerical models. The land surface model has three soil layers for the prediction of soil moisture and soil temperature. Model soil properties depend on soil texture and moisture content. A homogeneous distribution of vegetation is also included, so that transpiration may be included, as well as the interception of precipitation by vegetation elements. The simulated vegetation also affects the mean surface albedo and roughness characteristics.First ISLSCP Field Experiment (FIFE) data are used to verify the model. Three cases during the growing season were chosen, each case having different amounts of vegetation cover. “Stand alone” simulations, where observations of atmospheric and radiation variables are input to the land surface model, were performed. These simulations show that the model is able to reproduce observed surface energy budgets and surface temperatures reasonably well. The RMS differences between modeled and obsered turbulent fluxes of heat and moisture are quite comparable to those reported by more detailed land surface models.

Numerical analysis for hydrostatic and nonhydrostatic equations of inertial-internal gravity waves

Abstract A modified forward-backward scheme applied to the anelastic system is proposed. This modified scheme not only retains all the advantages of the conventional forward-backward scheme but also is more consistent with the original differential equations. This scheme is used to investigate inertial waves and internal gravity waves in three different lattices. It is found that the lattice C proposed by Deardorff is better than either lattice A or lattice B when applied to internal gravity waves and thermal convection in the atmosphere. The difference between a hydrostatic system and a nonhydrostatic system is also discussed in detail in this paper. Here we propose to apply Shumans smoothing on a hydrostatic system to filter out the undesirably, highly oscillatory short waves, or stroll small-scale convection, so that we may produce reasonable results compared with those of a nonhydrostatic system. The validity of this method has been proved by the numerical results of a study on the mesoscale cloud ba...

A Forward-Backward Time Integration Scheme to Treat Internal Gravity Waves

Abstract A forward time difference in the momentum equations and a backward time difference in the thermodynamic equation is conditionally stable according to von Neumann’s stability analysis of linearized anelastic equations. In an anelastic-hydrostatic system, this scheme is also conditionally stable in a staggered coordinate, but it is unstable for a two-grid-interval wave in a non-staggered coordinate. This unstable two-grid-interval wave can he avoided by applying the trapezoidal rule to calculate the pressure field from the hydrostatic equation.