William D. Hall

Explicit Forecasts of Winter Precipitation Using an Improved Bulk Microphysics Scheme. Part II: Implementation of a New Snow Parameterization

A new bulk microphysical parameterization (BMP) has been developed for use with the Weather Research and Forecasting (WRF) Model or other mesoscale models. As compared with earlier single-moment BMPs, the new scheme incorporates a large number of improvements to both physical processes and computer coding, and it employs many techniques found in far more sophisticated spectral/bin schemes using lookup tables. Unlike any other BMP, the assumed snow size distribution depends on both ice water content and temperature and is represented as a sum of exponential and gamma distributions. Furthermore, snow assumes a nonspherical shape with a bulk density that varies inversely with diameter as found in observations and in contrast to nearly all other BMPs that assume spherical snow with constant density. The new scheme’s snow category was readily modified to match previous research in sensitivity experiments designed to test the sphericity and distribution shape characteristics. From analysis of four idealized sensitivity experiments, it was determined that the sphericity and constant density assumptions play a major role in producing supercooled liquid water whereas the assumed distribution shape plays a lesser, but nonnegligible, role. Further testing using numerous case studies and comparing model results with in situ and other observations confirmed the results of the idealized experiments and are briefly mentioned herein, but more detailed, microphysical comparisons with observations are found in a companion paper in this series (Part III, forthcoming).

A Detailed Microphysical Model Within a Two-Dimensional Dynamic Framework: Model Description and Preliminary Results

Abstract A two-dimensional anelastic cloud model which incorporates detailed treatments of the water and ice phase is presented. The liquid phase processes considered include condensation, quasi-stochastic coalescence, fallout and breakup, while the ice phase processes include diffusional and accretional growth of ice particles. Results of two cloud simulations are presented. The first case assumes an atmosphere with maritime cloud condensation nuclei (CCN) activation characteristics and considers the warm rain processes only. It was found that with the appearance of precipitation, the model-predicted supersaturations within updraft regions often reach values larger than 5% with respect to water. The second case assumes an atmosphere with continental CCN characteristics and includes the ice phase processes leading to the formation of graupel. The results of the second case illustrate the importance of cloud vertical motions in transporting ice particles from preferential nucleation regions in the upper po...

Observations and Parameterizations of Particle Size Distributions in Deep Tropical Cirrus and Stratiform Precipitating Clouds: Results from In Situ Observations in TRMM Field Campaigns

Abstract This study reports on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled. Most of the spirals began at cloud top, with temperatures (T) as low as −50°C, and ended at cloud base or below the melting layer (ML)....

Cloud-Resolving Modeling of Cloud Systems during Phase III of GATE. Part II: Effects of Resolution and the Third Spatial Dimension

Abstract Two- and three-dimensional simulations of cloud systems for the period of 1–7 September 1974 in phase III of the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) are performed using the approach discussed in Part I of this paper. The aim is to reproduce cloud systems over the GATE B-scale sounding array. Comparison is presented between three experiments driven by the same large-scale conditions: (i) a fully three-dimensional experiment, (ii) a two-dimensional experiment that is an east–west section of the three-dimensional case, and (iii) a high-resolution version of the two-dimensional experiment. Differences between two- and three-dimensional frameworks and those related to spatial resolution are analyzed. The three-dimensional experiment produced a qualitatively realistic organization of convection: nonsquall clusters, a squall line, and scattered convection and transitions between regimes were simulated. The two-dimensional experiments produced convective organ...

Long-Term Behavior of Cloud Systems in TOGA COARE and Their Interactions with Radiative and Surface Processes. Part II: Effects of Ice Microphysics on Cloud–Radiation Interaction

Abstract A two-dimensional cloud-resolving model with a large domain is integrated for 39 days during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) to study the effects of ice phase processes on cloud properties and cloud radiative properties. The ice microphysical parameterization scheme is modified based on microphysical measurements from the Central Equatorial Pacific Experiment. A nonlocal boundary layer diffusion scheme is included to improve the simulation of the surface heat fluxes. The modified ice scheme produces fewer ice clouds during the 39-day simulation. The cloud radiative properties show significant improvement and compare well with various observations. Both the 39-day mean value (202 W m−2) and month-long evolution of outgoing longwave radiative flux from the model are comparable with satellite observations. The 39-day mean surface shortwave cloud forcing is −110 W m−2, consistent with other estimates obtained for TOGA COARE. The 39-day me...

Interactions between Topographic Airflow and Cloud/Precipitation Development during the Passage of a Winter Storm in Arizona

Abstract A case study showing comparisons between observations and numerical simulations of the passage of a winter storm over complex terrain is presented. The interactions between the mesoscale and cloud environments and the microphysical and dynamical processes are addressed using both observations and numerical simulations. A three-dimensional, time-dependent nested grid model was used to conduct numerical simulations of the three-dimensional airflow and cloud evolution over the Mogollon Rim and adjacent terrain in Arizona. The modeling results indicated that the flow patterns and cloud liquid water (CLW) were closely linked to the topography. To a large extent, gravity waves excited by the flow over the mountains determine the distribution of clouds and precipitation. The waves extend through deep layers of the atmosphere with substantial updrafts and downdrafts, at times exceeding 5 m s−1. The simulated vertical velocities and horizontal wavelengths of about 20 km were in good agreement with the air...

Multi-domain simulations of the time dependent Navier-Stokes equations: benchmark error analysis of some nesting procedures

This paper presents a benchmark error analysis of various approaches for treating multiple domain calculations within an anelastic finite difference model. One-way and two-way interactive nesting errors with and without temporal refinement are evaluated. The two-way interactive nesting approach is one where solutions between fine and coarse grid domains are matched through the simple post insertion of data. On the other hand, the equations can be matched by using the pressure defect correction approach. It is shown that, for the present model, the two-way interactive nesting method gives identical results to multi-domain solutions using the pressure defect correction approach. The present results indicate that in this type of analastic framework, a priori matching of the equations is equivalent to the a posteriori matching of the solutions. This result is attributed to the inflexible nature of the Neumann boundary conditions on the fine mesh pressure which need to be specified from the coarse mesh. Since a large number of meterological models employ the hydrostatic assumption, it is of interest to know of nesting errors attributable to this approximation. The results presented indicate essentially equivalent error levels for both the hydrostatic and nonhydrostatic systems of equations for the present case of airflow over an isolated mountain. It is shown how nesting technology can be used in a virtual sense to reduce the central memory requirements for large array sized numerical simulations. Nesting can be used in this sense to decompose the maximum memory working space required without affecting the results.

Two- and Three-Dimensional Simulations of the 9 January 1989 Severe Boulder Windstorm: Comparison with Observations

Abstract Simulations of the 9 January 1989 Colorado Front Range windstorm using both realistic three-dimensional (3D) orography and a representative two-dimensional (2D) east–west cross-sectional orography are presented. Both Coriolis forcing and surface friction (drag law formulation) were included for all experiments. The model results were compared with analyses of Doppler lidar scan data available from the surface to 4 km MSL provided by the Environmental Technology Laboratory of the National Oceanic and Atmospheric Administration (NOAA). The fully three-dimensional simulations with realistic orography used time-dependent inflow boundary conditions. These experiments were designed, in part, to assess the ability of mesoscale models to predict the onset and general characteristics of downslope windstorms. The present experiments highlight the sensitivity of wind storm onset and positioning of surface gusts to both model resolution and surface physics, which is in agreement with previous findings. These...

Terrain-Induced Turbulence over Lantau Island: 7 June 1994 Tropical Storm Russ Case Study

Abstract Numerical simulations of terrain-induced turbulence associated with airflow over Lantau Island of Hong Kong are presented. Lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level. This research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of Chek Lap Kok, which is located to the lee of Lantau. Intensive ground and aerial observations were taken from May through June 1994, during the Lantau Experiment (LANTEX). This paper focuses on flow associated with the passage of Tropical Storm Russ on 7 June 1994, during which severe turbulence was observed. The nature of the environmental and topographic forcing on 7 June 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction. High-resolution numerical simulations, initialized using local sounding data, were performed using the Clark model. ...

The design of smooth, conservative vertical grids for interactive grid nesting with stretching

Abstract This note describes how to generate vertically stretched grids within the context of vertical nesting that are consistent with the conservative interpolation formula used by Clark and Farley. It is shown that all nested grids derive their structure directly from the parent grid, where the only flexibility allowed for nested grids is their grid ratio relative to the parent grid. Formulas are presented that can he used to analyze resulting nested grid structures, and an example showing how these formulas were used to generate relatively smooth inner meshes is described. Suggestions for further improvements in grid design are also provided.