Robert L. Walko

A comprehensive meteorological modeling system—RAMS

SummaryThis paper presents a range of applications of the Regional Atmospheric Modeling System (RAMS), a comprehensive mesoscale meterological modeling system. Applications discussed in this paper include large eddy simulations (LES) and simulations of thunderstorms, cumulus fields, mesoscale convective systems, mid-latitude cirrus clouds, winter storms, mechanically- and thermally-forced mesoscale systems, and mesoscale atmospheric disperision. A summary of current RAMS options is also presented. Improvements to RAMS currently underway include refinements to the cloud radiation, cloud microphysics, cumulus, and surface soil/vegetative parameterization schemes, the parallelization of the code, development of a more versatile visualization capability, and research into meso-α-scale cumulus parameterization.

New RAMS cloud microphysics parameterization part I: the single-moment scheme

A new cloud microphysical parameterization is described. Features of this new scheme include: the use of generalized gamma distributions as the basis function for all hydrometeor species; the use of a heat budget equation for hydrometeor classes, allowing heat storage and mixed phase hydrometears; partitioning hydrometeors into seven classes (including separate graupel and hail categories) ; the use of stochastic collection rather than continuous accretion approximations and extension of the ice nucleation scheme to include homogeneous nucleation of ice from haze particles and cloud droplets. The versatility and credibility of the new scheme is explored, using sensitivity experiments for a simple two-dimensional convective cloud simulation.

Coupled Atmosphere–Biophysics–Hydrology Models for Environmental Modeling

The formulation and implementation of LEAF-2, the Land Ecosystem‐Atmosphere Feedback model, which comprises the representation of land‐surface processes in the Regional Atmospheric Modeling System (RAMS), is described. LEAF-2 is a prognostic model for the temperature and water content of soil, snow cover, vegetation, and canopy air, and includes turbulent and radiative exchanges between these components and with the atmosphere. Subdivision of a RAMS surface grid cell into multiple areas of distinct land-use types is allowed, with each subgrid area, or patch, containing its own LEAF-2 model, and each patch interacts with the overlying atmospheric column with a weight proportional to its fractional area in the grid cell. A description is also given of TOPMODEL, a land hydrology model that represents surface and subsurface downslope lateral transport of groundwater. Details of the incorporation of a modified form of TOPMODEL into LEAF-2 are presented. Sensitivity tests of the coupled system are presented that demonstrate the potential importance of the patch representation and of lateral water transport in idealized model simulations. Independent studies that have applied LEAF-2 and verified its performance against observational data are cited. Linkage of RAMS and TOPMODEL through LEAF-2 creates a modeling system that can be used to explore the coupled atmosphere‐biophysical‐ hydrologic response to altered climate forcing at local watershed and regional basin scales.

New RAMS cloud microphysics parameterization. Part II: The two-moment scheme

Abstract This paper is the second in a series of articles describing the new microphysics scheme in the Regional Atmospheric Modeling System (RAMS). In this part, a new two-moment microphysical parameterization is described. The proposed scheme predicts the mixing ratio and number concentration of rain, pristine ice crystals, snow, aggregates, graupel and hail. The general gamma distribution is the basis function used for hydrometeor size in each category. Additional features include: use of stochastic collection for number concentration tendency; breakup of rain droplets formulated into the collection efficiency; diagnosis of ice crystal habit dependent on temperature and saturation; evaporation and melting of each species assuming that the smallest particles completely disappear first; and more complex shedding formulations which take into account the amount of water mass on the coalesced hydrometeor. Preliminary sensitivity testing of the new microphysical scheme in an idealized convective simulation shows that the two-moment prediction scheme allows more flexibility of the size distribution enabling the mean diameter to evolve in contrast to the one-moment scheme. Sensitivity to the prescribed input parameters such as cloud droplet concentrations and the shape parameter v is demonstrated in the model results.

Elements of the Microphysical Structure of Numerically Simulated Nonprecipitating Stratocumulus

Abstract A set of 500 simulated trajectories and a simple parcel model are used to (i) evaluate the performance of a large eddy simulation model coupled to a detailed representation of the droplet spectrum (the LES-BM model) and (ii) gain insight into the microphysical structure of numerically simulated nonprecipitating stratocumulus. The LES-BM model reasonably reproduces many observed features of stratocumulus. The largest sources of error appear to be associated with limited vertical resolution, the neglect of gas kinetic effects and the inability of the model to properly represent mixing across cloud interfacial boundaries. The first two problems have simple remedies; for instance, a condensation–nucleation scheme is derived that includes gas–kinetic effects thus obviating the second problem. The third source of error poses a more vexing, and as yet unsolved, problem for models of the class described herein. Trajectories timescales are analyzed and in-cloud residence times are found to be, in the mean...

The influence of anthropogenic landscape changes on weather in south Florida

Using identical observed meteorology for lateral boundary conditions, the Regional Atmospheric Modeling System was integrated for July‐August 1973 for south Florida. Three experiments were performed—one using the observed 1973 landscape, another the 1993 landscape, and the third the 1900 landscape, when the region was close to its natural state. Over the 2-month period, there was a 9% decrease in rainfall averaged over south Florida with the 1973 landscape and an 11% decrease with the 1993 landscape, as compared with the model results when the 1900 landscape is used. The limited available observations of trends in summer rainfall over this region are consistent with these trends.

Cloud venting - A review and some new global annual estimates

Abstract In this paper we review observational and modeling studies of cloud venting by a wide variety of cloud types ranging from ordinary cumuli, to ordinary cumulonimbi, mesoscale convective systems and tropical and extratropical cyclones. We have used explicit cloud-resolving simulations with RAMS to illustrate the nature of the process of venting of boundary layer air by several cloud system types and to provide quantitative estimates of the transport rates for different storms. In order to help global modelers prioritize their efforts in developing and refining cloud transport parametrization schemes, we have also attempted to make global estimates of the contributions of the various storm types to venting of boundary layer air. We find that on a global-annual basis, the extratropical cyclone has the highest boundary layer mass flux of all cloud venting systems, followed by the general class of MCSs (excluding MCCs), ordinary thunderstorms, tropical cylcones, and MCCs. We estimate an annual flux of 4.95 × 10 19 kg of boundary layer air by these cloud systems, which represents a venting of the entire boundary layer about 90 times a year.

Simulations of marine stratocumulus using a new microphysical parameterization scheme

A new microphysical scheme that uses lognormal basis functions to represent cloud and drizzle . drop spectra is presented. The scheme is incorporated in a two-dimensional 2-D eddy-resolving model and applied to the simulation of a stratocumulus cloud deck based on a sounding from the Atlantic Stratocumulus Transition Experiment. Firstly, the philosophy behind the design of the scheme is discussed with emphasis on special problems that arise when simulating stratocumulus . clouds. These include but are not limited to simulation of collection at low liquid water contents y3 . a few tenths of g m , and the importance of correctly simulating drop sedimentation in weakly convective clouds. The scheme is then applied to two cases; one exhibiting weak precipitation y1 . y1 - 1 mm d at the surface and the other, a more strongly precipitating case 1-3 mm d at the . surface . Results show that the scheme captures the main features of a precipitating stratocumulus system when compared with the same model using a detailed bin microphysical scheme. The new scheme uses less than a fourth of the time required by the bin microphysical model and thus enables one to perform 2-D and 3-D simulations of the cloudy boundary layer over much larger domains, or much longer timescales than was previously possible. q 1998 Elsevier Science B.V. All rights reserved.

A Modeling Study of the Dryline

Abstract Results of a modeling study of the 24 May 1989 dryline are presented. A nonhydrostatic, two-dimensional version of the Colorado State University Regional Atmospheric Modeling System (CSU-RAMS) is used to deduce the impact of east-west variability of soil moisture and vegetation on convective boundary layer evolution and dryline formation. The effects of the initial moisture and wind fields and the impact of the Coriolis force on the model results are also examined. Model output is compared with special airborne and sounding observations of the 24 May dryline. Several findings of an earlier observational study of the 24 May dryline are supported in the present study. The modeled drylines are broadly comparable to the observed dryline with respect to the following properties: 1) width, 10 km; 2) strong horizontal moisture and virtual potential temperature gradients, >4 g kg−1/10 km and 2 K/10 km; 3) strong horizontal convergence, updraft, W–E shear of N–S wind component, 8 × 10−4 s−1, 1 m s−1, 10 m...

Polynomial fits to saturation vapor pressure

Abstract The authors describe eighth- and sixth-order polynomial fits to Wexlers and Hyland-Wexlers saturation-vapor-pressure expressions. Fits are provided in both least-squares and relative-error norms. Error analysis is presented. The authors show that their method is faster in comparison with the reference expressions when implemented on a CRAY-YMP.