Gary T. Bates

Development of a Second-Generation Regional Climate Model (RegCM2). Part I: Boundary-Layer and Radiative Transfer Processes

Abstract During the last few years the development of a second-generation regional climate modeling system (RegCM2) has been completed at the National Center for Atmospheric Research (NCAR). Based upon the National Center for Atmospheric Research-Pennsylvania State University Mesoscale Model (MM4), RegCM2 includes improved formulations of boundary layer, radiative transfer, surface physics, cumulus convection, and time integration technique, which make it more physically comprehensive and more computationally efficient than the previous regional climate model version. This paper discusses a number of month-long simulations over the European region that were conducted to test the new RegCM2 boundary-layer parameterization (the scheme developed by Holtsag et al.) and radiative transfer formulation [the package developed for the NCAR Community Climate Model 2 (CCM 2)]. Both schemes significantly affect the model precipitation, temperature, moisture, and cloudiness climatology, leading to overall more realist...

Development of a Second-Generation Regional Climate Model (RegCM2). Part II: Convective Processes and Assimilation of Lateral Boundary Conditions

Abstract In this paper we continue the description of a second-generation regional climate model (RegCM2) initiated in the companion paper by Giorgi et al. We first discuss the inclusion in the model of the cumulus cloud scheme developed by Grell (referred to as OCC). The sensitivity of summertime and wintertime results to different closures and parameter settings in the GCC scheme are examined in model simulations conducted over Europe. While wintertime precipitation is found to vary only slightly between the experiments, a wide range of results is found in the summer runs. The GCC scheme produces more rain than a Kuo-type scheme, responds strongly to variations in the surface energy and moisture fluxes, and performs best when used in conjunction with the most advanced physics processes of RegCM2. Modifications to the standard model relaxation boundary condition procedure, which allow smoother assimilation of driving lateral boundary data, are also discussed. Further testing of RegCM2, which is more phys...

A regional climate model for the western United States

A numerical approach to modeling climate on a regional scale is developed whereby large-scale weather systems are simulated with a global climate model (GCM) and the GCM output is used to provide the boundary conditions needed for high-resolution mesoscale model simulations over the region of interest. In our example, we use the National Center for Atmospheric Research (NCAR) community climate model (CCM1) and the Pennsylvania State University (PSU)/NCAR Mesoscale Model version 4 (MM4) to apply this approach over the western United States (U.S.). The topography, as resolved by the 500-km mesh of the CCM1, is necessarily highly distorted, but with the 60-km mesh of the MM4 the major mountain ranges are distinguished. To obtain adequate and consistent representations of surface climate, we use the same radiation and land surface treatments in both models, the latter being the recently developed Biosphere-Atmosphere Transfer Scheme (BATS). Our analysis emphasizes the simulation at four CCM1 points surrounding Yucca Mountain, NV, because of the need to determine its climatology prior to certification as a high-level nuclear waste repository.We simulate global climate for three years with CCM1/BATS and describe the resulting January surface climatology over the western U.S. The details of the precipitation patterns are unrealistic because of the smooth topography. Selecting five January CCM1 storms that occur over the western U.S. with a total duration of 20 days for simulation with the MM4, we demonstrate that the mesoscale model provides much improved wintertime precipitation patterns. The storms in MM4 are individually much more realistic than those in CCM1. A simple averaging procedure that infers a mean January rainfall climatology calculated from the 20 days of MM4 simulation is much closer to the observed than is the CCM1 climatology. The soil moisture and subsurface drainage simulated over 3–5 day integration periods of MM4, however, remain strongly dependent on the initial CCM1 soil moisture and thus are less realistic than the rainfall. Adequate simulation of surface soil water may require integrations of the mesoscale model over time periods.

The climatological skill of a regional model over complex terrain

Abstract As part of an ongoing study of the regional climate and hydrology of the southwestern United States, in this paper we investigate the systematic biases of two versions of the PSU/NCAR mesoscale model (MM4). These are a standard version and one that includes a more detailed treatment of radiative transfer, surface physics, and soil hydrology. We simulated the period 1–30 January 1979, in which nine Pacific storms moved across the western United States. Results from both model versions are compared to the large scale analysis used to provide initial and lateral boundary conditions. Both models show a lower tropospheric cold bias of 1–3 K near the surface over land and an upper tropospheric warm bias of less than 1 K, which suggest high model stability and reduced vertical mixing. The model atmospheres are wetter than that of the analysis, particularly in the lower troposphere and over the ocean. The wind magnitude bias is positive near the surface (∼1.5–3 m s−1), negative in the upper troposphere (...

Regional Climate Change Scenarios over the United States Produced with a Nested Regional Climate Model

Abstract In this paper two continuous 3½-year-long climate simulations over the continental United States are discussed, one for present-day conditions and one for conditions under double carbon dioxide concentration, conducted with a limited area model (LAM) nested in a general circulation model (GCM). The models used are a version of the NCAR Community Climate Model (CCM) at rhomboidal 15 spectral resolution and the climate version of the NCAR/Penn State mesoscale model (MM4) at 60-km gridpoint spacing. For present-day conditions the model temperatures are within 1°–2°C of observations except over the Great Lakes region, where temperature is overpredicted by 4°–6°C. The CCM overpredicts precipitation throughout the continental United States (overall by about 60%) and especially over the West (by up to 300%). The nested MM4 overpredicts precipitation over the West but underpredicts it over the eastern United States. In addition, it produces a large amount of topographically and lake-induced sub-GCM grid-...

NOAA's Second-Generation Global Medium-Range Ensemble Reforecast Dataset

A multidecadal ensemble reforecast database is now available that is approximately consistent with the operational 0000 UTC cycle of the 2012 NOAA Global Ensemble Forecast System (GEFS). The reforecast dataset consists of an 11-member ensemble run once each day from 0000 UTC initial conditions. Reforecasts are run to +16 days. As with the operational 2012 GEFS, the reforecast is run at T254L42 resolution (approximately 1/2° grid spacing, 42 levels) for week +1 forecasts and T190L42 (approximately 3/4° grid spacing) for the week +2 forecasts. Reforecasts were initialized with Climate Forecast System Reanalysis initial conditions, and perturbations were generated using the ensemble transform with rescaling technique. Reforecast data are available from 1985 to present. Reforecast datasets were previously demonstrated to be very valuable for detecting and correcting systematic errors in forecasts, especially forecasts of relatively rare events and longer-lead forecasts. What is novel about this reforecast dat...

Lake-Atmosphere Feedbacks Associated with Paleolakes Bonneville and Lahontan

A high-resolution, regional climate model nested within a general circulation model was used to study the interactions between the atmosphere and the large Pleistocene lakes in the Great Basin of the United States. Simulations for January and July 18,000 years ago indicate that moisture provided by synoptic-scale atmospheric circulation features was the primary component of the hydrologic budgets of Lakes Lahontan and Bonneville. In addition, lake-generated precipitation was a substantial component of the hydrologic budget of Lake Bonneville at that time. This local lake-atmosphere interaction may help explain differences in the relative sizes of these lakes 18,000 years ago.

The multiyear surface climatology of a regional atmospheric model over the western United States

Abstract This paper presents a validation analysis of the climatology of a version of the National Center for Atmospheric Research-Pennsylvania State University limited-area model (MM4) developed for application to regional climate simulation over the western United States. Two continuous multiyear simulations, for the periods 1 January 1982–31 December 1983 and 1 January 1988–25 April 1989, were performed over this region with the MM4 driven by ECMWF analyses of observations and run at a horizontal resolution of 60 km. The model used in these simulations includes horizontal diffusion on terrain-following σ coordinates, a Kuo-type cumulus parameterization, sophisticated radiative transfer and surface physics-soil hydrology packages, and a relaxation boundary- conditions procedure. Model-produced surface air temperatures, precipitation, and snow depths were compared with observations from about 390 stations distributed throughout the western United States. The base-model run reproduced the seasonal cycle o...

Toward the Simulation of the Effects of the Great Lakes on Regional Climate

Abstract This paper describes a set of numerical experiments aimed at evaluating the feasibility of applying a version of the National Center for Atmospheric Research-Pennsylvania State University regional model (MM4) to regional climate simulation over the Great Lakes Basin. The objectives of this initial modeling investigation are 1) to examine whether the MM4 can capture the primary forcing exerted by the Great Lakes on the regional climate and 2) to evaluate what model resolution and configuration are needed to simulate such forcing. Simulations over the Great Lakes region are conducted with and without representation of the lakes at four model gridpoint resolutions ranging from 15 to 90 km. One experiment at 60-km resolution is discussed in which a one-dimensional thermal eddy diffusion model is interactively coupled to the MM4 to represent the lakes. Initial and lateral boundary conditions necessary to drive these simulations are provided by European Centre for Medium-Range Weather Forecasts (ECMWF)...

The Effect of North Pacific Sea Surface Temperature Anomalies on the January Climate of a General Circulation Model

Abstract Four perpetual January integrations of an atmospheric general circulation model have been performed, in each of which a different sea surface temperature (SST) anomaly was specified in the North Pacific. The observed SST anomaly for the 1976/77 winter was chosen as the basic anomaly, and 1200-day runs were carded out in which this anomaly was multiplied by ±1 and ±2. A fifth run was performed which combined the basic midlatitude SST anomaly from 1976/77 with a tropical Pacific SST anomaly representative of the mature phase of a warm El Nino/Southern Oscillation (ENSO) episode. An ensemble of eight, independent 90-day averaged realizations was extracted from each simulation. Maps of ensemble-mean differences from the model climatology are presented in this paper, together with estimates of the statistical significance of some of the features which appear on these maps. The model response to the basic SST anomaly and to twice the basic SST anomaly is a midiatitude teleconnection pattern, the Pacifi...