Stanley G. Benjamin

Parameterization of cold‐season processes in the MAPS land‐surface scheme

A coupled atmospheric/land-surface model covering the conterminous United States with an associated 1-hour atmospheric data assimilation cycle, the Mesoscale Analysis and Prediction System (MAPS), has been improved to include a snow accumulation/melting scheme and also parameterization of processes in frozen soil. The new aspects of the land-surface model are described in this paper, along with detailed one-dimensional (1-D) tests using an 18-year observation data set from Valday, Russia. These tests show that the MAPS 1-D soil/vegetation/snow model is capable of providing accurate simulations over multiyear periods at locations with significant snow cover and frozen soil. A statistical analysis of the tests shows the expected improvement in snow depth, skin temperature, and especially in runoff from inclusion of these additional surface processes during the spring melting season. This performance in 1-D tests is a necessary prerequisite for robust long-term behavior of soil temperature and moisture fields and other components of the hydrological cycle in the 3-D MAPS coupled assimilation cycle.

Mesoscale Weather Prediction with the RUC Hybrid Isentropic–Terrain-Following Coordinate Model

Abstract A mesoscale atmospheric forecast model configured in a hybrid isentropic–sigma vertical coordinate and used in the NOAA Rapid Update Cycle (RUC) for operational numerical guidance is presented. The RUC model is the only quasi-isentropic forecast model running operationally in the world and is distinguished from other hybrid isentropic models by its application at fairly high horizontal resolution (10–20 km) and a generalized vertical coordinate formulation that allows model levels to remain continuous and yet be purely isentropic well into the middle and even lower troposphere. The RUC model is fully described in its 2003 operational version, including numerics and physical parameterizations. The use of these parameterizations, including mixed-phase cloud microphysics and an ensemble-closure-based cumulus parameterization, is fully consistent with the RUC vertical coordinate without any loss of generality. A series of experiments confirm that the RUC hybrid θ–σ coordinate reduces cross-coordinate...

Performance of Different Soil Model Configurations in Simulating Ground Surface Temperature and Surface Fluxes

Abstract This study compares three modifications to the one-dimensional planetary boundary layer scheme that is implemented in the σ–θ hybrid-b version of the Mesoscale Analysis and Prediction System (MAPS) and the Rapid Update Cycle (RUC). All three modifications are based on the incorporation of a simple soil model into the basic version to more accurately calculate the moisture and heat fluxes across the ground surface. The presented schemes are of increasing sophistication: the first model combines the soil model with heat and moisture budget equations for the ground surface and uses an explicit numerical scheme to compute the surface fluxes; the second model uses a more energy-conservative implicit solution for the latent and sensible surface fluxes and heat and moisture soil fluxes; the third model further incorporates a simple parameterization of the evapotranspiration process. The comparison includes the effect of different schemes on diurnal changes of surface temperature and soil heat flux. The ...

Elevated Mixed Layers in the Regional Severe Storm Environment: Conceptual Model and Case Studies

Abstract A conceptual model of the evolution of the severe local storm environment is discussed in conjunction with three case studies drawn from data obtained during the 1979 SESAME field program. The conceptual model describes how a particular configuration of topography and air flow can produce a low-level restraining inversion or “lid” which focuses the location and even enhances the intensity of severe local storms. While capping inversions have often been attributed to subsidence, in this model it is demonstrated that the lid frequently originates from differential advection of a hot, dry mixed layer from an elevated plateau over a cooler, moister layer advected northward ahead of a trough in the westerlies. For either the elevated mixed layer or subsidence inversion, this type of vertical stratification suppresses release of convective instability while, nevertheless, allowing the latent instability of the boundary layer to increase with time. However, for elevated mixed layer lids, the differing s...

Global Ensemble Predictions of 2009’s Tropical Cyclones Initialized with an Ensemble Kalman Filter

Abstract Verification was performed on ensemble forecasts of 2009 Northern Hemisphere summer tropical cyclones (TCs) from two experimental global numerical weather prediction ensemble prediction systems (EPSs). The first model was a high-resolution version (T382L64) of the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). The second model was a 30-km version of the experimental NOAA/Earth System Research Laboratory’s Flow-following finite-volume Icosahedral Model (FIM). Both models were initialized with the first 20 members of a 60-member ensemble Kalman filter (EnKF) using the T382L64 GFS. The GFS–EnKF assimilated the full observational data stream that was normally assimilated into the NCEP operational Global Statistical Interpolation (GSI) data assimilation, plus human-synthesized “observations” of tropical cyclone central pressure and position produced at the National Hurricane Center and the Joint Typhoon Warning Center. The forecasts from the two experimental ensembl...

A North American Hourly Assimilation and Model Forecast Cycle: The Rapid Refresh

AbstractThe Rapid Refresh (RAP), an hourly updated assimilation and model forecast system, replaced the Rapid Update Cycle (RUC) as an operational regional analysis and forecast system among the suite of models at the NOAA/National Centers for Environmental Prediction (NCEP) in 2012. The need for an effective hourly updated assimilation and modeling system for the United States for situational awareness and related decision-making has continued to increase for various applications including aviation (and transportation in general), severe weather, and energy. The RAP is distinct from the previous RUC in three primary aspects: a larger geographical domain (covering North America), use of the community-based Advanced Research version of the Weather Research and Forecasting (WRF) Model (ARW) replacing the RUC forecast model, and use of the Gridpoint Statistical Interpolation analysis system (GSI) instead of the RUC three-dimensional variational data assimilation (3DVar). As part of the RAP development, modif...

Regional Weather Prediction with a Model Combining Terrain-following and Isentropic Coordinates. Part I: Model Description

Abstract A short-range numerical prediction model, which is part of a real-time 3-h data assimilation and forecast system, is described. The distinguishing feature of the model is the use of terrain-following (σ) coordinate surfaces in the lower troposphere combined with isentropic (θ) surfaces aloft. Such a hybrid coordinate system allows modeling of processes in a convectively unstable boundary layer while retaining tile advantages of θ coordinates in representing upper-tropospheric frontal and jet-stream structures. The hybrid approach used in this model represents a in major departure from previous hybrid formulations in atmospheric models, oven though it has been used for more than ten years in oceanic modeling. Part I of this two-part paper contains a thorough description of the model and the results of validation experiments. Results of North American case studies wig be reported in Part II.

Some Effects of Surface Heating and Topography on the Regional Severe Storm Environment. Part I: Three-Dimensional Simulations

Abstract Model experiments have isolated several effects of surface heating and topography which may act in concert to focus the potential for severe thunderstorms in certain areas downstream of dry elevated terrain. These effects are examined primarily in results from 12 h and 36 h three-dimensional model simulations of the 9–11 April 1979 (SESAME I) case, but are also found in simulations of the 9–10 May 1979 (SESAME IV) case. The experiments are performed using the Pennsylvania State University mesoscale model, which includes a sophisticated boundary-layer component with horizontally varying surface characteristics and cloud effects on the surface radiation budget. Comparisons of simulations with and without surface fluxes of heat and moisture confirm that differential surface heating, topography, and differential advection may combine to produce a stabilizing effect. In certain synoptic situations, air strongly heated over the arid Mexican plateau rides over moist potentially cooler air downstream, th...

An Isentropic Three-Hourly Data Assimilation System Using ACARS Aircraft Observations

Abstract A 3-h intermittent data assimilation system (Mesoscale Analysis and Prediction System—MAPS) configured in isentropic coordinates was developed and implemented in real-time operation. The major components of the system are data ingest, objective quality control of the observation, objective analysis, and a primitive equation forecast model, all using isentropic coordinates to take advantage of the improved resolution near frontal zones and greater spatial coherence of data that this coordinate system provides. Each 3-h forecast becomes the background for the subsequent analysis; in this manner, a four-dimensional set of observations can be assimilated. The primary asynoptic data source used in current real-time operation of this system is air-craft data, most of it automated. Data from wind profilers, surface observations, and radiosondes are also included in MAPS. Statistics were collected over the last half of 1989 and into 1990 to study the performance of MAPS and compare it with that of the Re...

The Evolution of the Mesoscale Environment of Severe Local Storms: Preliminary Modeling Results

Abstract The development of mesoscale features in numerical model forecasts of the environment of severe local storms is examined for two of the SESAME-1979 cases. The results show that a 10-layer model with a horizontal resolution of about 100 km, simple physics and initialized with essentially synoptic-scale data, is capable of generating and maintaining mesoscale phenomena in the 0–24 h time period. These results indicate that some mesoscale phenomena are predictable for periods of time longer than the lifetime of the mesoscale feature itself. Mesoscale features produced in the forecasts of the 10–11 April and 25–26 April cases include low-level jets, mesoscale convective complexes, upper-level jet streaks, cold and warm frontogenesis, drylines, mountain waves and capping inversions (lids). The development and structure of these phenomena in the model forecast are examined in detail and the interactions among the phenomena are emphasized. The results strongly confirm the conclusions from earlier studie...