Limited-area Fine Mesh Model Research Articles

The Regional Analysis System for the Operational “Early” Eta Model: Original 80-km Configuration and Recent Changes

Abstract The analysis component of the National Centers for Environmental Prediction (NCEP) operational “early” 80-km eta model, as implemented in July 1993, is described. This optimum interpolation (OI) analysis is fully multivariate for wind and geopotential height (univariate for specific humidity) and is performed directly on the eta model's vertical coordinate. Although the eta OI analysis and model performance has been generally favorable when compared to the Limited-Area Fine Mesh Model (LFM) and the Nested Grid Model (NGM), deficiencies in the eta OI analysis fields have been observed, especially near the surface. A series of improvements to the eta OI analysis is described. A refinement to the eta model orography, which created a more realistic depiction of the model terrain, is also discussed along with the impact of these changes on analysis and model performance. These changes were implemented in the early eta system in September 1994. The operational configuration of the new mesoscale (29 km)...

A Case of Rapid Continental Mesoscale Cyclogenesis. Part I: Model Sensitivity Experiments

Abstract A rapid mesoscale cyclogenesis event took place over the southeastern United States during 28–29 March 1984. This small-scale cyclone, whose initial radius of circulation was approximately 120 km, was associated with a 3-h pressure fall of 11 mb, rainfall exceeding 60 mm, and numerous tornadoes. The development of this mesoscale cyclone was poorly forecasted by the operational Limited-Area Fine-Mesh Model (LFM). Later experiments with the Nested Grid Model (NGM) and eta model also experienced similar failure. In this paper, the authors present a series of numerical experiments using The Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 4 (MM4) with the goat of determining factors crucial to a successful prediction of the surface cyclogenesis. The control experiment simulated the rapid mesoscale cyclogenesis by using a 40-km grid spacing; explicit prediction of cloud water, rainwater, and cloud ice; subgrid cumulus parameterization developed by Grell; a...

Comparisons of LFM and NGM Lifted-Index Calculations

Several different forms of lifted index (LI) calculations are available from the Limited-Area Fine Mesh Model (LFM) and Nested-Grid Model (NGM) run operationally at the National Meteorological Center (NMC). The differences between the index calculations are presented, various applications of the LI are discussed, and a case demonstrating some of the strengths and weaknesses of each is presented.

Time Trends of VAS Satellite-derived Soundings

Time tendencies of operationally prepared Visible-Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) retrievals and derived products are evaluated by comparing them against corresponding tendencies from radiosonde soundings. Temperature and dewpoint trends from the two sources are compared, as are trends of thickness and precipitable water. VAS retrieval tendencies also are compared with those of the first-guess limited-area fine-mesh model (LFM) input to determine relationships and/or improvements. Time intervals of 6 h receive the greatest attention; however, 3- and 9-h periods also are considered. Agreements between VAS and radiosonde observation (RAOB) trends generally are found to be very poor, with correlations between two versions usually less than .5. VAS trends compare less favorably with the 'ground truth' than do trends of LFM data, which served as first guess. VAS-RAOB trends of vertically integrated parameters, that is, thickness and precipitable water, agree somewhat better than those of temperature and dewpoint, but correlations still are very poor. In evaluating 3-, 6-, and 9-h intervals, statistical agreements between VAS and radiosonde trends are found to improve considerably with increasing time intervals. VAS trends are found to degrade the first-guess (LFM) trend about as often as they improve it.

Some Aspects of Forecasting Severe Thunderstorms during Cool-Season Return-Flow Episodes

Abstract Historically, the Gulf of Mexico has been considered a primary source of water vapor that influences the weather for much of the United States east of the Rocky Mountains. Although severe thunderstorms and tornadoes occur most frequently during the spring and summer months, the periodic transport of Gulf moisture inland ahead of traveling baroclinic waves can result in significant severe-weather episodes during the cool season. To gain insight into the short-range skill in forecasting surface synoptic patterns associated with moisture return from the Gulf, operational numerical weather prediction models from the National Meteorological Center were examined. Sea level pressure fields from the Limited-Area Fine-Mesh Model (LFM), Nested Grid Model (NGM), and the aviation (AVN) run of the Global Spectral Model, valid 48 h after initial data time, were evaluated for three cool-season cases that preceded severe local storm outbreaks. The NGM and AVN provided useful guidance in forecasting the onset of ...

An Analysis of Sea-Level Cyclone Errors in NMC's Nested Grid Model (NGM) During the 1987-88 Winter Season

A study of sea-level cyclone errors which occurred in 24- and 48-h forecasts of the National Meteorological Center's nested grid model (NGM) is performed for the 1987–88 winter season (1 December 1987–31 March 1988). All available 0000 UTC and 1200 UTC forecast cycles are analyzed for North America and adjacent ocean regions. Errors in forecasted central pressure and position are computed. NGM forecasts of cyclone central pressure average 0.6 mb too deep at 24 hours and 0.3 mb too deep at 48 hours. The root-mean-square (RMS) errors for central pressure are 5.7 mb at 24 hours and 7.9 mb at 48 hours. The mean systematic displacement errors are 29 km at 24 hours and 51 km at 48 hours, and are directed towards the west at both times. The mean absolute displacement errors are much larger, 268 km at 24 hours and 393 km at 48 hours. Cyclone movement is forecasted too slow more frequently than too fast. Large variability in the skill of successive runs characterizes NGM cyclone forecasts though, with ∼70%–∼75% of the temporal variance being associated with fluctuations having periods shorter than seven days. The results for the 1987–88 NGM are compared to those for the limited-area fine-mesh model (LFM) for the 1978–79 winter season. The size of the systematic pressure error decreased 50%–75% over the past decade. The absolute displacement error only decreased 10%–15%. Forecast variability, as measured by the RMS error of central pressure, is ∼15% smaller at 48 hours but remains essentially the same at 24 hours. The comparison of the NGM and LFM results suggests that the nature of the short-range forecast problem for wintertime extratropical cyclones has changed somewhat over the past decade. It now appears that the problem is no longer one of primarily reducing the systematic error but also involves minimizing the impact of variability among individual forecasts.

A Statistical Comparison of the Forecasts Produced by the NGM and LFM for the 1987/88 Cool Season

Forecasts from the National Meteorological Center's (NMC) nested grid model (NGM) and limited-area fine-mesh model (LFM) were compared objectively for the 1987/88 cool season. Mean values of various predicted variables were computed for each model for each 6-h projection during the 0- to 48-h forecast period; the mean values from the two models were then compared to see how the models’ forecasts changed with increasing forecast projection. The results showed a tendency for the NGM to warm (cool) with increasing projection in the eastern (western) United States. The LFM warmed (cooled) with time in the central (western) United States. The relative humidity in the NGM tended to increase with increasing projection, especially at 500 mb. Both models generally overpredicted the frequencies of small amounts of precipitation; the NGM underpredicted the frequency of the larger amounts of precipitation. In addition to the mean value comparisons, the models’ forecasts were evaluated on the basis of correlation with observed temperature, cloud cover, and precipitation. In general, the forecasts produced by the NGM were slightly better correlated with the observed weather than the forecasts produced by the LFM.

New NGM-Based MOS Guidance for Maximum/Minimum Temperature, Probability of Precipitation, Cloud Amount, and Surface Wind

Abstract In this paper, we describe the development and use of new nested grid model (NGM)-based model output statistics (MOS) guidance that has been available since 26 July 1989 for 204 stations in the contiguous United States. The new guidance, which replaced the NGM-based perfect prog package that had been operational since May 1987, consists of forecasts of max/min temperature, probability of precipitation, cloud amount, and surface wind. Guidance for all four elements is available for projections of 1 and 2 days from 0000 and 1200 UTC. The limited-area fine-mesh model (LFM)-based MOS guidance package is still available and was not affected by this change. Verification on independent data shows that NGM-based MOS and LFM-based MOS temperature forecasts are about equally accurate and that both sets of MOS guidance are clearly superior to the NGM-based perfect prog guidance. For the probability of precipitation, the NGM-based MOS guidance is consistently more skillful than the perfect prog guidance, and...

The Regional Analysis and Forecast System of the National Meteorological Center

The three components of the Regional Analysis and Forecast System (RAFS) of the National Meteorological Center (NMC) are described. This system was implemented in March 1985 to supplement guidance from NMC's limited-area fine-mesh model (LFM), especially for precipitation forecasting. The three components of the RAFS are the regional optimum interpolation analysis, the Baer–Tribbia nonlinear normal mode initialization, and the nested grid model—a grid point, primitive-equation model in sigma coordinates. Postprocessing of model forecasts and plans for system improvement are also discussed.

Numerical Simulation of an Explosively Deepening Cyclone in the Eastern Pacific

Abstract A series of nine experiments were conducted using a version of the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. Objectives were: 1) to test the ability of a high resolution limited-area model to simulate an extraordinary cyclogenesis that occurred in the eastern Pacific in November 1982; 2) to examine the effects of various physical processes on the storm development and 3) to determine the reasons for the failure of the operational Limited-area Fine-mesh Model (LFM) to forecast the event. Six of the experiments employed a 40 km grid and three employed an 80 km grid. Initial data for seven of the experiments consist of fields interpolated from the National Meteorological Center (NMC) operational analysis supplemented by subjective soundings created by Reed and Albright. The supplementary data were withheld in two of the experiments. Principal findings are: 1) The control experiment, which utilized the supplementary dataset, a 40 km grid and an...

A Case Study of Trough Merger In Split Westerly Flow

Trough merger, defined as the amalgamation of two separate vorticity centers in distinct branches of westerlies into one coherent vorticity center, is studied for a case In November 1980. The analysis approach takes advantage of the failure of the operational Limited-Area Fine-Mesh Model (LFM) to simulate the mow process. A diagnostic analysis of the observed and forecast vorticity and thermodynamic structure is used to help isolate physical processes occurring during trough merger. During the period 17–19 November 1980 major cyclogenesis occurred along the coast of North America as a deepening 500 mb trough in the northern branch of the westerlies merged with a weaker trough in the southern branch of the westerlies. Cyclonic vorticity in the middle troposphere ahead of the northern trough was generated by: 1) horizontal convergence in the lower and middle troposphere and the upward advection of this vorticity and, 2) horizontal advection in the middle and upper troposphere. Frontogenesis along the 500 mb asymptote of confluence contributed to deep tropospheric ascent and enhanced the baroclinity. The overall baroclinic zone shifted northward ahead of the deepening northern trough as warming due to horizontal advection overwhelmed adiabatic cooling due to ascent. The LFM forecasts were deficient in all of these processes. In particular, the failure of the model to generate deep tropospheric ascent combined with an overprediction of the cooling due to adiabatic ascent precluded the model from shifting the principal baroclinic zone northward. A major quantitative precipitation forecast error resulted from the failure to predict trough merger.

A Comparison of Key Forecast Variables Derived from Isentropic and Sigma Coordinate Regional Models

Abstract When diabatic heating, frictional forces and radiative effects within the atmosphere are negligible then a parcel's entropy will remain nearly constant and its motion will be along isentropic surfaces. Because such conditions may extend over large areas on the synoptic scale, use of the isentropic coordinate in a regional forecast model offers the potential of more accurate predictions since numerical integration can largely be carried out in two rather than three dimensions. Forecasts from an isentropic-coordinate model employing a sigma-coordinate lower boundary are compared with those from two of the National Weather Service's operational models, the Limited-Area Fine Mesh Model and the Nested Grid Model. Five significant weather events are considered with attention given to those predicted quantities most important to each case. The operational models produced the best overall precipitation forecasts. The isentropic model displayed some advantages in its wind, specific humidity, and surface p...

The Effect of VISSR Atmospheric Sounder Data on Some LFM Analyses and Forecasts

The impact of VMR Atmospheric Sounder (VAS) temperature profiles upon the Limited-Area Finemesh Model used operationally at the National Meteorological Center (NMC) is investigated for a set of six cases during the winter or 1981/82. Pairs of analyses are prepared for each case, one using both VAS and conventional data (VAS analyses) and one using only conventional data (control analyses). VISSR Atmospheric Sounder data, inserted over the northeast Pacific Ocean, generally effected moderate changes in the 30–60 range, upon 500 mb height analyses. A comparison of one of the VAS analyses with a concurrent LFM analysis prepared using NOAA-7 data shows that the two observing systems compare favorably, implying that the VAS data for that case were at least as accurate as the NOAA-7 data in depicting the features of a complex man field. Six sets of VAS and control forecasts were prepared using the analyses described herein as initial data for the LFM. Four of the six forecasts made using VAS data improved in comparison with their control counterparts.

Synoptic Scale Forecast Skill and Systematic Errors in the MASS 2.0 Model

The synoptic scale performance characteristics of MASS 2.0 are determined by comparing filtered 12-24 hr model forecasts to same-case forecasts made by the National Meteorological Center's synoptic-scale Limited-area Fine Mesh model. Characteristics of the two systems are contrasted, and the analysis methodology used to determine statistical skill scores and systematic errors is described. The overall relative performance of the two models in the sample is documented, and important systematic errors uncovered are presented.

Automated Guidance for Forecasting Snow Amount

A new system is developed which gives both conditional and unconditional probability of snow amount forecasts and categorical forecasts at stations for the 12–24 h period after both 0000 and 1200 GMT. To derive the new equations, the Model Output Statistics technique is used with output from the Limited-area Fine Mesh model (LFM). First, experimental probability of snow amount forecast equations are developed for the 12–18, 18–24, 12–24 and 24–36 h projections. Verification of the experimental forecasts on independent data indicates that the scores generally deteriorated as compared to the scores for the developmental sample. The deterioration was worse for the 18–24 and 24–36 h projections than for the 12–18 and 12–24 h projections. It's concluded that the snow amount forecasts for the 24–36 h projection aren't skillful enough for operational implementation at this time. Next, to improve the stability of the forecast equations for the 12–24 h projection, the developmental and independent samples are combined (nine winter seasons) and new equations are developed for ≥5,≥10 and ≥15 cm snow amount categories. In this development, the results of a statistical screening procedure indicate that, generally, the most important predictor is the LFM precipitation amount forecast followed by the mean relative humidity (surface–500 mb) forecast. Other predictors which are relatively important include LFM forecasts of circulation intensity such as 700 mb vertical velocity, 700 mb east–west wind component, 850 mb divergence, 850 mb relative vorticity, and 850 mb east–west wind component. Verification of the new equations on another independent sample indicates that the scores were generally stable except for some deterioration for the ≥15 cm category. The new equations were implemented in the fall of 1982 and should be more useful than the old operational system, which provided forecasts for the ≥10 cm category only and which had been operational within the National Weather Service since October 1977.

An Evaluation of Soundings, Analyses and Model Forecasts Derived from TIROS-N and NOAA-6 Satellite Data

TIROS-N and NOAA-6 temperature soundings over North America during three days in January 1980, and synoptic analyses and numerical-model forecasts derived from them, are compared with conventional data and analyses from NMC's limited-area fine-mesh model (LFM). The collocated sounding comparison revealed significant errors, especially near the surface and the tropopause. Satellite-derived thermal gradients were found to be weak, and thickness-analysis difference fields to propagate eastward, suggesting that sounding errors are correlated with synoptic patterns. The same pattern of anomalies is seen in the model forecasts. More detailed determinations of the correlation detected here could be used to optimize the assimilation of satellite soundings to conventional data.

Improved Automated Surface Temperature Guidance

Abstract Currently, two sets of automated numerical-statistical forecasts of maximum/minimum (max/min) temperatures for calendar day periods are produced in the day-to-day operations of the National Weather Service. The “early” guidance forecasts are based on output from the Limited-area Fine Mesh (LFM) model, while the “final” guidance relies primarily on predictions from the hemispheric Primitive Equation (PE) model. This paper describes recent improvements to the early guidance surface temperature prediction system. The Techniques Development Laboratory recently developed new early guidance equations to forecast calendar day max/min temperatures for projections out to approximately 60 h and hourly temperatures at 3 h intervals out to 51 h for approximately 230 stations in the conterminous United States. A combination of LFM model output, surface weather observations and climatic factors were used in this development. We derived three sets of temperature prediction equations for both the 0000 and 1200 G...

A Model for Predicting Synoptic Weather Types Based on Model Output Statistics

An objective classification model was developed that can automatically predict synoptic weather types in southern California. Stepwise discriminant analysis was used to match the National Meteorological Center's Limited-area Fine Mesh (LFM) Model Output Statistics to subjectively classified weather types or patterns. The five classified weather types range from hot, dry, windy Santa Ana days to cool, rainy, cloudy days caused by a synoptic low. Discriminant function equations were developed for predicting each weather type 12 and 24 h in advance by screening 80 potential predictors consisting of forecasts at 500, 700 and 850 mb from the LFM model. Model output at nine grid points was used because that information adequately describes the meteorological patterns over southern California. Using independent LFM model forecasts valid 12 and 24 h in advance, the objective classification model predicted the probability of the days being in each of the five weather types, then the type with the highest probability was selected. Eighty-eight of 107 24 h periods (days) centered 12 h in advance (81%) were correctly predicted. Of 99 independent days centered 24 h in advance, 71 (72%) were correctly predicted. Hourly means and standard deviations of surface temperature and dew points at eight research sites in the San Bernardino Mountains computed by month for the five weather types had distinct diurnal variations corresponding with weather types. Summarizing hourly temperatures in August at the eight sites by weather type reduced their standard deviation by almost one-half. Measurements of mean daily maximum ozone at a San Bernardino Mountain crest site, where chronic ozone injury to ponderosa pine has occurred, showed significant differences between the weather types. The mean surface wind, temperature and dew-point patterns at 2100 GMT over southern California for type 1 (Santa Ana) days show strong offshore winds, high temperatures and very low dew points, whereas type 5 (synoptic low) days show strong onshore winds, low temperatures and high dew points.

Objective Prediction of Cloud Amount Based on Model Output Statistics

Abstract We have applied the Model Output Statistics (MOS) approach to the prediction of cloudiness. Final guidance warm and cool season forecasting equations were developed by screening forecast fields from the primitive equation and trajectory models. We derived separate equations for each of 233 stations to estimate the probability of clear, scattered, broken and overcast conditions 12 to 48 h in advance. The same predictors were used in all four equations for any given station and projection. In like manner, we also derived a set of early guidance equations for the warm season by screening forecasts from the limited-area fine mesh model. Here, separate equations were developed for 230 stations and projections of 6 to 24 h. Weather parameters from surface reports were also included as potential predictors for the first two forecast projections to provide the latest observed conditions for the early and final guidance systems. We verified both experimental and operational cloud forecasts made from the f...

Intercomparisons of Data Derived from Nimbus 5 Temperature Profiles, Rawinsonde Observations and Initialized LFM Model Fields

Abstract A 23–24 February 1975 case study is made comparing the results achieved using Nimbus 5 satellite temperature, profiles with those obtained from radiosonde data and the initial hour data of the Limited Area Fine Mesh (LFM) model of the National Meteorological Center. An objective analysis technique is used to construct and analyze isotropic cross sections through an intense baroclinic zone. A cross section based on 1700 GMT Nimbus 5 soundings is compared with those based on 1200 and 0000 GMT radiosonde and LFM data. Geostrophic shear calculations are used to compare the thermal gradients for various isobaric layers obtained from the three data acts. The Nimbus 5 results show somewhat less detail than those based on the radiosonde data, but wore than those obtained from the LPM data. Comparisons of the mean temperature in various isobaric layers for the three time periods indicate that the 1700 GMT Nimbus 5 data appear to be consistent with the changing synoptic pattern. Estimates of the wind compo...