Fine Mesh Model Research Articles

Direct and Indirect Effects of Latent Heat Release on a Synoptic-Scale Wave System

Abstract The primary goal of this paper is to diagnose, the “direct” and “indirect” effects of latent heat release on a synoptic-scale wave system containing an extratropical cyclone that developed over the eastern United States. To achieve this goal, comparisons are made between MOIST (full model physics) and DRY (latent heating removed) predictions of the wave system during the period 27–29 February 1984 using the National Meteorological Center's Limited-Area Fine Mesh (LFM) model. Both the MOIST and DRY models predict significant cyclone systems, suggesting that the background adiabatic forcing is quite important. However, the DRY model predict a weaker cyclone. The direct and indirect latent heating influences are diagnosed using eddy energy quantities and the extended height tendency equation. Direct effects are restricted to the diabatic generation of available potential energy and height tendencies forced by diabatic heating. Results show that latent heating exerts an important direct influence on ...

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...

Interpretation of Satellite Imagery of A Rapidly Deepening Cyclone

AbstractModel output and observational data are used to interpret, in detail, satellite images of a distinctive cloud system several hundred kilometres long which was observed before and during rapid cyclonic development that occurred over England. Conceptual models are forwarded which account for the structure of the major cloud areas and upper‐level moisture fields (as observed by water vapour imagery) immediately prior to and following the onset of vigorous cyclogenesis.The cloud system corresponded to what R. B. Weldon refers to as a baroclinic leaf cloud and was characterized by ‘ana’ cold frontal ascent. the baroclinic leaf lay within an area of enhanced ascent ahead of a major upper‐level trough. A jet streak which originated upstream, propagated rapidly around the base of the upper trough. A tongue of dry air centred on the left flank of this jet streak lay immediately upstream of the leaf cloud and had a major influence on its subsequent development. This intrusion of dry air was well represented by the Meteorological Office fine mesh model, which showed it as a pronounced tongue of recently descended air with high potential vorticity, which originated in the upper troposphere and lower stratosphere. Rapid cyclogenesis occurred as the dry intrusion overran low‐level air of high θw within a warm conveyor belt at the southern tip of the leaf cloud. Interaction of the dry intrusion and baroclinic leaf cloud is shown to be particularly important in accounting for the detailed distribution of weather. It also suggests a way of pinpointing the location and timing of vigorous cyclogenesis.

The Use and Interpretation of Numerical Weather Prediction Model Output in Identifying Synoptic-Scale Environments Associated with Development of Mesoscale Convective Systems

Abstract An experiment is reported in which derived diagnostic parameters computed from Limited-area Fine-Mesh (LFM) model gridpoint data were examined to determine subjectively whether their availability in real time would assist the forecaster in interpreting and understanding the model's forecast of the weather. Specifically, model products thought to relate to the development of mesoscale convective weather systems (MCSs) were combined into a composite forecast and compared with the standard ensemble of LFM products for 25 episodes of significant convective activity. An objective verification of the LFM forecasts themselves was not attempted. Both 12 and 24 h forecasts from the 1200 UTC run were considered. In a majority of cases, it was evident that derived diagnostic gridpoint data added information about parameter patterns and values important to MCS development that was not obvious from viewing the conventional model products alone. Two case studies demonstrate how information about 850 mb moistur...

A Case Study of Explosive Cyclogenesis in the Eastern Pacific

Abstract An explosive cyclogenesis that occurred in the eastern Pacific on 13 November 1981 is described and discussed on the basis of surface and upper air analyses and satellite imagery. During one 12-h period the storm deepened by nearly 40 mb and subsequently attained a central pressure of 950 mb or less. Peak surface winds were in the 45–50 m s−1 range. The Limited Area Fine Mesh (LFM) model 24-h forecast initialized at 0000 GMT on the thirteenth completely failed to predict the storm development. The development took place within a strong baroclinic zone, as a shallow frontal wave traveled from the relatively stable environment of a long-wave ridge to the unstable environment of a long-wave trough. Winds at jet stream level exceeded 90 m s−1 prior to the explosive deepening. The lifted index in and ahead of the storm diminished and became negative prior to the rapid intensification. Measurements revealed that sensible and latent heat fluxes from the ocean contributed significantly to the reduced sta...

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.

Numerical experiments on the genesis of vortices over the Qinghai-Tibet plateau

The formation of a vortex and shear line over the Qinghai-Tibet Plateau (QTP) was investigated with numerical experiments using the ECMWF grid point model. The case chosen occurred between 24 and 26 July, 1982, when the south-west monsoon invaded the QTP and a vortex developed. The correct simulation of the vortex in the model shows that it is possible to predict the atmospheric circulation over the QTP and the consequent weather over China. When the simulation was rerun with a dry model (i.e., without the release of latent heat), it was found that only a weak vortex formed. This shows that the release of latent heat is an extremely important factor in the development of the vortex over the QTP, and confirms earlier diagnostic and synoptic studies. The surface sensible heat fluxes, large near the northern and southern flanks of the QTP, appear to have a damping effect on the formation of the vortex; i.e., the vortex was greatly intensified in their absence. It was also found that the intensity of the vortex was dependent upon the height of the QTP: when the mountain height was reduced, the westerly trough invaded the QTP and no vortex formed at 500 mb; when it was increased, the south-west monsoon travelled along the southern boundary of the QTP and intensified the low-level jet south of the vortex area. The results of a 48-h integration with a fine-mesh model (resolution 0.5° of latitude and longitude) showed an improvement in forecasting the location of the vortex with respect to the coarse-mesh model (1.875° of latitude and longitude). However, when compared with the coarse-mesh analysis, the intensity of the vortex was overpredicted. This may be due to mesoscale orographic forcing and exaggerated latent heating. DOI: 10.1111/j.1600-0870.1986.tb00468.x

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.

Prediction of a typhoon using a fine-mesh NWP model

The ECMWF operational grid point model (with a resolution of 1.875° of latitude and longitude) and its limited area version (with a resolution of !0.47° of latitude and longitude) with boundary values from the global model have been used to study the simulation of the typhoon Tip. The fine-mesh model was capable of simulating the main structural features of the typhoon and predicting a fall in central pressure of 60 mb in 3 days. The structure of the forecast typhoon, with a warm core (maximum potential temperature anomaly 17 K). intense swirling wind (maximum 55 m s -1 at 850 mb) and spiralling precipitation patterns is characteristic of a tropical cyclone. Comparison with the lower resolution forecast shows that the horizontal resolution is a determining factor in predicting not only the structure and intensity but even the movement of these vortices. However, an accurate and refined initial analysis is considered to be a prerequisite for a correct forecast of this phenomenon. DOI: 10.1111/j.1600-0870.1985.tb00273.x

Prediction of a typhoon using a fine-mesh NWP model

The ECMWF operational grid point model (with a resolution of 1.875° of latitude and longitude) and its limited area version (with a resolution of !0.47° of latitude and longitude) with boundary values from the global model have been used to study the simulation of the typhoon Tip. The fine-mesh model was capable of simulating the main structural features of the typhoon and predicting a fall in central pressure of 60 mb in 3 days. The structure of the forecast typhoon, with a warm core (maximum potential temperature anomaly 17 K). intense swirling wind (maximum 55 m s -1 at 850 mb) and spiralling precipitation patterns is characteristic of a tropical cyclone. Comparison with the lower resolution forecast shows that the horizontal resolution is a determining factor in predicting not only the structure and intensity but even the movement of these vortices. However, an accurate and refined initial analysis is considered to be a prerequisite for a correct forecast of this phenomenon. DOI: 10.1111/j.1600-0870.1985.tb00273.x

Numerical Experiments in Mesoscale Prediction over Southeast Australia

Abstract A new movable fine-mesh model (MFM) developed at the Australian Numerical Meteorology Research Centre, for mesoscale data assimilation and forecasting, is described. For the present, it is a 10-layer, hydrostatic, primitive equations model with 60 km horizontal resolution nested in a one-way interactive sense within a coarse-mesh primitive equations model of 250 km horizontal resolution. The MFM model has been designed to take advantage of recent local increases in data availability and computer power which have hitherto prevented much attention being given to numerical forecasts of mesoscale features in Australia. Important features of the model include an efficient semi-implicit time differencing algorithm, the use on option of a vertical mode initialization procedure, and the incorporation of realistic topographic forcing. The performance of the MFM model has been assessed on eight case studies over southeastern Australia. Two forecasts are discussed here in detail. The firm is an example invo...

Severe Downslope Windstorm Calculations in Two and Three Spatial Dimensions Using Anelastic Interactive Grid Nesting: A Possible Mechanism for Gustiness

Abstract The Clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions. Tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model. Application of the conservation condition of Kurihara and others is shown to result in significant improvements in the treatment of interactive nesting. A significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow. This result, for the anelastic system, is in agreement with the earlier results of Phillips and Shukla, who considered the hydrostatic shallow water system of equations. The interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 January 1972 in Boulder using both two and three spatial dimensions. The three-dimensional simulation results in a gustiness signature in the ...

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.

A Three-Dimensional Planetary Boundary Layer Model for the Somali Jet

Abstract This paper is an extension of an earlier study on the planetary boundary layer dynamics of the low level monsoonal flow over the Arabian Sea (Krishnamurti and Wong, 1979), where the long term steady state motion field for a boundary layer was determined using a zonally symmetric model with a prescribed pressure field. In that study we examined the balance of forces in the surface layer and the planetary boundary layer for regions across the equator, across and along the low-level Somali jet, and across an intertropical convergence zone. The important role of advective accelerations in the near-equatorial balance of forces was demonstrated. The important study is based on a three-dimensional model that removes the restriction of zonal symmetry. This mesoscale fine mesh model, with a horizontal resolution of ∼55 km and a vertical resolution of 200 m, is integrated to examine the evolution of three-dimensional planetary boundary layer flows for prescribed three-dimensional pressure patterns. The obs...

An Improved Operational System for Forecasting Precipitation Type

Abstract A Model Output Statistics system for forecasting the conditional probability of precipitation type (PoPT) became operational within the National Weather Service in September 1978. Forecasts are provided for three precipitation type categories: snow or ice pellets, freezing rain, and rain. To develop the forecast equations, data are combined from different stations because of the limited amount of developmental data. To justify combining the data, the Limited-area Fine Mesh (LFM) model predictors are transformed from their original values through the use of the logit model. In one experiment, it is shown that probability of snow forecasts are made more accurate through an improved use of the logit model for predictor transformation. The new transformation procedure is then used in the development of a set of experimental PoPT forecast equations. The experimental equations differ from the operational equations in other ways also. The developmental sample for the experimental equations included appr...

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.

停滞したリッジ西縁の亜熱帯レインバンド

The largeand meso-scale features of a subtropical rainband which caused thunderstorms over western Japan (-33N/130E) in 25-7 July 1981 are studied. This case aroused our interest because the convective activities persisted for about three days in the west side of a slowmoving ridge.Radar observations showed a persistent convective echo band of -800km length extending in NNW-SSE direction. This echo band was not accompanied with front-like strong thermal gradient. Satellite IR imageries showed that the rainband was a part of a long subtropical cloud zone formed along the northern periphery of the extremely moist tropical maritime airmass in the lower troposphere. The enhancement of cloud zone and the formation of the rainband was owing to the incessant feeding of moisture accompanied with the northward intrusion of the moist tropical maritime air along the western hem of the slow- moving ridge.Real data forecast experiments on the cloud zone and rainband were made using a coarseand a fine-mesh primitive equation model. While only the large-scale zone of weak precipitation relevant to the observed cloud zone was produced in the coarse-mesh (380kmmesh) model, the narrow rainband with low-level convergence was simulated in the finemesh (77km-mesh) model. The "dry experiment" (a experiment without condensation) did not produce any low-level convergence zone. This indicated influence of condensation process on the formation of this subtropical rainband.

A Percentage of Possible Sunshine Forecasting Experiment at Albany, New York

Abstract The results of two regression experiments to predict percentage of possible sunshine (PoPS) one day in advance at Albany, New York are described. For the one experiment, predictors are derived from Albany radiosonde observations, while the other experiment uses predictors obtained from the daily 1200 GMT run of the operational Limited Area Fine Mesh (LFM) model at the National Meteorological Center (NMC). The most frequently chosen predictors from the radiosonde observations include layer-mean moisture and temperature information, 850 and 700 mb meridional wind components and stability indices. Similar predictors emerge from the LFM forecasts with the 700 mb vertical velocity replacing the meridional wind components. Ten years of twice-daily Albany radiosonde data and four years of LFM prognoses for Albany constituted the dependent data sample for both experiments. Skill levels are assessed relative to a persistence climatology through the ranked probability score for forecasts of POPS as a conti...

A Comparison of the Performance of Two Operational Dynamic Tropical Cyclone Models

This paper compares the performance of two multi-level high-resolution baroclinic tropical cyclone models which are currently in operational use. In the Atlantic, the National Hurricane Center utilizes the Movable Fine-mesh Model (MFM) for objective forecast guidance. In the western Pacific, the Navy's Nested Tropical Cyclone Model (NTCM) is employed by the Joint Typhoon Warning Center for this same purpose. However, the computer resource requirements, basic design and method of use are substantially different for the two models. Accordingly, two separate tests were conducted. In the first, or operational comparison, the NTCM was initialized with the Fleet Numerical Oceanography Center tropical analysis while the MFM was initialized with the National Meteorological Center (NMC) global analysis. Forecast errors for both models were very similar in this experiment. The other test isolated model differences by initializing both models with the NMC analysis only. Considerably degraded performance for the NTCM was noted. This result emphasizes the sensitivity of dynamic model performance to the large-scale analysis, and underlines the importance of analysis-prediction system co-development. Each model is briefly described and case-by-case performance evaluations are presented. The results indicate that dynamic models promise great potential for significant improvement in tropical cyclone movement forecasting, particularly for forecast intervals of greater than 36 h.

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...