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Naomi Surgi

Abstract

Three 20-day winter forecasts have been carried out using the Florida State University Global Spectral Model to examine the systematic errors of the model. Most general circulation models and global forecast models exhibit the same kind of error patterns even though the model formulations vary somewhat between them. Some of the dominant errors are a breakdown of the trade winds in the low latitudes, an overprediction of the subtropical jets accompanied by an upward and poleward shift of the jets, an error in the mean sea level pressure with overintensification of the quasi-stationary oceanic lows and continental highs, and a warming of the tropical mid- and upper troposphere. In this study, a number of sensitivity experiments have been performed for which orography and model physics are considered as possible causes of these errors.

A parameterization of the vertical distribution of momentum due to the subgrid scale orography has been implemented in the model to address the model deficiencies associated with orographic forcing. This scheme incorporates the effects of moisture on the wave induced stress. The parameterization of gravity wave drag is shown to substantially reduce the large-scale wind and height errors in regions of direct forcing and well downstream of the mountainous regions.

Also, a parameterization of the heat and moisture transport associated with shallow convection is found to have a positive impact on the errors, particularly in the tropics. This is accomplished by the increase of moisture supply from the subtropics into the deep tropics and a subsequent enhancement of the secondary circulations.

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Naomi Surgi
,
Hua-Lu Pan
, and
Stephen J. Lord

Abstract

An evaluation of the performance of the National Centers for Environmental Prediction Medium-Range Forecast Model was made for the large-scale tropical forecasts and hurricane track forecasts during the 1995 hurricane season. The assessment of the model was based on changes to the deep convection and planetary boundary layer parameterizations to determine their impact on some of the model deficiencies identified during previous hurricane seasons. Some of the deficiencies in the hurricane forecasts included a weakening of the storm circulation with time that seriously degraded the track forecasts. In the larger-scale forecasts, an upper-level easterly wind bias was identified in association with the failure of the model to maintain the midoceanic upper-tropical upper-tropospheric trough.

An overall modest improvement is shown in the large-scale upper-level tropical winds from root-mean-square-error calculations. Within a diagnostic framework, an improved simulation of the midoceanic tropical trough has contributed to a better forecast of the upper-level westerly flow. In the hurricane forecasts, enhanced diabatic heating in the model vortex has significantly improved the vertical structure of the forecast storm. This is shown to contribute to a substantial improvement in the track forecasts.

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T. N. Krishnamurti
,
S. K. Roy Bhowmik
,
Darlene Oosterhof
,
Gregg Rohaly
, and
Naomi Surgi

Abstract

This paper presents some recent results on physical initialization from the use of a very high resolution global model. Fundamentally this procedure improves the model-based initial rainfall, surface fluxes, and diagnostic cloud amount. Physical initialization is a useful procedure for the nowcasting of rainfall. Correlation between model-based initialized rain and satellite/rain gauge-based rain over the Tropics (for 6-h averages and averaged over transform grid squares) is of the order 0.85. This compares with a correlation of around 0.3 for models that do not include physical initialization. The day 1 tropical rainfall forecast skill is also relatively high for the physically initialized experiments; the correlation is of the order 0.55. It should be noted that the lifetime of mesoconvective systems is approximately 1 day, whereas more organized tropical disturbances may last substantially longer. A major portion of the tropical rainfall is associated with these short-lived systems, hence the skill beyond 1 day degrades somewhat. However, the model does seem to capture the 1-day passage of mesoconvective systems and their coupling to the large-scale, synoptic environment. The mesoconvective systems illustrated exhibit a robust vertical structure of divergence, heating, and vertical motion, which is absent without physical initialization.

The organization of mesoconvective systems (advected by the large-scale circulations and coalescence of the mesoscale elements) appears to play an important role in the formation of tropical storms. The vorticity associated with these mesoscale elements, however, does not exhibit any interesting organization during the forecast as the storms form. The Florida State University atmospheric global circulation model at the resolution T213 discerns the tight central circulation features and the outer rainbands of Hurricane Andrew (1992), which appear similar to the radar imagery; however, the storm as seen from the model is not on the exact scale as that of the radar that is shown. Further enhancement of resolution is needed to model tropical storms on a more realistic scale, which is well known in the modeling community. Overall this study demonstrates that mesoconvective elements are in fact simulated by very high resolution global models. It appears that very high resolution models with an augmented analysis using satellite data may soon aid in resolving the formation issue associated with tropical cyclones and cyclogenesis.

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T. N. Krishnamurti
,
Ricardo Correa-Torres
,
Greg Rohaly
,
Darlene Oosterhof
, and
Naomi Surgi

Abstract

Ensemble forecasting of hurricane tracks is an emerging area in numerical weather prediction. In this paper, the spread of the ensemble of forecast tracks from a family of different First Global GARP (Global Atmospheric Research Program) Experiment analyses is illustrated. All forecasts start at the same date and use the same global prediction model. The authors have examined ensemble forecasts for three different hurricanes/typhoons of the year 1979. The authors have used eight different initial analyses to examine the spread of ensemble forecasts through 6 days from the initial state. A total of 16 forecasts were made, of which 8 of them invoked physical initialization. Physical initialization is a procedure for improving the initial rainfall rates consistent with satellite/rain gauge based measures of rainfall. The main results of this study are that useful track forecasts are obtained from physical initialization, which is shown to suppress the spread of the ensemble of track forecasts. The spread of the tracks is quite large if the rain rates are not initialized. The major issue here is how one could make use of this information on ensemble forecasts for providing guidance. Toward that end, a statistical framework that makes use of the spread of forecast tracks to provide such guidance is presented.

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Robert Rogers
,
Sim Aberson
,
Michael Black
,
Peter Black
,
Joe Cione
,
Peter Dodge
,
Jason Dunion
,
John Gamache
,
John Kaplan
,
Mark Powell
,
Nick Shay
,
Naomi Surgi
, and
Eric Uhlhorn

In 2005, NOAA's Hurricane Research Division (HRD), part of the Atlantic Oceanographic and Meteorological Laboratory, began a multiyear experiment called the Intensity Forecasting Experiment (IFEX). By emphasizing a partnership among NOAA's HRD, Environmental Modeling Center (EMC), National Hurricane Center (NHC), Aircraft Operations Center (AOC), and National Environmental Satellite Data Information Service (NESDIS), IFEX represents a new approach for conducting hurricane field program operations. IFEX is intended to improve the prediction of tropical cyclone (TC) intensity change by 1) collecting observations that span the TC life cycle in a variety of environments; 2) developing and refining measurement technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and 3) improving the understanding of the physical processes important in intensity change for a TC at all stages of its life cycle.

This paper presents a summary of the accomplishments of IFEX during the 2005 hurricane season. New and refined technologies for measuring such fields as surface and three-dimensional wind fields, and the use of unmanned aerial vehicles, were achieved in a variety of field experiments that spanned the life cycle of several tropical cyclones, from formation and early organization to peak intensity and subsequent landfall or extratropical transition. Partnerships with other experiments during 2005 also expanded the spatial and temporal coverage of the data collected in 2005. A brief discussion of the plans for IFEX in 2006 is also provided.

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