Abstract

In 1982, the National Oceanic and Atmospheric Administration's Hurricane Research Division began a series of experiments to collect Omega dropwindsonde (ODW) observations within about 1000 km of the center of tropical cyclones. By 1992, 16 ODW datasets had been collected in 10 Atlantic basin hurricanes and tropical storms. Objective wind analyses for each dataset 10 levels from 100 mb to the surface, have been produced using a consistent set of analysis parameters. The objective analyses, which resolve synoptic-scale features in the storm environment with an accuracy and confidence unattainable from routine operational analyses, have been used to examine relationships between a tropical cyclone's motion and its surrounding synoptic-scale flow.

Tropical cyclone motion is found to be consistent with barotropic steering of the vortex by the surrounding flow within 3° latitude (333 km) of the cyclone center. At this radius, the surrounding deep-layer-mean flow explains over 90% of the variance in vortex motion. The analyses show vorticity asymmetries that strongly resemble the β gyres common to barotropic models, although other synoptic features in the environment make isolation of these gyres from the wind fields difficult. A reasonably strong relationship is found between the motion of the vortex (relative to its large scale surrounding flow) and the absolute vorticity gradient of the vortex environment.

Abstract

Mesoscale model simulations with and without diurnal planetary boundary layer heat flux are compared to a detailed surface analysis for a case of an isolated tornadic convective complex development. The case study, 3-4 June 1980, is of particular interest because of the development of several destructive tornadic storms within the Grand Island, Nebraska metropolitan area during a period of relatively weak synoptic scale forcing. This type of case presents an opportunity for the mesoscale numerical simulation of the subtle interactions between an upper tropospheric jet stream and surface diabatic heating. Model simulations runwith and without diurnal surface sensible heating show marked differences in processes both within and above the planetary boundary layer (PBL). The results of the simulations indicate that the evolution of the subsynoptic scale low pressure system and its accompanying low level jet streak, areas of moisture convergence, and regions of convective instability are influenced by the interaction of the deep surface-heated PBL with a weak synoptic scale jet streak. The model simulations show that the distribution and evolution of tropospheric velocity divergence cannot be realistically decoupled from the thickness changes caused by PBL heating in this case of relatively weak dynamic forcing. Modifications in the simulated velocity divergence and low level warm advection caused by PBL heating led to a more realistic pattern of pressure falls, low level jet formation, and a significant reduction of the lifted index values near the region of observed convection. Comparisons with observations, however, also indicate that the modeling system still requires: I) enhanced soil moisture information in the data base utilized for its PBL parameterization to achieve the proper amplitude and distribution of surface sensible heat flux and 2) the proper parameterization of convective scale processes such as latent heating to completely capture the evolution of the subsynoptic scale low pressure system into a mesoscale low pressure system. The most significant implication of these modeling results is that previous dynamical models of upper and lower tropospheric coupling during the pre-stormenvironment should include consideration of the effects of diurnal surface sensible heating upon a pre-existing jet streak.

Abstract

Atmospheric temperature profiles, obtained from spectral radiances of the earth between 2160 and 2360 cm⁻¹ measured by a balloonborne, multi-detector, grating spectrometer at 3.5 mb during a 6-hr flight, are described. Representative profiles obtained both before and after sunrise and for clear and cloudy skies show that atmospheric temperatures accurate to ∼2K can be inferred. The variations of surface temperature during the flight are discussed.

A state-of-the-science meso-β-scale numerical weather prediction model is being employed in a prototype forecast system for potential operational use at the Dallas–Fort Worth International Airport (DFW). The numerical model is part of a unique operational forecasting system being developed to support the National Aeronautics and Space Administration's (NASA) Terminal Area Productivity Program. This operational forecasting system will focus on meso-β-scale aviation weather problems involving planetary boundary layer (PBL) turbulence, and is named the Terminal Area PBL Prediction System (TAPPS). TAPPS (version 1) is being tested and developed for NASA in an effort to improve 1–6-h terminal area forecasts of wind, vertical wind shear, temperature, and turbulence within both stable and convective PBLs at major airport terminal areas. This is being done to enhance terminal area productivity, that is, aircraft arrival and departure throughput, by using the weather forecasts as part of the Aircraft Vortex Spacing System (AVOSS). AVOSS is dependent upon nowcasts or short-period forecasts of wind, temperature, and eddy dissipation rate so that the drift and dissipation of wake vortices can be anticipated for safe airport operation. This AVOSS system will be demonstrated during calendar year 2000 at DFW.

This paper describes the numerical modeling system, which has three basic components: the numerical model, the initial data stream, and the postprocessing system. Also included are the results of several case study simulations with the numerical model from a field program that occurred in September 1997 at DFW. During this field program, detailed local measurements throughout the troposphere, with special emphasis on the PBL, were taken at and surrounding DFW in an effort to verify the numerical model simulations. Comparisons indicate that the numerical model is capable of an accurate simulation of the vertical wind shear structure during the diurnal evolution of the PBL when compared directly to specific local observations. The case studies represent unambiguous examples of the dynamics of the Great Plains diurnal low-level jet stream. This diurnal jet stream represents the dominant low-level wind shear–production mechanism during quiescent synoptic-scale flow regimes. Five consecutive daily case studies, during which this phenomenon was observed over and in proximity to DFW, are compared to the products derived from TAPPS.

The Hurricane Research Division (HRD) is NOAA's primary component for research on tropical cyclones. In accomplishing research goals, many staff members have developed analysis procedures and forecast models that not only help improve the understanding of hurricane structure, motion, and intensity change, but also provide operational support for forecasters at the National Hurricane Center (NHC). During the 1993 hurricane season, HRD demonstrated three important real-time capabilities for the first time. These achievements included the successful transmission of a series of color radar reflectivity images from the NOAA research aircraft to NHC, the operational availability of objective mesoscale streamline and isotach analyses of a hurricane surface wind field, and the transition of the experimental dropwindsonde program on the periphery of hurricanes to a technology capable of supporting operational requirements. Examples of these and other real-time capabilities are presented for Hurricane Emily.

Abstract

The National Hurricane Center (NHC) uses a variety of guidance models for its operational tropical cyclone track, intensity, and wind structure forecasts, and as baselines for the evaluation of forecast skill. A set of the simpler models, collectively known as the NHC guidance suite, is maintained by NHC. The models comprising the guidance suite are briefly described and evaluated, with details provided for those that have not been documented previously. Decay-SHIFOR is a modified version of the Statistical Hurricane Intensity Forecast (SHIFOR) model that includes decay over land; this modification improves the SHIFOR forecasts through about 96 h. T-CLIPER, a climatology and persistence model that predicts track and intensity using a trajectory approach, has error characteristics similar to those of CLIPER and D-SHIFOR but can be run to any forecast length. The Trajectory and Beta model (TAB), another trajectory track model, applies a gridpoint spatial filter to smooth winds from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) model. TAB model errors were 10%–15% lower than those of the Beta and Advection model (BAM), the model it replaced in 2017. Optimizing TAB’s vertical weights shows that the lower troposphere’s environmental flow provides a better match to observed tropical cyclone motion than does the upper troposphere’s, and that the optimal steering layer is shallower for higher-latitude and weaker tropical cyclones. The advantages and disadvantages of the D-SHIFOR, T-CLIPER, and TAB models relative to their earlier counterparts are discussed.