Andrew Crook

Wind and Thermodynamic Retrieval from Single-Doppler Measurements of a Gust Front Observed during Phoenix II

Abstract The adjoint method to retrieve the three-dimensional wind and thermodynamic fields is applied to single-Doppler observations of a gust front measured during the Phoenix II experiment. This method uses a fluid dynamics model and its adjoint, and combines the retrieval with data assimilation into the prediction model. The wind and thermodynamic variables are determined by minimizing the difference between the model solution and the observations. Experiments are conducted first with radial velocity alone and then with both radial velocity and reflectivity to examine the quality of the retrieval with respect to radar location, boundary conditions, length of assimilation, data filtering, smoothing enforced by penalty functions, and model accuracy. Verification of these experiments is provided by a dual-Doppler analysis. Test results show that the adjoint method is able to retrieve the wind and thermodynamic fields of the gust front. The retrieved horizontal wind agrees very well with the dual-Doppler ...

THE PENTAGON SHIELD FIELD PROGRAM Toward Critical Infrastructure Protection

The Pentagon, and its 25,000+ occupants, represents a likely target for a future terrorist attack using chemical, biological, or radiological material released into the atmosphere. Motivated by this, a building-protection system, called Pentagon Shield, is being developed and deployed by a number of government, academic, and private organizations. The system consists of a variety of data-assimilation and forecast models that resolve processes from the mesoscale to the city scale to the building scale, and assimilate meteorological and contaminant data that are measured by remote and in situ sensors. This paper reports on a field program that took place in 2004 in the area of the Pentagon, where the aim was to provide meteorological data and concentration data from tracer releases, and to support the development and evaluation of the system. In particular, the results of the field program are being used to improve our understanding of urban meteorological processes, verify the overall effectiveness of the ...

Numerical Simulations Initialized with Radar-Derived Winds. Pail II: Forecasts of Three Gust-Front Cases

Abstract Numerical simulations of three gust-front cases that occurred in northeastern Colorado during the summers of 1991 and 1992 am presented. The simulations are initialized with radar-derived winds and, for the two cases in 1992, measurements from a surface mesonet. Thermodynamic retrieval is used to calculate the buoyancy in the boundary layer. The sensitivity of the retrieved buoyancy to the various constraints of real data was examined in Part I of this study. In the first case, a large-scale gust front moved southward over the Denver region at a speed of 8–9 m s−1. The retrieved buoyancy field for this case exhibits a broad baroclinic zone, with a width of approximately 20 km centered about the radar-detected fine line. This baroclinic zone collapses to a width of about 5 km as the numerical model is integrated forward. The simulated gust front propagates at 7 m s−1, which is slightly less than the observed speed. For the second and third cases, data from a 50-station surface mesonet were also av...

Numerical Simulations Initialized with Radar-Derived Winds. Part I: Simulated Data Experiments

Abstract Techniques to initialize the boundary layer in a numerical model with radar-derived wind fields are tested with simulated data of a gust front. Experiments show that Newtonian relaxation or “nudging” applied to the velocity fields is not capable of retrieving the buoyancy in the gust front. The traditional thermodynamic retrieval method is then applied to the simulated velocity data. Tests are performed to determine the sensitivity to the tendency terms, time smoothing, length of the data assimilation window, random error in the velocity fields, sloping terrain, and the nonhydrostatic terms. Two methods to initialize the flow in the data-void region above the boundary layer are also examined. The first solves the linear gravity wave equation, given the forcing at the top of the boundary layer. The second assumes irrotational flow and a divergence profile that exponentially damps the motion in the vertical. Both methods result in less than 10% arms error after 40 min compared with over 40% error i...