Christopher A. Davis

Potential Vorticity Diagnostics of Cyclogenesis

Abstract The assumption of dynamically balanced flow allows one to completely encase the dynamics of extratropical cyclones in a potential vorticity (PV) framework. This approach offers a conceptually simple interpretation of dynamics because PV is a conserved quantity (in the absence of heating and friction) from which the flow itself can be deduced (the property of invertibility). The conservation law allows one to identify developments significantly influenced by heating and friction, and the invertibility property can be used to quantitatively measure such effects. We develop a diagnostic system based on the relative smallness of the irrotational part of the horizontal wind, which allows us to calculate the balanced flow given the three-dimensional distribution of Ertels PV. The close agreement between the observed and balanced flows, even for intense cyclones, illustrates the practical utility of the PV approach. Furthermore, we present a technique for determining the flow associated with individual...

The role of vortical hot towers in the formation of tropical cyclone Diana (1984)

Abstract A high-resolution (3-km horizontal grid spacing) near-cloud-resolving numerical simulation of the formation of Hurricane Diana (1984) is used to examine the contribution of deep convective processes to tropical cyclone formation. This study is focused on the 3-km horizontal grid spacing simulation because this simulation was previously found to furnish an accurate forecast of the later stages of the observed storm life cycle. The numerical simulation reveals the presence of vortical hot towers, or cores of deep cumulonimbus convection possessing strong vertical vorticity, that arise from buoyancy-induced stretching of local absolute vertical vorticity in a vorticity-rich prehurricane environment. At near-cloud-resolving scales, these vortical hot towers are the preferred mode of convection. They are demonstrated to be the most important influence to the formation of the tropical storm via a two-stage evolutionary process: (i) preconditioning of the local environment via diabatic production of mul...

Prediction of Landfalling Hurricanes with the Advanced Hurricane WRF Model

Abstract Real-time forecasts of five landfalling Atlantic hurricanes during 2005 using the Advanced Research Weather Research and Forecasting (WRF) (ARW) Model at grid spacings of 12 and 4 km revealed performance generally competitive with, and occasionally superior to, other operational forecasts for storm position and intensity. Recurring errors include 1) excessive intensification prior to landfall, 2) insufficient momentum exchange with the surface, and 3) inability to capture rapid intensification when observed. To address these errors several augmentations of the basic community model have been designed and tested as part of what is termed the Advanced Hurricane WRF (AHW) model. Based on sensitivity simulations of Katrina, the inner-core structure, particularly the size of the eye, was found to be sensitive to model resolution and surface momentum exchange. The forecast of rapid intensification and the structure of convective bands in Katrina were not significantly improved until the grid spacing ap...

Experiences with 0-36-h Explicit Convective Forecasts with the WRF-ARW Model

Abstract Herein, a summary of the authors’ experiences with 36-h real-time explicit (4 km) convective forecasts with the Advanced Research Weather Research and Forecasting Model (WRF-ARW) during the 2003–05 spring and summer seasons is presented. These forecasts are compared to guidance obtained from the 12-km operational Eta Model, which employed convective parameterization (e.g., Betts–Miller–Janjic). The results suggest significant value added for the high-resolution forecasts in representing the convective system mode (e.g., for squall lines, bow echoes, mesoscale convective vortices) as well as in representing the diurnal convective cycle. However, no improvement could be documented in the overall guidance as to the timing and location of significant convective outbreaks. Perhaps the most notable result is the overall strong correspondence between the Eta and WRF-ARW guidance, for both good and bad forecasts, suggesting the overriding influence of larger scales of forcing on convective development in...

Object-Based Verification of Precipitation Forecasts. Part I: Methodology and Application to Mesoscale Rain Areas

Abstract A recently developed method of defining rain areas for the purpose of verifying precipitation produced by numerical weather prediction models is described. Precipitation objects are defined in both forecasts and observations based on a convolution (smoothing) and thresholding procedure. In an application of the new verification approach, the forecasts produced by the Weather Research and Forecasting (WRF) model are evaluated on a 22-km grid covering the continental United States during July–August 2001. Observed rainfall is derived from the stage-IV product from NCEP on a 4-km grid (averaged to a 22-km grid). It is found that the WRF produces too many large rain areas, and the spatial and temporal distribution of the rain areas reveals regional underestimates of the diurnal cycle in rain-area occurrence frequency. Objects in the two datasets are then matched according to the separation distance of their centroids. Overall, WRF rain errors exhibit no large biases in location, but do suffer from a ...

Piecewise potential vorticity inversion

Abstract The treatment of the potential vorticity (PV) distribution as a composite of individual perturbations is central to the diagnostic and conceptual utility of PV. Nonlinearity in the inversion operator for Ertels potential vorticity renders quantitative piecewise inversion (inversion of individual portions of the potential vorticity field) ambiguous. Several methods of piecewise inversion are compared for idealized and observed potential vorticity anomalies of varying strengths. Even as the Rossby number of the balanced solutions increases well past unity, relative differences among the more plausible methods do not increase significantly near the anomaly. These relative differences are also found to be smaller than those obtained by comparing any of the methods to quasigeostrophic inversion. However, differences above and below anomalies increase with increasing Rossby number, suggesting that one cannot uniquely diagnose the interaction of large amplitude PV anomalies.

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

The Integrated Effect of Condensation in Numerical Simulations of Extratropical Cyclogenesis

Abstract By combining traditional sensitivity studies with techniques that focus on the conservation and invertibility properties of Ertels potential vorticity (PV), we illustrate the effect of latent heating on the structure and evolution of three simulated extratropical cyclones. The cases include one continental cyclone development (15 December 1987), which we examine extensively, and two cyclones over the western Atlantic Ocean (6 January 1983 and 5 January 1985) of somewhat greater intensity, which are diagnosed to assess the generality of our findings for the continental case. Each storm featured a weaker cyclonic low-level circulation when latent heating was removed from the simulation, but the magnitude of the effect varied greatly. In all cases, the difference in intensity was attributed to velocities associated with a positive, condensation-produced PV anomaly above the warm front. The amplification of the surface thermal perturbations was not strongly altered in even the case most affected by ...

Object-Based Verification of Precipitation Forecasts. Part II: Application to Convective Rain Systems

The authors develop and apply an algorithm to define coherent areas of precipitation, emphasizing mesoscale convection, and compare properties of these areas with observations obtained from NCEP stage-IV precipitation analyses (gauge and radar combined). In Part II, fully explicit 12–36-h forecasts of rainfall from the Weather Research and Forecasting model (WRF) are evaluated. These forecasts are integrated on a 4-km mesh without a cumulus parameterization. Rain areas are defined similarly to Part I, but emphasize more intense, smaller areas. Furthermore, a time-matching algorithm is devised to group spatially and temporally coherent areas into rain systems that approximate mesoscale convective systems. In general, the WRF model produces too many rain areas with length scales of 80 km or greater. Rain systems typically last too long, and are forecast to occur 1–2 h later than observed. The intensity distribution among rain systems in the 4-km forecasts is generally too broad, especially in the late afternoon, in sharp contrast to the intensity distribution obtained on a coarser grid with parameterized convection in Part I. The model exhibits the largest positive size and intensity bias associated with systems over the Midwest and Mississippi Valley regions, but little size bias over the High Plains, Ohio Valley, and the southeast United States. For rain systems lastin g6ho rmore, the critical success index for matching forecast and observed rain systems agrees closely with that obtained in a related study using manually determined rain systems.

The Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) Experiment: Scientific Basis, New Analysis Tools, and Some First Results

The principal hypotheses of a new model of tropical cyclogenesis, known as the marsupial paradigm, were tested in the context of Atlantic tropical disturbances during the National Science Foundation (NSF)-sponsored Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment in 2010. PREDICT was part of a tri-agency collaboration, along with the National Aeronautics and Space Administrations Genesis and Rapid Intensification Processes (NASA GRIP) experiment and the National Oceanic and Atmospheric Administrations Intensity Forecasting Experiment (NOAA IFEX), intended to examine both developing and nondeveloping tropical disturbances. During PREDICT, a total of 26 missions were flown with the NSF/NCAR Gulfstream V (GV) aircraft sampling eight tropical disturbances. Among these were four cases (Fiona, ex-Gaston, Karl, and Matthew) for which three or more missions were conducted, many on consecutive days. Because of the scientific focus on the Lagrangian nature of the tropical cyclogen...