Showing papers by "Christopher A. Davis published in 2002"

Numerical Simulations of the Genesis of Hurricane Diana (1984). Part II: Sensitivity of Track and Intensity Prediction

Abstract: The authors examine numerous simulations that probe the dynamics governing the intensification and track of Tropical Cyclone Diana (1984) simulated in Part I. The development process is fundamentally dependent on a preexisting upper-tropospheric trough–ridge couplet. This couplet focuses mesoscale vertical motion that, in turn, produces lower-tropospheric potential vorticity (PV) anomalies, which form the seed for the tropical storm. Removal of this trough–ridge couplet from the initial conditions eliminates cyclogenesis. The simulated rate of development of Diana in the prehurricane phase depends principally on choices of cumulus parameterization, boundary layer treatment, sea surface temperature, and grid spacing. Simulations with cumulus schemes that allow more grid-scale precipitation on the 9-km grid exhibit unrealistic grid-scale overturning and slower intensification, primarily due to production of cyclonic vorticity anomalies at large radii. Use of an innermost nest with 3-km grid spacing...

Influence of Balanced Motions on Heavy Precipitation within a Long-Lived Convectively Generated Vortex

Abstract: The forcing of heavy precipitation within a long-lived convectively generated mesoscale vortex (MCV) is investigated with the aid of diagnoses from Rapid Update Cycle gridded analyses. Organized convection within the MCV followed a distinct diurnal cycle, which featured organized mesoscale convective systems (MCSs) that matured overnight near the MCV center on successive days. The MCV was typically most intense in the middle troposphere, but intensified within the lower troposphere during the episodes of organized nocturnal convection. The lower-tropospheric vertical shear was an important organizing factor in MCS development and sustenance, in the sense that its interaction with the cold temperature anomaly beneath the MCV center determined where balanced lower-tropospheric ascent occurred. From trajectory analyses, we estimate that balanced ascent accounted for approximately half of the total vertical displacement of the thermodynamically unstable air that eventually composed elevated saturated...

Detection and Prediction of Warm Season Midtropospheric Vortices by the Rapid Update Cycle

Abstract: The authors perform a statistical and dynamical analysis of midtropospheric mesoscale vortices captured by analyses from the Rapid Update Cycle, version 2 (RUC-2), during the period 1 May to 31 August 1999. A total of 203 vortices meeting conditions of an automated algorithm were found. Of these, 86 were observed to form within organized convection and were termed mesoscale convective vortices (MCVs). MCVs were favored over a broad area from eastern Colorado and western Nebraska to the Mississippi River valley, essentially collocated with the loci of organized convection. The remaining 117 vortices (termed dry vortices) clustered in the immediate lee of the Rocky Mountains and over the southeastern United States. Vortices arising from convection had considerably greater intensity and longevity than dry vortices. They were roughly five times more likely to be involved with the triggering of new convection. A relationship was found between intensity and longevity such that there appears to be a maximum vortex lifetime that can be estimated from its maximum intensity. Vortices arising from convection had markedly greater humidity and water vapor mixing ratio underneath their centers compared to dry vortices, consistent with many dry vortices having a topographic origin and MCVs arising from organized convection. Parameters such as horizontal scale, background vertical wind shear, and horizontal deformation were not systematically related to intensity or longevity. Prediction of mesoscale vortices by the RUC-2 was examined for a subsample of all cases. In general, the RUC-2 was able to predict the evolution of vortices once analyzed, but had virtually no skill at predicting (12 h in advance) the formation of the vortices. However, forecasts of organized convection should still benefit from accurate predictions of long-lived vortex tracks.

Cloud-Resolving Simulations of Mesoscale Vortex Intensification and Its Effect on a Serial Mesoscale Convective System

Abstract: The authors performed numerical simulations of the 27–28 May 1998 mesoscale convective vortex (MCV) and mesoscale convective system (MCS) to understand how the MCV modulated the convection, and how the convection intensified the vortex. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model (MM5) was used, initialized with the analysis from the Rapid Update Cycle version 2 (RUC-2) and nested to achieve 1.5-km grid spacing covering the MCS. The model successfully simulated a north–south convective band that formed near the center of the MCV in the early evening. The simulation then correctly reoriented the convection along a roughly east–west line in response to northward transport of warm, conditionally unstable air within the nocturnal low-level jet. Balanced vertical motion was found to contribute substantially to mesoscale lifting and thermodynamic destabilization, which localized the convection. Horizontal transport of moist, uns...