William M. Frank

Effects of Vertical Wind Shear on the Intensity and Structure of Numerically Simulated Hurricanes

A series of numerical simulations of tropical cyclones in idealized large-scale environments is performed to examine the effects of vertical wind shear on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic Pennsylvania State University‐National Center for Atmospheric Research fifth-generation Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When large-scale vertical shears are applied to mature tropical cyclones, the storms quickly develop wavenumber one asymmetries with upward motion and rainfall concentrated on the left side of the shear vector looking downshear, in agreement with earlier studies. The asymmetries develop due to the storm’s response to imbalances caused by the shear. The storms in shear weaken with time and eventually reach an approximate steady-state intensity that is well below their theoretical maximum potential intensity. As expected, the magnitude of the weakening increases with increasing shear. All of the storms experience time lags between the imposition of the large-scale shear and the resulting rise in the minimum central pressure. While the lag is at most a few hours when the storm is placed in very strong (15 m s21) shear, storms in weaker shears experience much longer lag times, with th e5ms 21 shear case showing no signs of weakening until more than 36 h after the shear is applied. These lags suggest that the storm intensity is to some degree predictable from observations of largescale shear changes. In all cases both the development of the asymmetries in core structure and the subsequent weakening of the storm occur before any resolvable tilt of the storm’s vertical axis occurs. It is hypothesized that the weakening of the storm occurs via the following sequence of events: First, the shear causes the structure of the eyewall region to become highly asymmetric throughout the depth of the storm. Second, the asymmetries in the upper troposphere, where the storm circulation is weaker, become sufficiently strong that air with high values of potential vorticity and equivalent potential temperature are mixed outward rather than into the eye. This allows the shear to ventilate the eye resulting in a loss of the warm core at upper levels, which causes the central pressure to rise, weakening the entire storm. The maximum potential vorticity becomes concentrated in saturated portions of the eyewall cloud aloft rather than in the eye. Third, the asymmetric features at upper levels are advected by the shear, causing the upper portions of the vortex to tilt approximately downshear. The storm weakens from the top down, reaching an approximate steady-state intensity when the ventilated layer can descend no farther due to the increasing strength and stability of the vortex at lower levels.

The Structure and Energetics of the Tropical Cyclone I. Storm Structure

Abstract A composite study of 10 years of northwest Pacific rawinsonde data is used to analyze the large-scale structure of tropical cyclones. The temperature, height, moisture, wind and vertical motion fields are analyzed for various storm regions. Mean soundings for all regions from the eye through 12° radius are presented. Rainfall characteristics of the area inside 4° radius are discussed. Hurricane flight data are used to augment the analyses in the inner regions. Many important features are noted. Strong persistent asymmetries in storm structure exist, particularly at large radii. The storm circulation has very broad horizontal extent and appears to conform to a constant scale regardless of inner core intensity. Inflow in the middle troposphere is substantial from 4° outward. A mean subsidence region is observed from about 4–6° radius. Humidities are extremely high in the inner regions, and conditional instability exists everywhere outside the eye. Significant diurnal variations in rainfall and temp...

Effects of Environmental Flow upon Tropical Cyclone Structure

Abstract Numerical simulations of tropical-cyclone-like vortices are performed to analyze the effects of unidirectional vertical wind shear and translational flow upon the organization of convection within a hurricane’s core region and upon the intensity of the storm. A series of dry and moist simulations is performed using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 5 (MM5) with idealized initial conditions. The dry simulations are designed to determine the patterns of forced ascent that occur as the vortex responds to imposed vertical wind shear and translational flow, and the mechanisms that modulate the vertical velocity field are explored. The moist simulations are initialized with the same initial conditions as the dry runs but with a cumulus parameterization and explicit moisture scheme activated. The moist simulations are compared to the dry runs in order to test the hypothesis that the forced vertical circulation modes modulate the convection...

The Role of Tropical Waves in Tropical Cyclogenesis

Abstract This paper analyzes relationships between tropical wave activity and tropical cyclogenesis in all of the earth’s major tropical cyclone basins. Twenty-nine years of outgoing longwave radiation data and global reanalysis winds are filtered and analyzed to determine statistical relationships between wave activity in each basin and the corresponding cyclogenesis. Composite analyses relative to the storm genesis locations show the structures of the waves and their preferred phase relationships with genesis. Five wave types are examined in this study, including mixed Rossby–gravity waves, tropical-depression-type or easterly waves, equatorial Rossby waves, Kelvin waves, and the Madden–Julian oscillation. The latter is not one of the classical tropical wave types, but is a wavelike phenomenon known to have a strong impact on tropical cyclogenesis. Tropical cyclone formation is strongly related to enhanced activity in all of the wave filter bands except for the Kelvin band. In each basin the structure o...

A Climatology of Waves in the Equatorial Region

Abstract Propagating anomalies of moisture and moist deep convection in the Tropics are organized into a variety of large-scale modes. These include (but are not limited to) the so-called intraseasonal oscillations, convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane, and tropical-depression-type disturbances. Along with the annual and diurnal cycles, these modes act and interact to control much of the variance of tropical convection. Analyses of 10 yr of outgoing longwave radiation (OLR) and precipitable water (PW) data are carried out to develop comparative climatologies of these wavelike modes. The analysis relaxes the commonly used cross-equatorial symmetry constraints, which allows study of the portions of the wavelike processes that are asymmetric across the equator. Mean background states are found for OLR and for PW as functions of day of the year. Examination of anomalies together with the background reveals much about how the waves are affec...

Large-Scale Influences on Tropical Cyclogenesis in the Western North Pacific

Abstract Objectively analyzed data from the European Centre for Medium-Range Weather Forecasts are used to examine the large-scale aspects of the formation of tropical cyclones. It is hypothesized that tropical cyclogenesis occurs when external atmospheric forcing on the synoptic or larger scale provides uplift through a deep layer, enhancing convection, in a region with environmental conditions favorable for genesis. Emphasis is placed on the roles of upper-level troughs, low-level wind surges, preexisting tropical cyclones, and propagating wave disturbances in triggering tropical cyclogenesis. Composites of the 200-hPa and 850-hPa flows reveal the presence of both upper-level troughs and low-level wind surges, respectively, prior to genesis. In the composites, the wind surges also appear to be related to the presence of a prior circulation located approximately 2000 km to the west of the genesis location. An examination of the individual cases demonstrates that approximately 85% of all storms had either...

The Cumulus Parameterization Problem

Abstract In the past two decades there has been extensive research into the nature of atmospheric convection and scale interactions in cumulus regimes. A major goal of these efforts has been to advance the state of the art in cumulus parameterization. This paper reviews the cumulus parameterization problem in terms of fundamental principles, goals and dynamics constraints as they apply to parameterization in mesoscale and large scale numerical models. Several popular current schemes are discussed in terms of their relationships to these overall aspects of the problem.

Relationships between Stability and Monsoon Convection

Abstract Rawinsonde data from the Australian Monsoon Experiment are analyzed to determine the manner in which the atmospheric stratification of density and moisture respond to large amounts of convective latent heat release. The study focuses on time series of data from a ship located at the northern end of the Gulf of Carpentaria during active and break periods of the monsoon. Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to midtropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occurred in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection. Conve...

The Interannual Variability of Tropical Cyclones

Abstract This paper examines the interannual variability of tropical cyclones in each of the earth’s cyclone basins using data from 1985 to 2003. The data are first analyzed using a Monte Carlo technique to investigate the long-standing myth that the global number of tropical cyclones is less variable than would be expected from examination of the variability in each basin. This belief is found to be false. Variations in the global number of all tropical cyclones are indistinguishable from those that would be expected if each basin was examined independently of the others. Furthermore, the global number of the most intense storms (Saffir–Simpson categories 4–5) is actually more variable than would be expected because of an observed tendency for storm activity to be correlated between basins, and this raises important questions as to how and why these correlations arise. Interbasin correlations and factor analysis of patterns of tropical cyclone activity reveal that there are several significant modes of v...

The Vertical Distribution of Heating in AMEX and GATE Cloud Clusters

Abstract Tropical cloud clusters that occurred during the Australian Monsoon Experiment (AMEX) are composited and compared to a composite of the GARP Atlantic Tropical Experiment (GATE) systems. The analysis focuses on the evolution of the life cycles and upon the vertical heating profiles. The AMEX and GATE systems were of comparable duration and magnitude, although the former produced more rainfall. However, AMEX convective systems produced maximum heating in the middle troposphere and showed only small variations in the heating with height. In contrast GATE systems began with heating concentrated in the lower troposphere and exhibited a marked upward shift in heating with time. GATE systems always had greater fractions of their total heating at lower levels than did AMEX systems, presumably due to differences in the large flow. The vertical stratification of the atmosphere in both regions resembles that of a reversible moist adiabat at lower levels and of a pseudoadiabat above the freezing level. This ...