Frank D. Marks

Low-Wavenumber Structure and Evolution of the Hurricane Inner Core Observed by Airborne Dual-Doppler Radar

Abstract The asymmetric dynamics of the hurricane inner-core region is examined through a novel analysis of high temporal resolution, three-dimensional wind fields derived from airborne dual-Doppler radar. Seven consecutive composites of Hurricane Olivia’s (1994) wind field with 30-min time resolution depict a weakening storm undergoing substantial structural changes. The symmetric and asymmetric mechanisms involved in this transformation are considered separately. To zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of Eliassen and Lystad for a neutrally stratified atmosphere. Vertical shear, however, increased dramatically during the observation period, leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector. Recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry. The role of asymmetric vorticity d...

Stationary and moving convective bands in hurricanes

Abstract Aircraft observations in hurricanes indicate that the hurricane vortex may be subdivided into an inner gyre where the air trajectories form closed paths and an outer envelope where they do not. In the closed gyre, a core of air moves with the vortex; in the envelope, environmental air passes through the vortex and around the core. A system of spiral bands, termed the stationary band complex (SBC), forms near the boundary between the core and the envelope where the Rossby number is of order unity. The SBC differs dynamically both from convective rings because it is asymmetric and from propagating gravity-wave bands because its Doppler-shifted frequency is below the local inertia frequency. In more intense systems with stronger convective instability, the SBC may evolve into a convective ring and move into the vortex core. Outward propagating gravity-wave bands have also been observed. Such bands are often associated with track oscillations as the storm makes landfall or recurves. Spiral-shaped ent...

Inner Core Structure of Hurricane Alicia from Airborne Doppler Radar Observations

Abstract Airborne Doppler radar measurements are used to determine the horizontal winds, vertical air motions, radar reflectivity and hydrometer fallspeeds over much of the inner-core region (within 40 km of the eye) of Hurricane Alicia (1983). The reconstructed flow field is more complete and detailed than any obtained previously. The data show both the primary (azimuthal) and secondary (radial-height) circulations. The primary circulation was characterized by an outward sloping maximum of tangential wind. The secondary circulation was characterized by a deep layer of radial inflow in the lower troposphere and a layer of intense outflow above 10 km altitude. The rising branch of the secondary circulation was located in the eyewall and sloped radially outward. Discrete convective-scale bubbles of more intense upward motion were superimposed on this mean rising current, and convective-scale downdrafts were located throughout and below the core of maximum precipitation in the eyewall. Precipitation particle...

Precipitation Distribution in Tropical Cyclones Using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager: A Global Perspective

TRMM microwave imager rain estimates are used to quantify the spatial distribution of rainfall in tropical cyclones (TCs) over the global oceans. A total of 260 TCs were observed worldwide from 1 January 1998‐31 December 2000, providing 2121 instantaneous TC precipitation observations. To examine the relationship between the storm intensity, its geographical location, and the rainfall distribution, the dataset is stratified into three intensity groups and six oceanic basins. The three intensity classes used in this study are tropical storms (TSs) with winds ,33 m s21, category 1‐2 hurricane-strength systems (CAT12) with winds from 34‐48 m s21, and category 3‐5 systems (CAT35) with winds .49 m s21. The axisymmetric component of the TC rainfall is represented by the radial distribution of the azimuthal mean rainfall rates ( R). The mean rainfall distribution is computed using 10-km annuli from the storm center to a 500-km radius. The azimuthal mean rain rates vary with storm intensity and from basin to basin. The maximum R is about 12 mm h21 for CAT35, but decreases to 7m m h 21 for CAT12, and to 3 mm h21 for TS. The radius from the storm center of the maximum rainfall decreases with increasing storm intensity, from 50 km for TS to 35 km for CAT35 systems. The asymmetric component is determined by the first-order Fourier decomposition in a coordinate system relative to the storm motion. The asymmetry in TC rainfall varies significantly with both storm intensity and geographic locations. For the global average of all TCs, the maximum rainfall is located in the front quadrants. The location of the maximum rainfall shifts from the front-left quadrant for TS to the front-right for CAT35. The amplitude of the asymmetry varies with intensity as well; TS shows a larger asymmetry than CAT12 and CAT35. These global TC rainfall distributions and variability observed in various ocean basins should help to improve TC rainfall forecasting worldwide.

Winter Monsoon Convection in the Vicinity of North Borneo. Part I: Structure and Time Variation of the Clouds and Precipitation

Abstract Radar and satellite observations in the vicinity of northern Borneo obtained during the International Winter Monsoon Experiment (WMONEX) showed that the convection in that region underwent an extremely regular diurnal cycle. Over the sea to the north of Borneo, the general level of convective activity was increased during monsoon surges and during the passages of westward-propagating near-equatorial disturbances. Convective activity was decreased during monsoon lulls. The diurnal cycle was well-defined, regardless of whether, the general level of convective activity was enhanced or suppressed by synoptic-scale events. The cycle of convection over the sea was especially well documented. It was typically initiated at about midnight when an offshore low-level wind began. Where this wind met the monsoonal northeasterly flow, usually just off the coast, convective cells formed. After midnight, the convection continued to develop and by 0800 LST it had evolved into an organized mesoscale system with a ...

Dual-Aircraft Investigation of the Inner Core of Hurricane Norbert. Part I: Kinematic Structure

Abstract On 24–25 September 1984 air motions and physical processes in the eyewall region of Hurricane Norbert were investigated with two National Oceanic and Atmospheric Administration (NOAA) Aircraft Operations Center (AOC) WP-3D research aircraft. One aircraft, equipped with the airborne Doppler radar, flew repeated radial penetrations of the eyewall, mapping the three-dimensional wind field throughout a 40-km radius of the storm center. These data provide the first complete documentation of the three-dimensional wind field of the inner core of a hurricane on spatial scales of 10–25 km in the horizontal and 1 km in the vertical and temporal scales of 1–2 h. The wind field was asymmetric, and the asymmetry varied with altitude. As altitude increased the location of the maximum tangential wind sloped upwind, from left of the storm track at 1-km altitude to right of the storm track above 3 km. The radial wind at 1-km altitude was inward in front of the storm and outward behind. This pattern in the radial ...

Effects of Vertical Wind Shear and Storm Motion on Tropical Cyclone Rainfall Asymmetries Deduced from TRMM

Abstract Vertical wind shear and storm motion are two of the most important factors contributing to rainfall asymmetries in tropical cyclones (TCs). Global TC rainfall structure, in terms of azimuthal distribution and asymmetries relative to storm motion, has been previously described using the Tropical Rainfall Measuring Mission Microwave Imager rainfall estimates. The mean TC rainfall distribution and the wavenumber-1 asymmetry vary with storm intensity and geographical location among the six oceanic basins. This study uses a similar approach to investigate the relationship between the structure of TC rainfall and the environmental flow by computing the rainfall asymmetry relative to the vertical wind shear. The environmental vertical wind shear is defined as the difference between the mean wind vectors of the 200- and 850-hPa levels over an outer region extending from the radius of 200–800 km around the storm center. The wavenumber-1 maximum rainfall asymmetry is downshear left (right) in the Northern ...

Vertical Motion Characteristics of Tropical Cyclones Determined with Airborne Doppler Radial Velocities

Abstract Vertical motions in seven Atlantic hurricanes are determined from data recorded by Doppler radars on research aircraft. The database consists of Doppler velocities and reflectivities from vertically pointing radar rays collected along radial flight legs through the hurricane centers. The vertical motions are estimated throughout the depth of the troposphere from the Doppler velocities and bulk estimates of particle fallspeeds. Portions of the flight tracks are subjectively divided into eyewall, rainband, stratiform, and “other” regions. Characteristics of the vertical velocity and radar structure are described as a function of altitude for the entire dataset and each of the four regions. In all of the regions, more than 70% of the vertical velocities range from −2 to 2 m s−1. The broadest distribution of vertical motion is in the eyewall region where ∼5% of the vertical motions are >5 m s−1. Averaged over the entire dataset, the mean vertical velocity is upward at all altitudes. Mean downward mot...

Evolution of the Structure of Precipitation in Hurricane Allen (1980)

Abstract Reflectivity data from the airborne radar systems on board the three NOAA aircraft were gathered during six consecutive days in Hurricane Allen of 1980. The data have been used to specify the horizontal and vertical precipitation distribution within 111 km radius of the hurricane center. The evolution of the structure and intensity of the precipitation in the storm is described from representative time composite radar maps for seven research flights made during the 6-day period. The eyewall was characterized by a narrow ring (12–15 km wide) of intense reflectivity (42–47 dBZ) surrounding the center of the storm at a radius that varied in time from 12–40 km. The eyewall had steep radial gradients of reflectivity (4–5 dB km−1) and tilted radially outward in height. The rainbands were characterized by areas of enhanced reflectivity embedded in a region of stratiform rainfall that contained a distinct bright band at the height of the 0°C isotherm. The most striking changes in structure during the 6-d...

On the Characteristic Height Scales of the Hurricane Boundary Layer

In this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to studythecharacteristicheightscalesofthehurricaneboundarylayer.Theheightscalesaredefinedinavariety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximumwind speed and the inflow layerdepth are referred to asthe dynamical boundarylayer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.