Jeffrey D. Kepert

Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part I: Hurricane Georges

Abstract The GPS dropsonde allows observations at unprecedentedly high horizontal and vertical resolution, and of very high accuracy, within the tropical cyclone boundary layer. These data are used to document the boundary layer wind field of the core of Hurricane Georges (1998) when it was close to its maximum intensity. The spatial variability of the boundary layer wind structure is found to agree very well with the theoretical predictions in the works of Kepert and Wang. In particular, the ratio of the near-surface wind speed to that above the boundary layer is found to increase inward toward the radius of maximum winds and to be larger to the left of the track than to the right, while the low-level wind maximum is both more marked and at lower altitude on the left of the storm track than on the right. However, the expected supergradient flow in the upper boundary layer is not found, with the winds being diagnosed as close to gradient balance. The tropical cyclone boundary layer model of Kepert and Wan...

Choosing a Boundary Layer Parameterization for Tropical Cyclone Modeling

AbstractThe boundary layer in a tropical cyclone is in some respects unlike that elsewhere in the atmosphere. It is therefore necessary to evaluate boundary layer parameterizations for their suitability for use in tropical cyclone simulation. Previous work has shown substantial sensitivity to the choice of scheme and identified specific shortcomings in some schemes, but without recommending which schemes are most suitable. Here, several schemes, representative of those available in popular modeling systems, are reviewed and applied in a simplified modeling framework. Based on a comparison with observations and on theoretical grounds, one popular class of schemes is shown to be badly flawed in that it incorrectly predicts the near-surface wind profile, and therefore should not be used. Another is shown to be sensitive to diagnosis of the boundary layer depth, a difficult problem in the core of the tropical cyclone, and caution is advised. The Louis boundary layer scheme and a higher-order closure scheme ar...

Estimating Maximum Surface Winds from Hurricane Reconnaissance Measurements

Radial profiles of surface winds measured by the Stepped Frequency Microwave Radiometer (SFMR) are compared to radial profiles of flight-level winds to determine the slant ratio of the maximum surface wind speed to the maximum flight-level wind speed, for flight altitude ranges of 2‐4 km. The radius of maximum surface wind is found on average to be 0.875 of the radius of the maximum flight-level wind, and very few cases have a surface wind maximum at greater radius than the flight-level maximum. The mean slant reduction factor is 0.84 with a standard deviation of 0.09 and varies with storm-relative azimuth from a maximum of 0.89 on the left side of the storm to a minimum of 0.79 on the right side. Larger slant reduction factors are found in small storms with large values of inertial stability and small values of relative angular momentum at the flight-level radius of maximum wind, which is consistent with Kepert’s recent boundary layer theories. The global positioning system (GPS) dropwindsonde-based reduction factors that are assessed using this new dataset have a high bias and substantially larger RMS errors than the new technique. A new regression model for the slant reduction factor based upon SFMR data is presented, and used to make retrospective estimates of maximum surface wind speeds for significant Atlantic basin storms, including Hurricanes Allen (1980), Gilbert (1988), Hugo (1989), Andrew (1992), and Mitch (1998).

How Does the Boundary Layer Contribute to Eyewall Replacement Cycles in Axisymmetric Tropical Cyclones

AbstractThree diagnostic models of the axisymmetric tropical cyclone boundary layer, with different levels of approximation, are applied to the problem of tropical cyclones with concentric eyewalls. The outer eyewall is shown to have an inherently stronger frictional updraft than the inner because it is in an environment of lower vorticity. Similarly, a relatively weak local enhancement of the radial vorticity gradient outside the primary radius of maximum winds can produce a significant frictional updraft, even if there is no outer wind maximum. Based on these results, it is proposed that the boundary layer contributes to the formation of outer eyewalls through a positive feedback among the local enhancement of the radial vorticity gradient, the frictional updraft, and convection. The friction-induced secondary circulation associated with the inner eyewall is shown to weaken as the outer wind maximum strengthens and/or contracts, so boundary layer processes will contribute, along with the heating-induced...

The Boundary Layer Winds in Hurricanes Danielle (1998) and Isabel (2003)

Abstract This paper describes the boundary layer wind structure and dynamics of Hurricanes Danielle (1998) and Isabel (2003), based on the analysis of high-resolution global positioning system dropwindsonde data and simulation of the flow by a three-dimensional boundary layer model produced by Kepert and Wang. The observations show that the hurricane boundary layer has a complex three-dimensional structure with large variability over small distances. The analysis emphasizes three aspects: the degree of gradient-wind balance, the radially varying depth of the boundary layer, and the strength of the near-surface wind speed relative to that at a higher level. Each aspect is compared both with results obtained in a simulation of the individual storm by Kepert and Wang’s model and with theoretical predictions. The observations show that the boundary layer depth decreases toward the center of the storm, consistent with theoretical arguments. The strongest azimuthal winds occur near the top of, but still within,...

Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part II: A Diagnosis of Tropical Cyclone Chris Formation

This is the second of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS). The primary TC-LAPS vortex enhancement mechanism (convergence/stretching and vertical advection of absolute vorticity in convective updraft regions) was presented in Part I. In this paper (Part II) results from a numerical simulation of TC Chris (western Australia, February 2002) are used to illustrate the primary and two secondary vortex enhancement mechanisms that led to TC genesis. In Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. During the first 18 h of the simulation, a mature vortex of TC intensity developed in a monsoon low from a relatively benign initial state. Deep upright vortex cores developed from convergence/stretching and vertical advection of absolute vorticity within the updrafts of intense bursts of cumulus convection. Individual convective bursts lasted for 6–12 h, with a new burst developing as the previous one weakened. The modeled bursts appear as single updrafts, and represent the mean vertical motion in convective regions because the 0.15° grid spacing imposes a minimum updraft scale of about 60 km. This relatively large scale may be unrealistic in the earlier genesis period when multiple smaller-scale, shorter-lived convective regions are often observed, but observational evidence suggests that such scales can be expected later in the process. The large scale may limit the convection to only one or two active bursts at a time, and may have contributed to a more rapid model intensification than that observed. The monsoon low was tilted to the northwest, with convection initiating about 100–200 km west of the low-level center. The convective bursts and associated upright potential vorticity (PV) anomalies were advected cyclonically around the low, weakening as they passed to the north of the circulation center, leaving remnant cyclonic PV anomalies. Strong convergence into the updrafts led to rapid ingestion of nearby cyclonic PV anomalies, including remnant PV cores from decaying convective bursts. Thus convective intensity, rather than the initial vortex size and intensity, determined dominance in vortex interactions. This scavenging of PV by the active convective region, termed diabatic upscale vortex cascade, ensured that PV cores grew successively and contributed to the construction of an upright central monolithic PV core. The system-scale intensification (SSI) process active on the broader scale (300–500-km radius) also contributed. Latent heating slightly dominated adiabatic cooling within the bursts, which enhanced the system-scale secondary circulation. Convergence of low- to midlevel tropospheric absolute vorticity by this enhanced circulation intensified the system-scale vortex. The diabatic upscale vortex cascade and SSI are secondary processes dependent on the locally enhanced vorticity and heat respectively, generated by the primary mechanism.

A Simple Model of the Australian West Coast Trough

Abstract The Australian west coast trough forms near the west coast in the easterly flow over Australia in the warmer months of the year. Its development and movement is the major synoptic influence on the weather in those months, particularly in the production of extreme maximum temperatures and subsequent “cool changes.” This paper begins with a brief discussion of the climatology of the trough, followed by a case study. The main focus is on the development and interpretation of a simple dynamical model of the trough. The model is an adaptation of the linear diabatic equatorial β-plane models of Matsuno and of Gill for the particular situation of the trough. The study leads to a new hypothesis on the role of the sea breeze in the dynamics of the trough.

Reply to ''Comments on 'How Does the Boundary Layer Contribute to Eyewall Replacement Cycles in Axisymmetric Tropical Cyclones?'''

Kepert (2013, hereafter K13) presented a series of idealized simulations of the boundary layer (BL) flow in axisymmetric tropical cyclones (TCs) with secondary wind maxima of various strengths using a hierarchy of diagnostic BL models. Three models, with different levels of approximation, were used because each has its own advantages. In particular, the nonlinear model of Kepert and Wang (2001, hereafterKW01)is expected to bethemostaccurateofthesebecauseitcontainsmoreof the relevant physics, including more realistic parameterizations of the turbulence and surface fluxes, while the simplermodels ofKepert(2001)andOoyama(1969) trade off some physical realism against the benefits of an analytical solution and the consequent ease of physical interpretation. These models, in common with many other models of the TC BL, diagnose the steady-state boundary layer flow in response to an applied pressure field representative of a TC. That is, they assume that the BL is, to good approximation, ‘‘slaved’’ to the rest of the cyclone. Whilethere are influencesin the opposite direction—for instance, the frictional convergence within the BL strongly influences the distribution of convection and hence heating within the storm—the aim with these simplified models is to study one side of a two-way interaction. Perhaps the most surprisingfinding ofK13was that, in a TC with concentric wind maxima of similar strength, the outer wind maximum produces a much stronger frictional updraft. Similarly, an outer wind bump—too weak to be a local maximum—can produce a frictional updraft as strong as that of the primary eyewall. These results were introduced using the nonlinear BL model, and a physical interpretation of their cause offered based on the updraft equation from the linear model

On the Use of Potential Vorticity Tendency Equations for Diagnosing Atmospheric Dynamics in Numerical Models

AbstractThis study critically assesses potential vorticity (PV) tendency equations used for analyzing atmospheric convective systems. A generic PV tendency format is presented to provide a framework for comparing PV tendency equations, which isolates the contributions to PV tendency from wind and mass field changes. These changes are separated into forcing terms (e.g., diabatic or friction) and flow adjustment and evolution terms (i.e., adiabatic motions).One PV tendency formulation analyzed separates PV tendency into terms representing PV advection and diabatic and frictional PV sources. In this form the PV advection is shown to exhibit large cancellation with the diabatic forcing term when used to analyze deep convective systems, which compromises the dynamical insight that the PV tendency analysis should provide. The isentropic PV substance tendency formulation of Haynes and McIntyre does not suffer from this cancellation problem. However, while the Haynes and McIntyre formulation may be appropriate fo...

Why is the tropical cyclone boundary layer not "well-mixed"?

AbstractPlausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer ad...