Fifth International Workshop on Tropical Cyclones

Topic 2.4 Coastal and Inland Wind Structure Changes

Rapporteur: Jeffrey D. Kepert
Bureau of Meteorology Research Centre
GPO Box 1289K
150 Lonsdale St
Melbourne Vic 3001

Email: J.Kepert@bom.gov.au
Phone: +61+3+96694492 (voice) +61+3+96694660 (fax)

Working Group: Bruce Harper, Kendal McGuffie.

2.4.1 Introduction

Tropical cyclone landfall has become a ‘hot topic’ over the past few years. The changes in structure, the eventual decay, and rainfall, are all important topics with significant impacts on people and structures. The purpose of this report is to summarize the current state of knowledge on the landfall-induced changes in the boundary-layer winds. Rainfall, inland flooding, and related topics are included in subtopics 2.1 to 2.3, and storm surge in 2.5.

At landfall, the cyclone usually encounters a rougher surface with increased friction. At the same time, the surface energy flux that fuels the storm is disrupted. These changes lead to changes in the cyclone on time-scales ranging
from the formation of convective cells due to frictional convergence at the coast, to the slow decay of the storm as a whole due to the substantial drop in the surface enthalpy flux.

The land surface properties are changed significantly by the cyclone. In particular, rain cools the surface directly and when re-evaporated by high winds. The cooler surface further reduces the surface energy flux. Increased adiabatic cooling due to the stronger frictional inflow, coupled with the reduced heat transfer, leads to a marked cooling of the boundary layer. These stability changes in turn contribute to a more rapid reduction in the surface winds, relative to those aloft.

A variety of meso- to micro-scale phenomena are known or hypothesized to affect the winds and concomitant damage during and following landfall. These include boundary layer rolls, eye asymmetries, tornadoes, and convective downdrafts.

Landfall becomes particularly complex in the presence of significant mountains. While fresh-water flooding becomes the major concern, interaction of the winds with the terrain can lead to locally much stronger winds. In addition, elevated sites may be closer to the upper boundary-layer wind speed maximum. Finally, the orography may cause changes in the structure and track of the cyclone.

The aims here are to briefly summarize, and to provoke discussion. More comprehensive recent reviews may be found in Powell (2000) and Foley (2002).

2.4.2 Impacts of landfall

The discussion covers four relevant ways the land differs from the ocean: it is rougher, more heterogeneous, colder, and hillier. Other relevant aspects of boundary-layer winds are then reviewed.

a) The land is rougher:

The most obvious difference between land and sea is that the former is rougher. Even at hurricane-force winds, current parameterizations indicate the roughness length of the sea is of the order of a few millimeters, while land roughness lengths are typically several centimeters for open fields, and greater for forested or urban areas. Although our knowledge of air-sea interaction at these wind speeds is incomplete, there is clearly a substantial change in the surface friction at landfall. However, it may not be as abrupt as the above suggests, since shallow water is known to produce shorter, steeper waves, with a concomitant increase in roughness (M. Powell, 2002, personal communication).

Boundary-layer flow over a roughness change produces an internal boundary layer (IBL) in which the flow has adjusted to the new surface condition, while the flow above retains the characteristics of the old surface. In between is a transition zone, and the depth of the IBL increases downwind of the change. A substantial body of theory, based on wind-tunnel and atmospheric observations, and theoretical and numerical modeling, describes IBL’s in nontropical cyclone onditions. Powell et al. (1996) have discussed their application to Hurricane Andrew’s landfall in Florida. The effect is to give a quite rapid decrease in wind speed at the coastline. The analyses in Powell and Houston (1996a) suggest this is of the order of 5 m s-1. Similarly, Kaplan and DeMaria’s 1995, 2001) empirical decay models include an immediate 10% reduction at landfall to account for this.

The reduction of wind speed at the coast in onshore flow acts to generate or enhance convergence there, which then has an impact on convection. Radar evidence in Hurricane Andrew suggested cells formed at the coast and developed
as they propagated inland around the storm, and may have contributed to enhanced damage to the left of the track (Black, 2002). Frictional convergence can also be important when the cyclone is some distance from land. Modelling studies show it may lead to the formation of a rainband between the cyclone and the coast, when the latter is a couple of hundred kilometres offshore. Subsequent evolution can include an eyewall contraction or replacement cycle, which may then have an impact on the intensity.

The reduced wind speed at landfall upsets the mass-wind-friction balance in the boundary layer and results in increased inflow. This implies an increase in angular momentum advection, which will act to maintain the boundary-layer wind speed, so the dynamics farther downstream of the coast are more complex than classical IBL theory suggests. Kepert (2002a, b) has suggested that the frictional asymmetry in a stationary cyclone circulation that is half over land is analogous to that in a moving cyclone. The azimuthal deceleration and increased inflow in the onshore flow, coupled with the azimuthal acceleration and decreased inflow in the offshore flow, generates a frictionally modified wavenumber one inertia wave. The phase rotates anticylonically with height, such that the frictional terms keep the wave stationary (Kepert 2001). The net effect of a strong near-surface low-level jet in the offshore flow, and a weaker higher one in the onshore flow, can be seen in Blackwell’s (2000) observational study of the near-stationary Hurricane Danny at landfall. Similar structures were also apparent in Hurricane Floyd at landfall (Peter Dodge, 2002, personal communication; Kepert, 2002b).

The estimation of surface winds from flight-level observations over the ocean is an important topic that is discussed in more detail below. Typical marine values for the ratio of surface wind speed to that aloft are 0.7 to 0.8 except near the radius of maximum winds (RMW). Dobos et al. (1995) used profiler data to calculate regression relationships, and found the ratio was lower (0.66 from 1500 m during the day, and 0.43 at night) than over the sea due to the increased roughness. The difference between day and night is due to the diurnal variation of boundary layer stability over the land.

b) The land is more heterogeneous:

As well as being rougher than the sea, the land surface is also much more heterogeneous. The alternation of urban, suburban, forested and farmed areas creates a patchwork of highly variable surface roughness, which can cause large variations in measured winds over short distances. Therefore wind observations must be adjusted to a common exposure before analysis to remove this effect (Powell et al. 1996). Similarly, the anemometer height, and averaging of the data, has significant impacts on the measurements that must be accounted for. The converse applies in forecasts and warnings: the exposure of the site may be a relevant parameter for sufficiently site-specific forecasts. These factors may cause controversy in comparing model results with observations (Powell and Houston 1999; Zhang et al 1999).

c) The land is cooler:

The land also differs from the sea in being substantially cooler. This occurs because the land has much lower effective heat capacity than the ocean, which is partly that soil has lower heat capacity than water, but also that the ocean mixes, so energy is drawn from a substantial depth, while the land relies on conduction to replenish the heat lost through the surface. The surface cooling has at least two important effects. Firstly, the feedback between strong winds and large surface enthalpy flux, which is a vital part of the cyclone’s energy budget (the WISHE process, see Emanuel, 1986), is disrupted. Tuleya (1994) is the latest in a series of papers by the Geophysical Fluid Dynamics Laboratory (GFDL) modeling group (Tuleya et al. 1984, Tuleya and Kurihara 1978) that examine this problem. Further illumination is given by Shen et al. (2002), who look at the effect of landfall when the land is covered by a layer of water of varying depths. They
found that the decay rate is greatest when the water is shallow, due to the heat capacity effect. In such studies, other components of the surface energy budget, and in particular the diurnal variation of radiation, are also important.

The second effect of the surface cooling is that it changes the boundary layer from being unstable or near-neutral over the sea to stable over the land. The damping of turbulence by the increased static stability leads to a reduction in the downward flux of momentum through the boundary layer. Thus the searsurface winds weaken further. This effect was modelled by Tuleya (1994), where his Fig. 10 shows a slow decay in the maximum winds at the boundary layer top after landfall, but a much more rapid decay at the surface. The effect of stability on the relative strength of winds at the surface and aloft in tropical cyclones has been shown observationally by Powell and Black (1990)
and Dobos et al. (1995). The increase in shear across the boundary layer may lead to increased gustiness, if higher momentum air is transferred to the surface by turbulence or boundary-layer rolls. It can also result in the sudden onset of more widespread strong winds, if convective downdrafts transfer high momentum air to the surface.

Empirical models of landfall wind decay contain the effect of both reduced surface momentum transfer, and overall storm decay. Kaplan and DeMaria (1995, 2001) suggest an e-folding decay time scale of 10 hours following tropical and id-latitude landfall, with more rapid decay for storms in the New England area. This more rapid decay seems be due to higher topography, and a more hostile environment in terms of environmental shear and the like.

It is worth mentioning that the thermodynamics of the boundary layer, which are so important to the storm decay, are controlled by more than just the surface thermodynamic fluxes. Adiabatic cooling due to the enhanced downgradient flow, advection of cooler and drier continental air, downdrafts from enhanced rainband convection, and fluxes through the top of the boundary layer are also important, and subject to change at landfall. Powell (1987) discusses these issues further in analyzing the landfall of Hurricane Alicia. Heating due to turbulent dissipation may also play a role (Zhang and Althuser 1999, Andreas and Emanuel 2001).

Although land surface cooling causes weakening by the mechanisms discussed above, there are occasionally cases where baroclinic sources of energy help to maintain or even intensify the cyclone. The most spectacular of these is when the storm undergoes extratropical transition. However, less rapid but still significant re-intensifications have occurred in the presence of weak baroclinicity, such as Hurricane David of 1979 (Bosart and Lackmann 1995, Hurricane Danny of 1997 (Rappaport 1999), and Typhoon Yancy (Liang et al. 1995).

Konrad (2001) studied the landfall time of hurricanes in the eastern United States and found a marked diurnal tendency, with the evening and midmorning being the favoured times. The pattern was stronger for weaker storms. This is presumably relevant to the boundary layer thermodynamics.

d) The land is hillier:

Topography has a major impact. Deflection and channeling of winds by mountains can not only produce locally much stronger winds, but also affect the track and intensity of the storm as a whole. Rainfall enhancement is also important, but is outside the scope of this report. Numerous modeling and observational studies have explored the effects, with a particular focus on landfall on Taiwan. The track typically deflects to the north of the island, but the details depend on factors such as the size of the storm and its speed of movement. The cyclone generally weakens on interacting with topography, and the centre may show a discontinuous jump in position. The latter is often associated with the formation of a meso-vortex to the lee of the range. The physics are complex, with various authors invoking vorticity arguments, warming in Foehn flows, PV generation in wakes, and turbulence and diffusion, to explain them. A good introduction and review may be found in Wu and Kuo (1999). Other recent contributions include He and Yang (1995) Huang and Hsu (1998), Li et al. (1997), Yeh et al. (1997), Tscheng and Wang (1999), Wang (1999), Wu et al. (199a, b), Geerts et al. (2000), Lee et al. (2000), and Wu (2001).

e) Other factors:

Boundary-layer roll vortices are known to be a very common feature of the atmospheric boundary layer, and have been extensively studied observationally,
analytically, and numerically. Etling and Brown (1993) give a recent review. Wurman and Winslow (1998) have presented high-resolution Doppler radar evidence that similar roll vortices occur in the core region of landfalling tropical cyclones. Their observations show horizontally fairly uniform winds of 50 to 60 m s
-1 above 500 m are replaced by alternating bands of light (15-35 m s-1) and strong (40 to 60 m s-1) winds on a highly variable horizontal scale of about 600 m, aligned with the mean flow. A study by Gall et al.(2001) has shown bands on somewhat larger scales, which may or may not be boundary-layer rolls. Morrison et al. (2002) noted small-scale features in radar data, but the nature of these is unclear.

The past few years have seen a substantial improvement in our level of knowledge about the boundary layer over the ocean. Better knowledge of the initial condition will inevitably improve our ability to understand the changes that occur at landfall. Recent advances include analytical (Kepert 2001) and numerical (Kepert and Wang 2001; Smith 2002) models, and observational studies (e.g. Black et al. 2002, Dodge et al. 2002, Franklin et al. 2002, Kepert 2002a, Knupp et al. 2000). Observational progress has been facilitated by the introduction of new instruments, particularly the Global Positioning System (GPS) dropsonde (Hock and Franklin, 1999).

Tropical cyclones may spawn tornadoes at landfall. These occur preferentially in the right front quadrant in outer rainbands. McCaul (1991) gives a comprehensive climatology; see also Bluestein (1985) and Vescio et al. (1996).
Theory and modeling (e.g., Schubert et al. 1999) have shown that the tropical
cyclone eye is frequently barotropically unstable, and can generate mesoscale vortices on the reversed vorticity gradient on the inner edge of the eyewall. Observations have shown these may be of significant intensity. Clearly, should these occur at landfall, they would result in significantly stronger winds on small horizontal scales. Black (2002) quotes Shoemaker as saying the very damaging winds to the left of Hurricane Andrew’s track occurred before the second eyewall passage. Although Black ascribes this to supercell formation triggered by frictional convergence, it seems to this author that an eyewall mesovortex is a plausible alternate explanation. In support of this, reflectivity data from the Miami radar shows Andrew’s eye becoming markedly triangular at the point of landfall.

2.4.3 Observational Issues

Research-grade observations of the boundary layer have substantially improved.
These include over the sea (GPS dropsonde, Stepped Frequency Microwave Radiometer (SFMR), scatterometer) and at landfall (mobile tower program: Howard et al. 2002, mobile Doppler radars: Wurman et al. 1997, etc). This data should continue to promote rapid progress. Routine analyses of surface wind fields (Powell and Houston, 1996) are a useful product when observation density permits their preparation, and provided the issues (such as surface wind reduction, and exposure) are understood.

Near-surface winds are one of the most difficult meteorological parameters to measure well. Significant variations can arise due to surface roughness heterogeneities, measurement height, channeling or sheltering by near-by buildings or trees, instrument response characteristics, and averaging techniques. Moreover, the standard meteorologist’s wind (10-min average, 10-m
height) may not be the most relevant for a wind engineer, for example.

Suitable models may be used to remove some of the variation. For example, Powell et al. (1996) adjust all their land-based measurements to a common averaging period, height and surface roughness using simple schemes based on well-known boundary-layer theory. More sophisticated schemes may include numerical or wind tunnel modeling to account for channeling effects and the like. A wind measurement is meaningless unless the detailed circumstances under which it was made, and hence what processes influence it, are known.

Novel instruments introduce new issues. Doppler radars and wind profilers give a volume (and time) averaged wind. No consensus has emerged as to what averaging period anemometer wind a GPS dropsonde wind is equivalent to.

Calibration is a particular issue. Anemometers are typically `more mechanical’
than other instruments, with concomitant maintenance issues. `No-moving-parts’
instruments may be unsuitable in tropical cyclone conditions. For example, rain contamination caused a hot-film anemometer to report a spurious extreme wind record in Typhoon Paka, and also causes signal degradation in sonic anemometers. The current Australian record wind gust was measured in Severe Tropical Cyclone Vance in March 1999 by a high-speed Dines anemometer with a strip chart recorder. The fact that two nearly co-located, modern, electronically logged cup anemometers each recorded peak gusts some 10 m s
-1 lighter than this illustrates the potential for variability between instruments, and the importance of routine post-event calibration of extreme events.

All the wind damage is caused by speeds at the high-speed tail of the frequency distribution. Characterizing the tails of any distribution has inherent sampling difficulties. Clearly it is vital that instrumental issues be very carefully considered in analyzing extreme observations. That instruments should be designed so that they and their measurements survive the
event goes without saying.

A further issue is the use of aircraft data to estimate surface winds. This has a long history over the oceans, with schemes ranging from the empirical (multiply by a surface wind factor (SWF) of, say, 0.8) to one-dimensional models. Several factors affect the SWF, with Powell and Black (1990) showing the impact of stability, and Dobos et al. (1995) the effect of landfall. Recently, theoretical work by Kepert (2001) and Kepert and Wang (2001) suggests the SWF should vary spatially within the storm, being higher towards the centre, and on the left (right) of track in the Northern (Southern) Hemisphere. Analysis of GPS observations by Franklin et al. (2002) and Kepert (2002) supports these theoretical findings, as do model-observation comparisons by Vickery et al. (2000). The issues discussed above apply not only to the surface winds in such analyses, but also to the aircraft, where the different turbulence characteristics above the boundary layer, and its rapid movement across the tight gradient near the RMW, need to be considered.

The application of Franklin et al’s eyewall SWF of 0.9, which is derived from open-ocean GPS dropsonde observations, to aircraft data was a major part of the justification for the recent upgrade of Hurricane Andrew to Category 5 at landfall. Landsea (2002) discusses the rationale behind the decision and includes links to the submissions to the reanalysis committee. Further work will no doubt better elucidate the behaviour of the SWF at landfall.

2.4.4 Research Recommendations

Marks et al. (1998) gives a comprehensive overview of research issues for
landfalling tropical cyclones, and includes some winds-specific recommendations. Wu et al. (1999) discuss research issues affecting Taiwan,
again including winds.

1. Scientific confidence in the correctness of most aspects of boundary layer
parameterization decreases at high wind speeds. Efforts to remedy this over the sea are underway, with the Coupled Boundary Layer Air-Sea Transfer CBLAST) program, sea spray modeling efforts, and so forth.
Land surface schemes may need similar verification at extreme wind speeds.

2. Extreme wind speeds are hard to measure, and are rare. Therefore, measurements must routinely be properly characterized and calibrated, so that they can fully and validly contribute to understanding.
Similarly, instruments and logging systems should be designed to survive the event.

3. Targeted observations, air- and ground-based, direct and remotely sensed, have yielded a wealth of data that needs full analysis to characterize the mean and fluctuating (on all scales) components of the winds at landfall.

4. The GPS dropsonde has recently, and will continue to, contributed enormously to our understanding of boundary layer structure and dynamics. We need to know what, in terms of averaging period and so forth, a dropsonde wind is equivalent to.

5. The World Wide Web is a great tool for sharing data sets. The availability of high quality data, analyses and reports on the websites of Hurricane Research Division, the National Hurricane Center, the Naval Research Laboratories, and the Cooperative Institute for Meteorological Satellite Studies (to name a few) is of enormous benefit to individual researchers, and the community as a whole. These and other similar efforts should be applauded, encouraged, and continued.

6. Landfall change processes occur on scales ranging from the IBL, through wave-number one frictional asymmetries, to land-surface cooling with concomitant energy flux and boundary layer stability changes, to decay of the storm as a whole due to reduced moisture supply. Much of our knowledge here is based on application of theory derived at modest wind speeds, and on models. Further observational verification or rejection of these matters is needed.

7. Potentially, all of the environmental factors such as the synoptic situation, topography, roughness, cooling, diurnal effects, cyclone intensity and size, affect spin-down. How do these interact to determine the spin-down of a particular storm? At the surface? And above the boundary layer?

8. Radar reflectivity data in landfalling Hurricanes Andrew and Gustav, and in Severe Tropical Cyclone Vance, show that eyewall asymmetries characteristic of eyewall mesovortices formation formed rapidly at the moment of landfall. Were mesovortices present? Is the asymmetric friction of landfall a sufficiently large perturbation to release the instability? If so, can we forecast when this might occur?

9. Other small-scale events, such as tornadoes and convective downdrafts, also need better characterization and improved forecasting ability.

2.4.5 Acknowledgments

I am grateful to Bruce Harper, Mark Powell, Mark DeMaria, and John Kaplan for
their helpful comments on an early draft of this report.


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