A Modeling Study of Hurricane Landfall in a Dry Environment

Home , Hurricane Danny (1985), Landfall

JULY 2006 K I MBALL 1901

SYTSKE K. KIMBALL Department of Earth Sciences, University of South Alabama, Mobile, Alabama

(Manuscript received 24 February 2005, in final form 7 October 2005)

ABSTRACT

The effects of dry air intrusion on landfalling hurricanes are investigated using eight numerical simulations. The simulations differ in the initial amount of moisture in the storm core and its horizontal extent from the storm center. The storms evolve very differently during the 36-h simulation. Storms with a small radial extent of moisture develop minimal rainbands, intensify rapidly in the first 3 h, and weaken as dry air from the 800–850-hPa layer wraps cyclonically and inward around the storm core. As the air approaches the core, it sinks (possibly by eyewall downdrafts or as a result of evaporative cooling), reaches the storm’s inflow layer, and entrains into the eyewall updrafts. Storms with large radial extent of moisture develop into larger storms with large rainbands, having smaller intensification rates initially, but continue to intensify for a longer period of time. Rainband downdrafts release low equivalent potential temperature air into the moat region. Low-level convergence into the rainbands reduces the magnitude of eyewall inflow. Both factors reduce storm intensification initially. Simultaneously, the rainbands act as a barrier between the moist core and the dry environment, preventing dry air from penetrating the storm core. As land is approached, inflowing air is no longer replenished with heat and moisture. Eventually, rainband convection erodes and dry air approaches the storm core from the landward side causing the storms to weaken. Without the presence of land, a hurricane can sustain itself in a dry environment, provided its moist envelope is large enough.

1. Introduction gan to weaken. Dry air was observed to the east and northwest of Hurricane Georges as it made landfall on Several recent hurricanes (Opal, 1995; Georges, the Mississippi coast (Curtis 2004). Hurricane Lili 1998; Lili, 2002; Ivan, 2004) weakened just before mak- (2002) made landfall on the Louisiana coast as a cat- ing landfall on the U.S. Gulf of Mexico coast. This egory 1 hurricane after a period of rapid decay during weakening spared Gulf Coast residents from even more which its eyewall collapsed (Pasch et al. 2004). Satellite extensive damage than was done. In each of these cases, images of Lili before landfall indicate a region of very dry air was present in the vicinity of the storms and may dry air to the north of the storm. When Ivan ap- have contributed to their weakening. Opal has been proached the Alabama coast on 15 September 2004, dry studied extensively, and its weakening phase just before air existed in its western half and an erosion of the landfall occurred in high vertical wind shear and low- southwestern eyewall was observed in the Mobile, Ala- ered sea surface temperatures (SSTs) as the system bama, Regional Airport (KMOB) Weather Surveil- moved away from an eddy of warm water (Rodgers et lance Radar-1988 Doppler radar imagery and reported al. 1998). However, an area of dry air existed west of by a National Oceanic and Atmospheric Administra- the axis of a midlatitude trough that was also respon- tion research aircraft. sible for the high shear values. The dry air intruded Hurricane Alicia (1983), on the other hand, contin- cyclonically around the western and southern regions of ued to intensify during landfall in spite of the presence Opal and came within 222 km of Opal’s center in its of dry air. Dry air was present beyond about 350 km southwestern quadrant at the same time the storm be- from the storm center, but there was no evidence of dry air intrusion according to Curtis (2004). On the other hand, Powell (1987) observed that Alicia’s eyewall was Corresponding author address: Sytske Kimball, Dept. of Earth Sciences, LSCB 136, University of South Alabama, Mobile, AL open to the south before and during landfall; however, 36688. Alicia continued to intensify in spite of half of the E-mail: [email protected] storm’s circulation being located over land. Powell

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(1987) noted that the storm’s rainbands may have TABLE 1. Experiments ranked according to initial moisture served as a boundary between the moist core and the content. dry environment. Expt name A (g kgϪ1) B (km) Previous modeling studies of hurricane landfall have A15B250 DRY 15.0 250 focused on the sensitivity of the hurricane to surface A16B250 16.5 250 boundary conditions and not dry air intrusion (e.g., Tu- A18B250 18.0 250 leya and Kurihara 1978; Tuleya et al. 1984; Tuleya A18B400 18.0 400 1994). Bender et al. (1987) modeled the effects of island A19B250 19.0 250 terrain on tropical cyclones and observed how dry air A19B400 19.0 400 A19B500 19.0 500 intrusion from above mountaintops caused the model A19B600 WET 19.0 600 storms to weaken. Chan and Liang (2003) noted that reduction of the surface moisture flux over land in their idealized numerical study would lead to advection of relatively dry air into the storm center as it approached Ϫr2 q͑r, z͒ ϭ q ͑z͒ ϩ A͑z͒ expͩ ͪ, ͑1͒ land. They found that such an intrusion during landfall b B2 did not always lead to reduced convective activity. The drier air was advected cyclonically and upward from the where qb(z) is a dry environmental sounding, A(z)is landward side, decreasing column static instability on the amplitude of the perturbation (which decreases the offshore flow side just off the coast, but increasing with increasing height), B is the e-folding radius, and r it on the onshore flow side over land as the dry air is the radial distance from the center of the vortex. The moved over low-level moist air. dry environment is loosely based on the shape of a When a hurricane approaches a dry environment, sounding from the Geophysical Fluid Dynamics Labo- forecasters are faced with a difficult decision as to the ratory (GFDL) analysis at 1200 UTC 18 July 1997 at effect of the dry air on the storm’s intensity. Further- 36°N and 83°W. Roughly a third of the values observed more, moisture is not well assimilated into hurricane in that location are taken as qb(z). The moist envelope forecast models because of a lack of observations over is defined as the area where q exceeds qb. The magni- the tropical oceans, especially within the dangerous en- tudes of A and B are varied to obtain different initial vironment of a hurricane core. An additional concern moist envelopes. Table 1 lists the eight different com- about dry air is that its presence and subsequent intru- binations of values for A and B (at the lowest model sion at midlevels has been linked to tornado outbreaks level) along with the case name of each experiment. during hurricane landfall (e.g., Curtis 2004). The plain solid line in Fig. 1 represents the dry envi- This study will use a numerical modeling approach to ronmental sounding; the other four soundings are taken explore dry air intrusion in landfalling hurricanes. Spe- at 0, 225, 450, and 675 km away from the center of the cific questions to be addressed are 1) how and where vortex. In the driest case (A15B250), the sounding at does dry air enter the storm, 2) can a larger moist en- r ϭ 450 km coincides with that of the dry, unperturbed velope protect a storm from dry air intrusion, and 3) to environment, indicating the edge of the moist envelope. what extent does the proximity to land enhance the In the other extreme (A19B600), the sounding at r ϭ adverse effects of dry air intrusion on hurricane inten- 675 km is still more moist than the original unperturbed sity? The design of the numerical experiments and environment, indicating a moist envelope with a radial model configuration are described in the next section. extent larger than 675 km. Figure 2 is a water vapor In section 3, the results are presented; followed by a satellite image of Hurricane Ivan at 1915 UTC on 15 discussion and a list of conclusions in section 4. September 2004. Very dry air can be seen around 500 km from the center of the vortex to the southwest and around 1000 km to the northeast of the storm center. 2. Method and numerical model configuration Therefore, the values for B are within a realistic range. In Fig. 3, mixing ratio soundings for each experiment a. Design of the numerical experiments at r ϭ 100 km from the vortex center are compared with Eight experiments are compared, each with the same observations from Slidell, Louisiana, at 1200 UTC on initial hurricane vortex and large-scale analysis (see 18 July 1997, which was, at that time, located 100 km section 2b), but with a different moist envelope sur- from the center of Hurricane Danny (1997). Danny was rounding the storm. The moist envelope is constructed a category 1 storm at the time, as are the initial model by adding a Gaussian mixing ratio perturbation to the vortices in this study. A comparison between the ob- center of the vortex: served and model soundings reveals that below about

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Ϫ FIG. 1. Initial mixing ratio (g kg 1) soundings for all experiments at 0 (open circles), 225 (filled circles), 450 (open squares), and

675 km (filled squares) from the vortex center. The plain line is the sounding for the background dry environment (qb).

800 hPa the observed sounding lies between those of The simulation ends at 0000 UTC on 20 July. The the cases with B Ն 400 km and the smaller B cases. The Danny environment is chosen because of the absence of observed 800–600-hPa layer is about as moist as that of strong steering currents and vertical wind shear. In this the B Ն 400 km cases. Above 600 hPa, the observations manner it becomes possible to investigate the effects of fluctuate between being about1gkgϪ1 more moist just dry air intrusion upon the simulated storms as they than the wettest experiment and 1.5 g kgϪ1 drier than approach land. the driest experiment. Overall, the observations agree b. The numerical model reasonably well with the experimental soundings, providing confidence that the Gaussian perturbations are The fifth-generation Pennsylvania State University– realistic. National Center for Atmospheric Research (PSU– The model vortices are embedded in the temperature NCAR) Mesoscale Model (MM5) is a nonhydrostatic, and wind fields that surround Hurricane Danny (1997) primitive equation model for a fully compressible at- at 1200 UTC 18 July 1997, which is when the simulation mosphere (Grell et al. 1994). In this study, the MM5 is begins. The storm is located over water to the south of initialized using GFDL atmospheric analysis fields and the coast of Mississippi at that time and is steered U.S. Navy Fleet Numerical Meteorology and Oceanog- slowly northeastward toward Mobile Bay. Danny en- raphy Center SST fields from 1200 UTC 18 July to 0000 ters Mobile Bay around 0900 UTC 19 July. Steering UTC 20 July 1997. This time period captures Hurricane currents weaken further and the storm remains in the Danny (1997) moving from a location southeast of New bay until it makes landfall near Mullet Point, Alabama, Orleans to Mobile Bay and eventually making landfall around midday local time on 19 July (Rappaport 1999). in Alabama. The GFDL analysis includes a bogus vor-

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FIG. 2. A Geostationary Operational Environmental Satellite water vapor image of Hurricane Ivan at 1915 UTC 15 Sep 2004.

tex to represent Hurricane Danny, which is located boundaries are used (Grell et al. 1994). The high- about 10 km to the north of the observed center of resolution Blackadar planetary boundary layer param- Danny at 1200 UTC on 18 July 1997. This misplacement eterization scheme (Blackadar 1979; Zhang and Anthes is most likely a result of interpolating the coarser- 1982) is used on both grids. The surface flux param- resolution GFDL fields (approximately 18 km) to the eterization makes use of a form of the bulk aerody- finer-resolution MM5 model fields used here. The namic formulations. The modest intensity of the mod- GFDL vortex is also much larger than the observed eled hurricanes in this study makes it unlikely that the Danny because the coarse resolution of the operational fluxes are overestimated because of extrapolation of GFDL model at that time could not resolve an initial the surface exchange coefficients into high-wind re- vortex with a small (45 km) radius of maximum winds gimes. The magnitude of the wind speed above the (RMW). For these reasons, the GFDL vortex is re- boundary layer beyond which this becomes a problem moved and replaced by a new artificial vortex spun up is 40–50 m sϪ1 (Franklin et al. 2003); the hurricanes in by MM5, following the method described by Kimball this study only cross that threshold by a small amount and Evans (2002). A vortex constructed in the above and for a short period of time. manner has a consistent internal structure (moisture, As low-level winds over the warm tropical ocean con- wind, temperature fields, etc. will be in balance with verge toward the hurricane center, surface sensible and one another), it will resemble the corresponding real latent heat fluxes supply the low-level inflow with moist storm, and it will be compatible with the numerical entropy. This moist entropy is often measured in terms ␪ model physics, computational schemes, and grid reso- of equivalent potential temperature e because of its ␪ lution (Kurihara et al. 1993). conservational properties and because the mean e of The MM5 simulations make use of two two-way the eyewall column has been related to tropical cyclone nested domains with horizontal resolutions of 9 and 3 (TC) intensity (Malkus and Riehl 1960; Betts and Simp- ␪ km, respectively, and 24 vertical levels, 7 of which are son 1987). Furthermore, e combines temperature and located in the first 1.5 km above the surface of the moisture content in one variable. To make an impact on model. Convection is modeled explicitly on both hurricane intensity, a rising parcel needs both a high meshes. Microphysics is modeled using the Reisner temperature and high moisture content. The former graupel scheme (Reisner et al. 1993, 1998) and includes will allow a parcel to rise in the first place, the latter will snow, supercooled water, graupel, and ice number pre- allow it to continue to rise and will supply fuel to the diction equations. Time inflow–outflow-dependent storm in the form of latent heat release. For these rea-

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Ϫ FIG. 3. Initial mixing ratio (g kg 1) soundings for all experiments at 100 km from the vortex center. The line style refers to the size of B (dotted, 250 km; dashed, 400 km; solid gray, 500 km; solid black, 600 km), while the symbol denotes the size of A (no symbol, 15 g kgϪ1; plus signs, 16 g kgϪ1; open circles, 18 g kgϪ1; open squares, 19 g kgϪ1). Plain gray line is the observed sounding at Slidell, LA, at 1200 UTC 18 Jul 1997, which was 100 km away from the center of Hurricane Danny at that time.

sons, dry air intrusion will be mostly discussed in terms period (Xiao et al. 2000). Storm intensity is measured in ␪ of e. terms of minimum sea level pressure (PSMIN; Fig. 4) and it is readily seen that hurricanes with different ini- 3. Results tial moisture contents evolve very differently. Increas- ing the low-level initial moisture content in the core of a. Storm evolution a TC, as well as its radial extent, leads to an increased ␪ The evolution of the storms’ intensity and wind radii low-level e under the eyewall. At 300 hPa, the initial are presented in Figs. 4–6. An initial adjustment period moisture content of all cases is almost equal in value ␪ is seen in the first 3–6 h of the simulation. During this (Fig. 1); hence, higher boundary layer e implies higher time, the larger-scale environment of the model adjusts convective available potential energy (CAPE). As ␪ to the inserted bogus vortex (e.g., Liu et al. 1997). The high- e air ascends in the eyewall, latent heat release use of nested domains for prediction and a single do- occurs, enhancing parcel buoyancy. The strong ascent main for initialization may contribute to the adjustment in the eyewall is compensated for by low-level radial

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FIG. 4. Time series showing the PSMIN (hPa). The legend shows experiment name and line styles. inflow toward the eyewall and subsidence in the eye and over water only, because increased friction over (Willoughby 1998). This subsidence warming leads to a land causes an abrupt change in wind speed and smaller hydrostatic surface pressure drop in the eye, a subse- radii over land. This eliminates changes in wind radii as quent increase in the low-level radial pressure gradient, a result of different landfall times, but also causes the and an increase in low-level winds via gradient wind RMW of some storms (Fig. 5) to increase as the storms balance. Hence, it would seem reasonable to expect an track inland. As the storm center moves away from the initially wetter (in terms of larger A and/or B) storm to coastline, the distance between storm center and maxi- become more intense. If B is held constant and the mum winds (i.e., the RMW) over water increases. After initial mixing ratio amplitude A is increased, this ex- around t ϭ 24 h, the RMWs of the two largest B cases pectation is indeed realized. For the four storms with (A19B500 and A19B600) suddenly increase to very B ϭ 250 km (dotted lines in Fig. 4), the storms become large sizes (close to 45 km at t ϭ 36 h). Both these more intense as A increases. The same holds true for storms track farther from the coastline after landfall the two cases with B ϭ 400 km (dashed lines in Fig. 4). than the other cases (see track discussion below) and, However, a larger horizontal extent (B) of initial mois- hence, their centers move farther away from their maxi- ture, while A is kept constant, does not necessarily lead mum winds over water. Before moving inland, the to a more intense storm. For example, A19B400 is a RMW of each case contracts as the simulation pro- more intense storm than the initially wetter A19B500 gresses, even if the storm is weakening. Usually, con- throughout most of the 36 h of the simulation. Intensi- tracting RMWs are associated with intensifying tropical fication rates (Fig. 4) differ throughout the simulation cyclones (e.g., Willoughby 1998), but cases where the and are not necessarily greater for wetter storms. In opposite occurs have been observed in real hurricanes fact, the B ϭ 250 km cases (dotted lines) show a large (e.g., Kimball and Mulekar 2004). The two cases with intensification rate during the first3hofthesimulation, the largest e-folding radii (A19B500 and A19B600) followed by mostly weakening. Larger B cases intensify have the largest RMW for most of the simulation time more slowly initially but continue to intensify for a and are significantly larger in size (Fig. 6). The B ϭ 250 longer period of time. km cases have smaller RMWs during most of the simu- The RMW and 17 m sϪ1 winds (or size) are calcu- lation time and are smaller in size. lated at the lowest model level (40 m from the surface) Figure 7 shows that the tracks, landfall locations, and

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FIG. 5. Time series showing the RMWs (km) 40 m above the surface, azimuthally averaged over water only. landfall times of the storms differ. This is to be expected different levels and shallower or deeper layers of atmo- given the differences in size and intensity evolution. spheric steering flows. The impact of the ␤ effect also Past research (e.g., Elsberry 1995) has shown that depends on storm structure. Landfall times (i.e., the storms of different sizes and intensities are steered by time the storm’s center crosses the coastline) and the

Ϫ FIG. 6. As in Fig. 5 but for a radius of 17 m s 1 (size, km).

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FIG. 7. Tracks of the eight simulations. Line styles and symbols are as in Fig. 3. time of the onset of storm weakening are listed in Table marks the storm’s eyewall. Temperatures over land 2. All of the storms begin to weaken before they make (Fig. 8d) are significantly lower than water tempera- landfall; column 3 in Table 2 indicates how many hours tures (because of overcast conditions and the presence before landfall this occurs. This number ranges from 9 of vegetation). This leads to significantly lower sensible to 21 h. The smallest A cases (A ϭ 15 and 16 g kgϪ1) heat fluxes over land than over water for the same low- begin to weaken early (t ϭ 3 h) but also make an early level wind speed (Figs. 8b and 8d). The same applies to landfall because of their faster motion during the first 9 the surface moisture content (not shown) and latent h. This explains why their onset of weakening precedes heat fluxes (Fig. 8c). Over water the surface fluxes fall their moment of landfall by a relatively small amount. off as the low-level wind decreases; the largest fluxes Considering just the large A cases (A ϭ 18 and 19 g occur near the storm’s RMW. However, the model la- kgϪ1) suggests that the smaller the value of B, the larger tent and sensible heat fluxes depend on low-level wind the amount of time by which the onset of weakening speed, as well as the moisture q and temperature T precedes landfall. This may indicate that dry air intru- difference, respectively, between the surface and lowest sion plays a stronger role in the small B cases than in model level (Blackadar 1979; Zhang and Anthes 1982). storms with an initially larger moist envelope. The correlation is not statistically significant, however, because of the small sample size. TABLE 2. Number of hours into the simulation when the storm All storms begin to weaken before their center begins to weaken and when the storm center crosses the coastline. The last columns lists the difference in time between the two crosses the coastline (Fig. 4, Table 2); therefore, it is events. possible that land effects begin to play a role before landfall, in addition to or instead of, dry air intrusion. Time center across Ϫ However, this moment is difficult to measure. When Onset of Center across coastline onset Case weakening (h) coastline (h) of weakening (h) hurricanes encounter land, friction increases and surface fluxes reduce because of the reduced heat and A15B250 3 17 14 moisture content of the land surface compared with the A16B250 3 20 17 A18B250 3 24 21 sea surface. Figure 8 shows the low-level wind speed, A18B400 12 30 18 surface fluxes, and surface temperature for A19B600 at A19B250 3 21 18 t ϭ 18 h when part of the storm’s eyewall has crossed A19B400 12 30 18 the coastline. The white dashed line marks the 33 m sϪ1 A19B500 9 26 17 wind contour at 40 m from the surface and roughly A19B600 12 21 9

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Ϫ Ϫ Ϫ FIG. 8. Maps of (a) 40-m wind speed (m s 1), (b) sensible heat flux (W 2), (c) latent heat flux (W 2), and (d) surface temperature (°C) at t ϭ 18 h for case A19B600. The white dotted line is the 17 m sϪ1 wind contour; the white dashed line is the 33 m sϪ1 wind contour. The thick black line denotes the model coastline.

Therefore, the surface fluxes change in response to low- cases. This would indicate that these cases probably level T and q changes that may occur in dry air intru- begin to weaken as a result of dry air intrusion alone; sion events. This can be seen in Fig. 8, where the surface that is, well before land begins to play a role. However, fluxes are larger under the western eyewall than the all cases begin to weaken before the RMW crosses the eastern eyewall in spite of equal values in wind speed coastline. Therefore, dry air intrusion probably plays a (Fig. 8a), surface temperature (Fig. 8d), and surface significant role in all of these simulated hurricanes. The moisture content (both are over water). Furthermore, if low-level energy supplied to the storms seems insuffi- a storm weakens, its low-level winds and, hence, the cient to counteract the negative effects from dry air surface fluxes decline. This makes it difficult to decide whether reduced fluxes cause storm weakening or vice versa. Therefore, a change in surface flux magnitude is TABLE 3. Number of hours into the simulation when the RMW crosses the coastline and the difference between the times of not a good indicator of when land effects begins to play RMW landfall and storm weakening. a role. Figure 8 illustrates that surface friction immediately RMW across Ϫ reduces a storm’s strongest low-level winds and that the RMW across coastline onset Case coastline (h) of weakening (h) fluxes directly under the eyewall decrease by a large amount when the RMW crosses the coastline. Table 3 A15B250 9 6 lists the time the RMW crosses the coastline for each A16B250 12 9 A18B250 16 13 case. Also shown is the amount of time by which storm A18B400 15 3 weakening (Table 2) precedes this moment. Again dis- A19B250 15 12 regarding the two smallest A cases, it is clear that the A19B400 17 5 B ϭ 250 km cases begin to weaken a significantly longer A19B500 17 8 time before RMW landfall than do the B Ն 400 km A19B600 16 4

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entrainment on storm intensification, even when storms have an initially large moist envelope. Some real hurricanes have been observed to continue intensifying (e.g., Alicia, 1983) or to remain in steady state (e.g., Frederic, 1979) until the time when their center crossed the coast in spite of reduced access to surface energy and an increase in land-induced friction beneath por- tions of their eyewalls. In these cases, the reduced supply of energy from the surface was apparently still sufficient to sustain the storm.

b. Dry air intrusion

1) A19B250 AND A19B600 These two cases have the same initial moisture amplitude (A) and make landfall at the same time, but their initial moist envelopes (B) differ substantially in radial extent. Their evolution in terms of intensity and structure also differs dramatically. Both storms approach the coastline at roughly the same pace and their centers reach the coastline at t ϭ 21 h, within 10 km of one another (Fig. 7, Table 2). Hence, different land effects (reduced surface moisture and heat fluxes and enhanced friction) do not explain the differences in storm evolution. Case A19B250 has an initially small moist envelope and remains a small storm (Fig. 6). The storm intensifies at a greater rate than any other in the first 3 h (Fig. 4), but this is followed by steady weakening before rapid weakening occurs after landfall at t ϭ 21 h. In contrast, A19B600, with a large moist envelope, remains a large storm throughout the simulation. This case intensifies slowly in the first 3 h followed by continued intensification until t ϭ 12 h. Between t ϭ 6 and 12 h, this storm intensifies more rapidly than any other FIG. 9. Equivalent potential temperature (shaded; K) and wind vectors at 950 mb and t ϭ 6 h for (a) A19B250 and (b) A19B600. case. In the remainder of this section, the above two cases will be closely compared with the aid of Figs. 9–14. environment. In the southeastern half of the storm an- ␪ Figure 9 compares the e at 950 hPa of A19B250 and other dry arc is seen, this one connected to the dry A19B600 at t ϭ 6 h. Figure 10 shows a vertical cross environment. At higher levels, between 900 and 800 ␪ ϭ section of e at t 6 h for case A19B250 at the 90° hPa, this arc is still present and wraps farther around azimuth. Figure 11 compares the reflectivity of the two the storm center to reach the western side. Both arcs cases at t ϭ 6 h, while Fig. 12 compares the radial com- spiral cyclonically and inward toward the storm center. ponent of the wind at that same time. Figures 13 and 14 The northern end of the southeastern arc wraps inward ϭ ␪ display the vertical wind speed at t 6 h and the e at and approaches the storm center very closely. Radial– ϭ ␪ t 12 h, respectively, for case A19B600. height cross sections of e at 15° azimuth intervals In the small radius case (Fig. 9a), a central moist through this arc (i.e., between the 135° and 75° azi- ␪ Ն ␪ region of e 330 K exists, beyond which e drops off muths, using meteorological coordinates) reveal that ␪ rapidly to less than 316 K. About 30 km to the north of low- e air sinks and moves cyclonically toward the eye- ␪ the storm an arc of very low e can be seen embedded wall of the storm. At the 135° azimuth, the dry air is in warmer and moister air. The arc is isolated from the located 65 km away from the storm center at ϳ800 hPa, dry environment surrounding the storm at this level, whereas by 75° it has approached the surface and the but between 900 and 850 hPa it is connected to the dry storm center within a distance of 35 km, which coin-

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␪ ϭ FIG. 10. Vertical cross section of e (shaded; K) at t 6 h for A19B250 at 90° azimuth (meteorological coordinates). Black ar- rows represent wind vectors of radial (m sϪ1) and vertical wind (ϫ10 m sϪ1) components.

cides with the outer periphery of the storm eyewall. ␪ ϭ Figure 10 shows a vertical cross section of e at t 6h at the 90° azimuth. The eyewall is marked by strong updrafts between r ϭ 15 and 30 km, while weak subsidence occurs in the eye. Near the surface, high values of ␪ e are seen in the eye because of a combination of low pressure and large moisture content that typically exists at low levels in hurricane eyes (Willoughby 1998). The intruding dry air can be seen at r ϭ 40 km accompanied by sinking and radially inward flow. As the dry air enters the eyewall, it lowers eyewall CAPE and spirals cyclonically and upward within eyewall updrafts, possibly explaining the eroded western eyewall seen in Fig. 11a. FIG. 11. Reflectivity (dBZ) and wind vectors at 850 hPa and Although the dry air surrounding A19B250 is seen to t ϭ 6 h for (a) A19B250 and (b) A19B600. extend upward to almost 600 hPa (Fig. 10), it seems to have little impact on the storm above about 800 hPa because of the lack of inward motion at those levels. storm center is substantially weaker in the northwest- The dry air circles around the storm center until it ern half of the storm than the southeastern half. This is comes into contact with a pocket of sinking motion, most likely caused by a combination of 1) outflowing which brings it down to the inflow layer of the storm. air from strong convection in the northern eyewall and This sinking motion may be an eyewall convective 2) air flowing toward outer convective cells to the north downdraft. Alternatively, the dry air may have been and west of the storm core. This can be seen by com- cooled by evaporating hydrometeors causing an in- paring Figs. 11a and 12a, which show a good correlation crease in density, allowing it to sink spontaneously. between converging air and convective activity. To the Eyewall entrainment of dry air like that described southeast of the storm, a pattern of weaker and stron- above continues throughout the simulation of ger inflowing air (i.e., convergence and divergence) co- A18B250. incides with the small convective bands in that quad- The dry arc to the north of the storm center does not rant. In spite of the reduced inflow to the north, strong manage to approach the storm eyewall by t ϭ 6 h. Fig- tangential winds allow the moisture content of the dry, ure 12a shows the radial component of the wind at t ϭ subsiding air in the northern arc to be replenished via 6 h and 40 m above the surface. Inflow toward the latent and sensible heat fluxes over the warm sea waters

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Ϫ FIG. 13. Vertical wind speed (shaded, m s 1)att ϭ 6 h and 850 hPa for A19B600.

␪ pockets of low value e air embedded in a much larger ␪ Ͼ region of e 330 K than A19B250. To the southeast, ␪ very low e air attempts to penetrate this large moist envelope. A map of the low-level divergence (not shown) reveals a band of strong convergence coincident with the

FIG. 12. As in Fig. 11 but for the radial component of the wind (shaded, m sϪ1). The thin black line is the 0 m sϪ1 contour. Nega- tive (positive) values indicate flow toward (away from) the storm center.

to the west of the storm, before it enters the storm eyewall. In spite of being initialized with more moisture in terms of e-folding radius, A19B600 intensifies significantly more slowly during the first3hofthesimulation than A19B250. The most striking difference between the two cases is the larger size of A19B600 (Fig. 6) and the presence of longer and broader rainbands (Fig. 11). Apparently the presence of a larger initial moist envelope and, hence, a larger area of unstable air surrounding the storm center, allows the formation of large rain- ␪ bands. As a result, the low-level e field of this case takes on an entirely different picture (Fig. 9). Collo- FIG. 14. Equivalent potential temperature (shaded; K) at cated with the convective cells in the rainband are t ϭ 12 h and 950 hPa for A19B600.

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rainband and eyewall. Low-level convergence into the winds at large radii remain stronger than in A19B250, convective cells of the rainband causes air beyond the maintaining strong surface fluxes at those radii. Both band to flow toward the band, in a direction toward the the demise of the rainbands and the persistence of Ͻ storm center, or negative radial wind ur 0 in Fig. 12b. strong surface fluxes at large radii allow more energy to This is especially prominent to the southwest of the be fed into the storm’s eyewall, increasing the convec- storm over water. At the same time, air in the moat tive activity and allowing rapid intensification of region between the eyewall and rainband also flows A19B600 between t ϭ 6 and 12 h. ϭ ␪ toward the band, but away from the storm center, or By t 12 h, the low- e air seen to the southeast of the Ͼ ϭ ur 0 in Fig. 12b. Powell (1987) observed a similar storm at t 6 h has rotated cyclonically and inward pattern of radial flow between rainband and eyewall as around the TC center to reach to the northeastern side Hurricane Alicia (1983) was making landfall. As a re- of the TC (Fig. 14). Three hours later (not shown), dry sult, the overall inflow into the storm eyewall is reduced air is observed to entrain into the eyewall at low levels compared with a case without rainbands (Fig. 12a), (900 hPa and below) on the landward side of the storm which may explain the slow intensification rate of where protective rainbands have been eroded away by A19B600 during the first6hofthesimulation. The the impinging dry air and reduced surface fluxes over vertical motion field (Fig. 13) reveals that the rainbands land. ␪ consist of strong up- and downdrafts. High- e (or un- Limited rainband activity in A19B250 allows early ␪ stable) air ascends in the updrafts, while low- e air de- and continuous penetration of dry air into the storm scends in convective downdrafts (Powell 1990). This eyewall. The onset of A19B250’s weakening occurs ␪ explains the pockets of low- e air seen in Fig. 9b. Over while surface fluxes remain high (not shown) since the ␪ land, low- e air in the moat region is even more promi- RMW and strongest surface fluxes do not reach the nent because of the reduced surface fluxes over land. A coastline until 12 h later. Vertical shear over the system ␪ Ϫ1 pool of lower- e air over land, between the core and is below the threshold value of 8.5 m s (Fitzpatrick eyewall, was also observed in Hurricane Alicia (Powell 1996). All three points suggest that dry air intrusion was ␪ 1987). The inflow of lower- e air from the moat further the main contributor to the onset of weakening in the reduces the strength of the eyewall convection and B ϭ 250 km case. may, therefore, contribute to the slow intensification In both cases, dry air intrusion into the eyewall is ␪ rate of A19B600 in the first 6 h. Besides low- e air, seen at low levels (mostly below 900 hPa) because in- rainband downdrafts may also bring down stronger flow at higher levels is weak. This lowers CAPE and winds from aloft (Powell 1982), contributing to the reduces both the latent heat release in the eyewall and storm’s larger size (Fig. 6). In addition to vertical ad- compensating subsidence in the eye. Both contribute to vection of air with higher tangential momentum, such a rise in central surface pressure and storm weakening. air may also be radially advected from the core of the In observed storms, dry air intrusion was seen at 500 storm toward the rainbands by the outward-flowing air and 700 hPa (e.g., Curtis 2004) and Special Sensor Mi- Ͼ (ur 0) that was observed in parts of the moat region crowave Imager images of total precipitable water (Fig. 12b), further explaining the larger size of the around Hurricane Opal (1995) indicate a dry air intru- storm. sion to within 222 km of the core (Rodgers et al. 1998). From the above, it appears that the rainbands may These observations, however, were at too coarse a reso- initially act to reduce eyewall convection, resulting in lution to precisely observe at which levels and radii the low initial intensification rates of A19B600. At the dry air intruded. Chan and Liang (2003) observed a same time, dry air intrusions like those observed in cyclonic and upward rotation of dry air from the land- A19B250 were not observed in the large B case (Fig. 9) ward side of an idealized landfalling TC. Their land during the initial 12 h. In other words, the rainband surface was flat with a straight coastline and the model seems to act as a thermodynamic boundary between the storm approached land at a perpendicular angle. In this dry environmental air at midlevels and the storm core. study there is no evidence of upward advection of dry The rainbands functioning as a boundary between the air, possibly because of the more complex configuration hurricane core and its environment was also suggested of the experiments and the fact that once dry air en- by Willoughby et al. (1984) and observed in Hurricane trains into the eyewall it is modified by microphysical Alicia (1983) by Powell (1987). This blocking role of processes. the rainbands is what may allow A19B600 to intensify 2) A19B500 for a longer period of time than A19B250. Between t ϭ 6 and 12 h, rainband convection gradually reduces, es- This case behaves similarly to A19B600 discussed pecially over land (not shown). However, low-level above. The storm is slightly smaller (Fig. 6) than its B ϭ

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600 km counterpart and follows a similar track for most of the simulation (Fig. 7), but makes landfall about 20 km farther east. Deepening during the first9his slightly larger, but filling commences 3 h earlier. The ␪ Ͼ area of e 332 K is slightly smaller as is to be expected given the smaller B. Rainbands form, but convection is somewhat weaker and dissipates earlier than in A19B600. Hence, the rainband’s initial dampening effect on eyewall convection is reduced, but so is its protective role against environmental dry air intrusion as compared with the larger B case. This likely explains the larger initial intensification rate and the earlier start of the filling process of A19B500. Similarly to A19B600, dry air wraps cyclonically around the storm from the southeastern quadrant. After the rainbands over land have eroded, dry air is observed to intrude into the eyewall on the landward side of the storm at t ϭ 12 h, 3 h after the onset of weakening. However, because of the 3-hourly model output times, the exact onset of dry air intrusion and weakening cannot be de- termined and may have occurred anywhere between t ϭ 9 and 12 h.

3) A18B400 AND A18B250 Like small-radius case A19B250, A18B250 displays FIG. 15. Time series of A19B400 (solid) and NO-LAND (dashed) for (a) PSMIN (hPa) and (b) size (km). minimal rainband development and dry air intrusion begins at t ϭ 6 h. In a similar manner to A18B250, dry air from the environment spirals cyclonically and in- eventually penetrates the storm eyewall from the landward toward the TC center where it eventually entrains ward side. into eyewall updrafts. The storm begins to weaken at 4) A19B400 AND NO-LAND t ϭ 6 h or 13 h before the RMW reaches the coastline. This suggests weakening is caused by dry air intrusion To investigate the role of land in the weakening pro- and not by the effects from land. The storm’s large B cess of the B Ͼ 250 km cases, a simulation identical to counterpart, A18B400, has an initially smaller deepen- A19B400 but without land (NO-LAND) is performed. ing rate but the deepening lasts longer: until t ϭ 12hor If land is not present, the storm may be able to continue 3 h before the RMW crosses the coastline. A18B400 is to protect itself again dry air intrusion. The minimum a larger storm with more rainband activity than central pressure and size evolution of NO-LAND is ␪ Ͼ A18B250. A large area of e 330 K exists with pock- compared with that of A19B400 in Fig. 15. There is a ␪ Ͻ ets of lower values of e in the moat region, especially small difference ( 2 hPa) during the initial 12 h of the over land. Weaker inflow into the eyewall occurs to the simulation during which period both storms intensify. northwest of the storm center and inflow into the rain- At t ϭ 12 h the PSMIN differences become larger as band occurs on both sides of the band. This points to a A19B400 begins to weaken while NO-LAND continues similar dual role of the rainband to that discussed in the to intensify. At t ϭ 24 h, NO-LAND appears to reach a previous sections. Initially the band reduces storm in- steady state with an intensity of about 974 hPa. For the tensification by 1) reducing the magnitude and 2) low- first 27 h, the two storms remain similar in size (Fig. ering the energy level of the eyewall inflow. Simulta- 15b). At t ϭ 27 h, A19B400 suddenly drops in size as neously, the band forms a barrier against the dry envi- land approaches (Fig. 7). Reflectivity images reveal that ronmental air allowing the storm to continue over water both storms possess rainbands of similar intensifying for a longer period of time. As the storm horizontal extent and intensity. After t ϭ 12 h, rainband approaches land, dry environmental air wraps around convection ceases over land in A19B400, but the storm the storm from the southeast to reach the landward side remains large in size. Since size is calculated over water of the storm. Over land, reduced surface fluxes cannot only, this is in agreement with the observed rainband replenish the air with heat and moisture and the dry air activity.

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␪ ϭ Figure 16 compares the 950-hPa e field at t 12hof both cases and shows a tongue of dry air to the southeast of both storms. However, the presence of rainbands and their convective activity and accompanying ␪ high surface winds creates a zone of high- e air around the storm center that prevents this dry air from reaching the core. In A19B400 this zone is considerably less moist over land because of the reduced surface fluxes. As a result, drier air is observed to spiral cyclonically and inward from the landward side of the TC to its southwestern side. This continues through the remainder of the simulation and the drier land air eventually connects with the extremely dry environmental air to ␪ the southeast. In NO-LAND, low- e pockets exist in the moat area as a result of rainband outflow, but the moat air continues to be replenished by strong surface fluxes over water. The tongue of dry air to the southeast surges back and forth and manages to reach the northern side of the TC by t ϭ 21 h; however, by t ϭ 33hit has receded back to the southeast corner as it cannot overcome strong surface fluxes. The azimuthally averaged sum of the latent and sensible heat fluxes are displayed in Figs. 17a and 17b as a function of radius and time. Also shown are the lowest- level wind speeds for both cases (Figs. 17c and 17d). As was discussed in section 3a, the surface fluxes are strongest in the eyewall region where the strongest winds occur. Initially, the surface fluxes are stronger in this area in the NO-LAND case consistent with the stronger low-level wind speeds of that case, but between t ϭ 11 and 18 h, the reverse is true despite weaker wind speeds in A19B400. In both cases, the eyewall is located over water during this time, making the surface values of T and q identical in both cases. Therefore, the larger surface fluxes must be caused by FIG. 16. Equivalent potential temperature (shaded; K) and wind vectors at 950 mb and t ϭ 12 h for (a) A19B400 and (b) NO- a larger vertical gradient in T and q between the surface LAND. The thin black line is the 336-K contour. and the lowest model level (Blackadar 1979; Zhang and Anthes 1982) in A19B400. This means that T and q at the lowest model level are smaller in A19B400, which is whose convection brings strong wind speeds to low lev- ␪ ␪ most likely caused by the low- e air that was seen ro- els at large radii and mixes high- e air from the surface tating around the storm core from the landward side into the low-level hurricane inflow layer. This prevents (Fig. 16a). By t ϭ 15 h, the region of maximum surface the entrainment of dry environmental air into the eye- fluxes has crossed land (black line in Fig. 17a) and the wall. When land is present, reduced surface fluxes fluxes reduce while the storm begins to weaken. Sur- break up the protective barrier on the landward side of face fluxes at radii beyond the eyewall are consistently the storm. Dry air reaches the storm core before the larger in the NO-LAND case compared with A19B400. storm center makes landfall but for a while surface This is because of the presence of land to the north of fluxes in the core continue to replenish the air with the storm center in the latter case. energy before it ascends in the eyewall. As the storm approaches land, however, the core surface fluxes de- 4. Discussion and conclusions crease and at some point lose their battle against in- It seems that a storm with an initially large moist creasing dry air intrusion. envelope can survive in a dry environment without sig- Real Gulf Coast landfalling hurricanes have been ob- nificant wind shear. Such a storm develops rainbands served to start weakening before their centers crossed

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Ϫ FIG. 17. Azimuthally averaged time–radius series for (a) sensible plus latent heat flux (shaded; W 2) for A19B400, (b) sensible plus latent heat flux (shaded; WϪ2) for NO-LAND, (c) 40-m wind speed (shaded; m sϪ1) for A19B400, and (d) 40-m wind speed (shaded; m sϪ1) for NO-LAND. The black line indicates the shortest distance between the hurricane center and the coastline at each time. the coastline, for example, Opal (1995) and Ivan (2004). environmental air above approximately 850 hPa In both cases the convection was eroded away in part of rotates cyclonically and inward around the storm the eyewall as a result of dry air intrusion. Both these center. When it encounters an eyewall downdraft storms, however, also interacted with a midlatitude or increases its density because of evaporational trough as they were approaching the coast, the same fea- cooling, the dry air sinks to lower levels, reaches ture that introduced the dry air. Trough interaction can be strong low-level inflow, and is entrained into eye- unfavorable to hurricane intensification because of large wall updrafts. This reduces eyewall CAPE and, vertical wind shear, but also favorable (e.g., Molinari hence, latent heat release in the eyewall as well as and Vollaro 1989; Sadler 1976). The results from this compensating subsidence in the eye. Both effects study indicate that the already complex problem and cause weakening of the warm core and a rise in difficult predictability of hurricane–trough interaction central surface pressure. may be further complicated by the presence of dry air. Over water, full access to surface energy may allow a On examining the intensity and structure evolution hurricane to fend off negative effects from dry air of eight model storms, initialized with different mois- intrusion. This is likely a function of the size of the ture profiles, several important conclusions regarding initial moist envelope, which should be further in- the role of environmental dry air can be drawn: vestigated in future work. Storms with initially small moist envelopes cannot When the proximity to land causes a reduction in protect themselves against the intrusion of dry air surface fluxes and an increase in surface friction, from the environment and weaken well before in- the surface energy supply to the hurricane may be creased friction and reduced surface fluxes from insufficient to counteract the negative effects from land begin to play a role. enhanced dry air entrainment. The outcome will The scenario for dry air intrusion is as follows. Dry depend on the size of the initial moist envelop sur-

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rounding the storm and the degree of dryness of present between the eyewall and rainband, be- the environment. cause lower surface fluxes over land prevent the air ␪ Different initial moisture distributions impact TC in- from being replenished with high values of e. tensity, structure, and motion. This is because Rainbands can also prolong the intensification of steering depends on TC intensity and structure. TCs, by acting as a thermodynamic boundary be- ␪ Hence, TC landfall location and damage potential tween the high- e core and dry environmental air. are sensitive to initial moisture content. Landfall While the above rainband conclusions seem to con- locations differ by at most a 20-km distance (Fig. tradict, the extent to which either plays a role de- 7). While this may seem small, this lies within the pends on the structure of the individual storm as range of the values of the RMWs at landfall (Fig. well as its surrounding environment. The protec- 5). Therefore, this could mean the difference be- tive role of the rainbands will likely dominate in a tween hurricane force winds or barely tropical storm in a dry environment; whereas in moist sur- storm force winds at a given location. Storm sizes roundings, rainbands may prevent storms from at landfall range from about 90 to 145 km (Fig. 6); reaching their maximum potential intensity. In the this means that the area of tropical storm force or latter case, strong rainbands feeding off the moist higher winds ranges from 25 447 km2 to 66 052 environment simultaneously discharge dry, cold km2. This is a substantial increase in the surface downdraft air toward the storm core. area receiving damaging winds. A fine balance exists between dry air intrusion and Moisture is not well measured operationally and, eyewall CAPE production. Over warm water, eye- therefore, not directly assimilated in operational wall CAPE production is sufficient (because of hurricane models, yet this study shows that TC strong surface fluxes) to overcome small amounts models are highly sensitive to the amount and dis- of dry air intrusion caused by (i) the proximity to tribution of initial moisture. If moisture is not cap- land, (ii) surrounding dry environmental air, and/ tured correctly in the analyses or shifted to differ- or (iii) rainband outflow. The TC decays, however, ent scales because of model resolution, the track, if dry air intrusion increases above a certain thresh- intensity, and structure of the TC varies substan- old and/or local eyewall CAPE production falls be- tially. To obtain more accurate TC model predic- low a certain threshold—because of landfall or a tions, it is crucial that accurate initial moisture decrease in SST. The value of these thresholds de- measurements are incorporated both in the core pends on both the nature of the environment and and around the TC. This is supported by a study the structure of the hurricane. The problem can be done by Kamineni et al. (2003) who assimilated further complicated by the presence of a midlati- Lidar Atmospheric Sensing Experiment moisture tude trough, often accompanied by dry air in ad- profile data into the forecasts of three hurricanes. dition to strong vertical wind shear, as well as In each case, a significant improvement in track mechanisms that possibly enhance hurricane inten- and intensity forecasts was obtained. sification. Therefore, predicting if, when, and how More moisture in the core of the storm, with every- much storm weakening occurs before landfall thing else kept equal, leads to a more intense poses a challenging problem. storm, as expected. More low-level moisture in the core increases CAPE and convective activity in the Acknowledgments. The author thanks GFDL for eyewall. This leads to stronger eye subsidence, a supplying the atmospheric analysis and the U.S. Navy warmer core, and hence a lower PSMIN. for providing the OTIS SST field. Much gratitude is due Increasing the radial extent of the initial moisture to Keith Blackwell for in-depth discussions and sugges- content enhances the formation of rainbands. tions. Holly Allen, Christopher Dyke, and Robert Bar- Rainband activity causes wind speeds to increase bre generated many of the graphics to analyze model at larger radii and, hence, causes the storm to be output. All simulations and analyses were performed larger in size. This, in addition to larger areal cov- on Sun Microsystems Inc. machines that were acquired erage of heavy rainfall, makes the storm poten- through a SUN Mircosystems Inc. equipment grant. tially more destructive. 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