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766 WEATHER AND FORECASTING VOLUME 26

A Review of Three Signiﬁcant Wake Lows over Alabama and Georgia

TIMOTHY A. COLEMAN AND KEVIN R. KNUPP Department of Atmospheric Science, University of Alabama in Huntsville, Huntsville, Alabama

(Manuscript received 15 February 2011, in ﬁnal form 24 March 2011)

ABSTRACT

The kinematics and thermodynamics of wake lows have been extensively examined in the literature. However, there has been relatively little focus on the widespread, sometimes very strong winds associated with wake lows. Some wake lows are, essentially, severe local storms, producing widespread and sometimes intense damage, similar to that of a derecho, but they occur in environments supporting elevated convection, a phenomenon not often perceived as a signiﬁcant wind damage threat. Three signiﬁcant wake lows that affected Alabama and/or Georgia, producing widespread (25 000–50 000 km2) wind damage, and local wind gusts near 25 m s21, are reviewed in detail. The environments and morphology of the wake lows are ad- dressed, using radar, surface, and upper-air data.

1. Introduction lows where the parent MCS moves over a surface in- version or stable layer (e.g., Bosart and Seimon 1988). A wake low may be described simply as a mesoscale Johnson and Hamilton (1988) proposed that the area of low pressure at the rear of a mesoscale convective subsidence is associated with the rear-inflow jet (RIJ) system (MCS). Wake lows were initially examined by in the MCS. Many studies have shown that wake lows Fujita (1955) and have since been studied by many au- are most intense when the RIJ is ‘‘blocked’’; that is, it thors (e.g., Handel and Santos 2005; Gaffin 1999; Johnson does not flow through the entire stratiform precipita- 2001). Yet, a comprehensive understanding of their ther- tion area (e.g., Stumpf et al. 1991; Johnson and Bartels modynamics remains ‘‘elusive’’ (Johnson 2001). Loehrer 1992). This blocking is associated with large conver- and Johnson (1995) noted their strong winds. The con- gence aloft and a rapid descent of the RIJ. Micro- sensus is that wake lows are associated with subsidence at physical processes help produce the downward vertical the rear of an MCS; cooling due to sublimation, melting, motion. Schmidt and Cotton (1990) suggest that gravity and evaporation in the descending air is more than offset waves play a role in MCS-related surface pressure by adiabatic warming, producing an ‘‘overshooting’’ bot- perturbations. tom associated with strong positive buoyancy (e.g., The intent of this paper is to examine three significant Gallus and Johnson 1995; Smull and Jorgensen 1990; wake lows over Alabama and Georgia. We will examine Smull et al. 1991; Johnson and Hamilton 1988). The warm their environments, typically including a low-level stable perturbation hydrostatically produces negative pressure layer and midlevel dry air behind the MCS. Radar and perturbations at the surface. This overshooting is ther- surface observations will be examined. The magnitudes modynamically similar to a heat burst (e.g., Bernstein and of the surface winds in the wake lows, and the damage Johnson 1994). Stumpf et al. (1991) found that significant they produced, will be observed, including factors con- pressure falls in a wake low could be directly attributed to tributing to the presence of these winds. It should be the downward depression of a surface-based cold pool noted that the strong winds associated with most severe (Johnson 2001; see Fig. 1), and suggested that such squall lines are from a westerly direction, while those in a process may explain the very large pressure falls in wake wake lows are from an easterly direction. This may allow for more efficient downing of trees in wake lows, given that a different subset of trees is exposed/susceptible Corresponding author address: Dr. Timothy A. Coleman, Dept. of Atmospheric Science, University of Alabama in Huntsville, to the highest winds in wake lows than those exposed/ NSSTC, 320 Sparkman Dr., Huntsville, AL 35805. susceptible to the more frequent strong westerly winds E-mail: [email protected] accompanying squall lines.

DOI: 10.1175/WAF-D-11-00021.1

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NCDC 2011). At least 150 000 Alabama Power Company customers lost power, and damage was reported in more than 50 counties in Alabama and Georgia (NWS 2009; NCDC 2011). Pressure falls of 8–10 hPa in 2 h were common, as were wind gusts greater than 20 m s21, with the highest recorded wind gust (26 m s21) occurring at an elevated site in downtown Birmingham, just south of the most intense damage swath (for surface observations in all threewakelows,seeFig.2).Thewakelowwaspropa- FIG. 1. Schematic cross section of an MCS with a mesohigh at gating toward the southeast (from 3108)at16m s21. Since the front and a wake low at the back edge of the stratiform pre- the perturbation winds in a wake low are in the opposite cipitation area (adapted from Johnson 2001). The gray circled area illustrates the area where the surface-based cool air (cold direction of the wake low propagation, ambient easterly pool or stable layer) is depressed downward, causing a fall in low-level winds (also opposite of the propagation di- surface pressure. Vectors are not necessarily to scale; the stron- rection) enhanced the net winds in the wake low (e.g., gest winds in the wake low are located at the point of lowest Coleman and Knupp 2009). pressure. At least one of the high wind warnings issued by the National Weather Service for this event referred to the 2. A review of the events wake low as a ‘‘gravity wave.’’ This points to an inter- a. 13 April 2009 esting and often confusing distinction between ducted gravity waves and wake lows. As shown in Fig. 3a, the An intense wake low propagated across much of lower levels of the atmosphere were rather stable, with northern Alabama and northern Georgia during the average N between 0.015 and 0.020 s21, but slightly less morning hours of 13 April 2009. This wake low was a stable air was located above 1500 m MSL. Coleman widespread, signiﬁcant weather event, downing thousands (2008) showed that, for any vertical displacements above of trees (many onto homes and automobiles), causing an a point in the atmosphere, estimated $4 million in property damage, several injuries, ð ‘ and even one fatality in the Atlanta metropolitan area 2 p95 N r0dzdz, (1) (due to a tree falling on the person’s vehicle) (NWS 2009; z

21 FIG. 2. Observed surface pressure (hPa, solid dark line) and wind gusts (m s , gray dots) from the three wake lows examined herein: (a) Atlanta, GA (ATL), 13 Apr 2009; (b) BHM, 20 Dec 2007; and (c) BHM, 22 Feb 1998. (d) The analog wind trace from BHM, showing a peak wind gust of 49 kt (25 m s21) on 22 Feb 1998 [(c) and (d) are adapted from Bradshaw et al. (1999)].

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21 FIG. 3. Vertical profiles of the Brunt–Vaïsa¨la¨ frequency (N,s ), representing static stability. Data are derived from soundings at (a) 1200 UTC 13 Apr 2009, Peachtree City, GA (FFC); (b) 0000 UTC 21 Dec 2007, Calera, AL (EET); and (c) 1200 UTC 22 Feb 1998 at EET. where p9 is the pressure perturbation at height z, N is the precipitation. This was likely associated with the deep, 1/2 Brunt–Vaïsa¨la¨ frequency fN5[(g/u)(du/dz)] g, r0 is descending RIJ (Fig. 5b) that did not penetrate the entire the unperturbed density at each z, and dz is the vertical stratiform precipitation area, meaning it was ‘‘blocked,’’ as displacement at each height. Therefore, for a given is often the case in intense wake low events (see section 1). vertical displacement, the pressure perturbation below it The evaporation of precipitation was likely aided by dry will have larger magnitude when the static stability is air in the RIJ. Figures 6a and 6b shows an 8-km-deep and higher, consistent with the discussion in section 1. An 400-km-wide region of dry air (relative humidity , 30%) atmosphere with a surface-based stable layer topped by to the west of the MCS, near the main source region of RIJ a conditionally unstable layer, like the one on 13 April air. The 0.58 PPI velocity image (Fig. 4b) shows the strong 2009, is favorable for elevated convection and intense low-level winds at the back edge of the stratiform rain wake lows. However, this same atmosphere is favorable area; the zoomed-in area indicates winds near 30 m s21. for ducted gravity waves (e.g., Koch and O’Handley To eliminate the ‘‘cone of silence’’ (COS) in the radar 1997; Lindzen and Tung 1976), often leading to ambi- cross sections, a method was employed that assumed the guity as to the nature of the pressure and wind event in wake lows were two-dimensional and quasi–steady state this environment. It is possible that the subsidence re- over a short period of time, 10–20 min (valid assumptions sponsible for a wake low may also initiate ducted gravity in each of these cases). A time-to-space conversion (thus waves in the low-level stable layer; further discussion of requiring the steady-state assumption) was performed. that mechanism is beyond the scope of this paper. The data from an earlier volume scan, when mesoscale Radar observations from 13 April 2009 (Figs. 4a and 5a) features obscured by the COS at the main volume scan show the classical features of a wake low. The lowest- time were visible, were advected at the speed of movement elevation plan-position indicator (PPI) reflectivity scan of the wake low, and substituted only in the COS region. indicates an asymmetric MCS (e.g., Skamarock et al. 1994; The data were then smoothed and fitted onto a Cartesian Loehrer and Johnson 1995, especially their Fig. 22). There grid, with dx 5 1kmanddz 5 250 m. was a long, fairly sharp gradient in radar reflectivity at the Another inference may be made from Eq. (1). The p9 back edge of the MCS (Fig. 4a) that extended almost 3 km at a given height in the atmosphere is determined by the vertically (Fig. 5a), underneath anvil precipitation. These integral of all vertical displacements above that point. Also, sharp reflectivity gradients are a common feature in wake the vertical displacements at lower levels (with higher lows (e.g., Haertel and Johnson 2000). The horizontal density r0) have the largest effect on the pressure pertur- gradient in reflectivity at 2 km MSL was 4 dB km21, bation below. Therefore, the maximum p9, and therefore indicating significant subsidence and evaporation of the maximum wind perturbations, should occur at or near

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FIG. 4. The 0.58-elevation PPI radar scans (100-km range rings). (a) Reflectivity (dBZ; see legend at bottom) from the BMX Weather Surveillance Radar-1988 Doppler (WSR-88D) at 0637 UTC 13 Apr 2009. (b) As in (a), but for base radial velocity (m s21; see legend at bottom). (c) The reflectivity from BMX at 2227 UTC 20 Dec 2007. (d) As in (c) but for velocity. (e) Reflectivity from the FFC WSR-88D at 1808 UTC 22 Feb 1998. (f) As in (e), but for velocity. The two white squares in (b) and (d) indicate the area of the zoomed-in base velocities in the insets (m s21, legend at right in each panel). In these insets, deep purple indicates radial velocities . 25 m s21, and peach colors indicate velocities . 30 m s21.

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FIG. 5. Cross sections (as described in the text) of radar data (a) along the azimuth from 3108 (negative range values) to 1308 (positive range values) of reﬂectivity (dBZ) from the BMX radar at 0637 UTC 13 Apr 2009. (b) As in (a), but for radial velocity (m s21), with positive values representing winds toward positive ranges on both sides of the radar. (c),(d) Similar to (a),(b), but at 2227 UTC 20 Dec 2007, along the azimuth from 3008 to 1208. (e),(f) As in (a),(b), but at 1808 UTC 22 Feb 1998 from the FFC radar, along the azimuth from 2258 to 458. See text for an explanation of the lack of COS. the surface. This was clearly the case in this event. Doppler and a roof was blown off of a business in Decatur, Alabama velocity images (Fig. 5b) show winds decreasing from (NWS 2007). The surface pressure dropped 8 hPa at 225 m s21 at 750 m AGL (the sign is relative to the wake BHM in about 1 h, with measured wind gusts at BHM low propagation direction) to about 15 m s21 in the RIJ reaching 16.5 m s21. Higher gusts of 20–24 m s21 were at 3 km AGL, producing a vertical wind shear of recorded at slightly elevated stations (e.g., Elliott 2010). O(1022 s21). This wind shear in wake lows may present The wake low was propagating toward the southeast at a hazard to aviation (e.g., Johnson 2001; Meuse et al. 1996). 13 m s21. In this case, velocity–azimuth display (VAD) analysis indicates ambient 5 m s21 winds at 150 m AGL b. 20 December 2007 from the east-southeast; therefore, these winds also Another signiﬁcant wake low affected much of northern added to the wake low perturbation winds. The atmo- Alabama on 20 December 2007, including the Birming- sphere over northern Alabama was again characterized ham (BHM) metropolitan area during the evening rush by conditional instability aloft, with a layer of more hour. Numerous trees and power lines were blown down, stable air near the surface (the average N below 1800 m

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FIG. 6. (a) National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis of 500-hPa RH (%) at 0600 UTC 13 Apr 2009. (b) North American Mesoscale Model (NAM) east–west cross section of RH along the line in (a), where arrows indicate the approximate location of RIJ, based on radar data. (c) NCEP–NCAR reanalysis of 500-hPa RH (%) at 1800 UTC 20 Dec 2007. (d) As in (a), but at 1800 UTC 22 Feb 1998. [Legends for (a),(c), and (d) are below (c) (Kalnay et al. 1996).]

MSL was approximately 0.02 s21; the average N from 22 February 1998, producing widespread structural dam- 1800 to 5000 m was less than 0.01 s21; see Fig. 3b). age, downed trees, and power outages (e.g., Bradshaw The parent MCS in this case was similar in morphol- et al. 1999). Pressure falls were not only large, but oc- ogy to that in section 2a, with an asymmetrical shape. curred very quickly (more than 8 hPa in less than 30 min Radar cross sections (Figs. 5c and 5d) show a strong, at Birmingham and Anniston, Alabama; see Fig. 2c). Peak descending RIJ, not penetrating the stratiform rain area, wind gusts at most Automated Surface Observing Sys- associated with rapid descent and a sharp horizontal tem (ASOS) stations were 15–20 m s21, but wind dam- reﬂectivity gradient (up to 5 dB km21) at the back edge age at higher elevations around Birmingham indicated of the rain. Once again, dry air was present to the wind gusts in excess of 30 m s21, and Birmingham– northwest of the MCS, in the source region of RIJ air Shuttlesworth International Airport’s analog wind re- (Fig. 6b). The RIJ and wake low combined to produce corder indicated a peak wind of 25 m s21 (49 kt; Fig. 2d) large vertical wind shear of around 0.016 s21. (Bradshaw et al. 1999). One possible reason for the rapid pressure falls was c. 22 February 1998 likely the depth of the low-level stable layer. In this case, A very large-amplitude wake low moved across almost the sounding from BMX (station located south of Bir- the entire state of Alabama, and parts of Georgia, on mingham) indicates an average N of 0.014 s21 up to

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4000 m MSL, with much less stable air (average N 5 conducive to elevated thunderstorms and large pressure 0.009 s21) above that. The parent MCS was structurally falls at the surface; 2) a strong, descending RIJ ‘‘blocked’’ different from than those discussed in sections 2a and 2b, in the stratiform precipitation region (shown in Doppler in that the wake low itself was only about 100 km di- radar velocity cross sections), producing strong down- rectly behind the most intense convection. This MCS drafts there; 3) dry air at upper levels behind the parent and trailing wake low moved toward the NE (from 2258) MCS; and 4) a sharp horizontal reflectivity gradient (4– at a rather fast 25 m s21. Bradshaw et al. (1999) point 5dBkm21) at the back edge of the stratiform precipita- out that the synoptic environment was favorable for tion region due to subsidence. Two of the three wake lows gravity wave generation (e.g., Uccellini and Koch 1987). were also accompanied by ambient surface winds in the The wind event discussed herein was a wake low, but it is opposite direction of the wake low propagation. Opera- possible that the entire MCS could have been associated tional forecasters may apply these observations to specific with a long-wavelength internal gravity wave, in a wave- MCSs to ascertain the possibility of a wake low event. conditional instability of the second kind (CISK; e.g., Lindzen 1974) process. The fairly rapid movement of Acknowledgments. The authors wish to thank Declan this wake low, and the ambient low-level winds being Cannon (NWS) for thoughtful discussions. The authors nearly parallel to the wake low’s movement, likely re- also wish to thank the reviewers. This research was funded duced the winds somewhat (e.g., Coleman and Knupp by a grant from the National Oceanic and Atmospheric 2009). However, the very large magnitude of the pres- Administration (NOAA Grant NA07OAR4600493). sure falls still indicates that wind perturbations should be around 20 m s21, according to the nonlinear imped- REFERENCES ance relation (Coleman and Knupp 2010), and this is Bernstein, B. C., and R. H. Johnson, 1994: A dual-Doppler radar consistent with surface observations. study of an OK PRE-STORM heat burst event. Mon. Wea. Radar data (Figs. 4c and 5c) show a tight reflectivity Rev., 122, 259–273. gradient extending over 300 km horizontally. This wake Bosart, L. F., and A. Seimon, 1988: Case study of an unusually low was associated with a very intense, descending RIJ, intense atmospheric gravity wave. Mon. Wea. Rev., 116, 1857– 21 1886. with wind speeds in the RIJ of 50 m s .However,asin Bradshaw, J. T., R. A. Murphy, and K. J. 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