P7.10 EYEWALL EVOLUTION OF HURRICANE KATRINA NEAR LANDFALL USING NEXTRAD REFLECTIVITY AND RADIAL VELOCITY DATA analysis features are outlined in section 4 along Kimberly D. Campo and Thomaswith M.specific Rickenbach adequacies and inadequacies of Doppler radar data use with the context of Department of Meteorology, Sanhurricane Jose State eyewall University strength and structure. Section 1. Introduction1 5 provides details of the results found in the examination of the evolution of the eyewall region Katrina was an exceptionally large and of Hurricane Katrina as the system approached intense hurricane causing an estimated $75 billion and made landfall, followed by concluding remarks in damage and 1,336 deaths across five states. in section 6. The sheer physical size of Katrina caused devastation far from the eye, with an estimated radius of maximum winds extending nearly 56 km, 2. Background and previous studies making Katrina quite possibly the largest hurricane of its strength on record. On 29 August, Katrina’s One of the main goals of NOAA’s storm surge breached the levee system that Hurricane Research Division (HRD) is to augment protected New Orleans, flooding nearly 80% of the the ability to predict TC intensity upon landfall. A city and permanently destroying 275,000 to TC undergoes large intensity changes as it 300,000 homes. The extent, magnitude, and approaches land, but little is understood in regards impacts of the damages caused by Katrina are to mechanisms for these intensity changes. astounding, resulting in public outrage in the U.S. Accurate TC wind field derivations and surface government’s response to the disaster, and a measurements near and post-landfall are essential realization that further studies are vital to better to understanding landfalling TC decay. In addition, understand landfalling Gulf Coast hurricanes. the United States Weather Research Program Since the late 1960s, numerous studies (USWRP) Fifth Prospectus Development Team have been conducted to better understand the (PDT-5) identified six key research objectives to rapid intensification (RI) a tropical cyclone (TC) better understand and forecast landfalling TCs undergoes over the open ocean. While forecasting (Marks et al. 1998), one of which is to research TC intensity changes prior to landfall is imperative, and develop techniques to improve wind field and understanding the structure and decay patterns of precipitation patterns of landfalling systems. TCs post-landfall and when the TC is only a few A major source of difficulty in predicting hundred kilometers off the coast, is crucial since hurricane intensity, wind fields, and storm surge at populations continue to increase in these landfall in the past have been the inability to vulnerable coastal regions. measure the surface wind field directly and to TCs are driven by latent heat flux over the predict how the system changes in response to warm ocean surface. This heat flux increases both external and internal forcing (Marks et al. rapidly with increasing wind speed, making mature 1998). Currently, surface wind field parameters TCs effective heat engines. Upon landfall, this are estimated using flight-level wind observations latent heat flux shuts down and frictional conducted by both NOAA’s HRD and air force dissipation decreases the storm’s kinetic energy, reconnaissance (AFRES) aircraft. These flight- causing the storm to dissipate and the eye to fill level observations are adjusted to estimate with clouds and precipitation. surface wind speeds, using a planetary boundary This study aims to document the decay layer (PBL) model (Zhang et al. 1999). Multiple patterns of Hurricane Katrina as it made landfall flight-level research experiments and Doppler along the Gulf Coast using land-based NEXRAD radar measurements suggest that maximum mean radar reflectivity and radial velocity data. Section 2 wind speeds are found at 0.5-2.0 km above sea provides a literary background of previous level, while HRD flights are conducted at 2.5-3.2 landfalling TC studies, followed by a synoptic km levels. This method of estimating the surface summary of Katrina and a discussion of RI periods wind field is problematic due to variations with in section 3. The WSR-88D radar and NCAR Solo height of the storms’ structure, environmental wind shear, and our uncertainty involving the internal 1 Corresponding Author Address: Kimberly Campo, dynamics of TCs. Department of Meteorology, San Jose State University, Since 1982, NOAA’s HRD has conducted One Washington Square, San Jose, CA 95192 a series of experiments with research aircraft using multiple Global Positioning System (GPS) flight-level Air Force reconnaissance aircraft data, dropwindsondes which measure the vertical assimilated into a PLB model, to describe changes profiles of wind, temperature, and humidity from in the surface wind field of several landfalling flight level to the surface. Dropwindsondes provide hurricanes. Dodge et al. (1999) used the pseudo- the high-quality data source needed for the dual Doppler radar on HRD’s WP-3D aircraft to improvement of TC winds and thermodynamic derive the wind field of Hurricane Danny as it analysis. These data have reduced 12-60h model made landfall along the Gulf Coast, and included forecast track errors by as much as 30% (Marks et NEXRAD radial velocities in their synthesis for al. 1998). Franklin et al. (1999) estimated the comparison. Geerts et al. (2000) used ground- quantitative relationship between flight-level and based radar, EDOP, and ER-2 passive microwave surface winds by analyzing GPS dropwindsonde imagery to observe the changing surface wind data. Their results suggest that to reasonably field of Hurricane Georges as it made landfall in assess the surface wind field, the aircraft would the Dominican Republic. have to fly at the 800-1200 m level. By using a combination of WSR-88D land- The next goal the PDT-5 intends to based Doppler velocities with the damage track of accomplish is the improvement of forecasting the Hurricane Andrew, Wakimoto and Black (1994) near surface wind field during landfall (6 hours were able to identify important information before and after landfall), pre-landfall (the period regarding the structural changes that occurred as up to 48h prior to landfall), and post-landfall (>6h Andrew made landfall. Tuttle and Gall (1999) after landfall). To complete this task, a mobile found it useful to complement single-Doppler observing system was developed which is based velocities with estimates of the horizontal winds of upon the NOAA (WP-3D and G-IV), NASA, and landfalling hurricanes, by objectively tracking AFRES aircraft, portable profiles, Doppler radars, reflectivity echo patterns as they rotate about the and surface mesonets. This mobile unit is placed eye. This method was found to work reasonably in the landfalling path of an approaching TC, well for well organized hurricanes, but not in the providing a unique opportunity to sample the case of Katrina whose eye became significantly destructive winds of a TC offshore before and asymmetrical near landfall with the deterioration of during landfall. the eyewall in the southwest quadrant. Although the advent of new technological tools used to better understand and predict landfalling TC wind fields and decay patterns has 3. Storm synoptic history proved promising to our understanding of landfalling TCs, the continued use of classical a. Synoptic history methods of observing TC changes remains vital. The land-based Weather Surveillance Radar-1998 On 23 August 2005 the National Hurricane Doppler (WSR-88D) has been used to verify the Center (NHC) reported that Tropical Depression new observing systems (e.g., Geerts et al. 2000, Twelve had formed over the southeastern Dodge et al. 1999). WSR-88D units routinely scan Bahamas, upgrading to Tropical Storm Katrina on regions out to ~400 km range every 6 minutes, the morning of 24 August. The storm continued to thus have been used to monitor TCs that track intensify along its slow northwesterly then westerly near populated coastlines. Lee and Marks (2000) track through the Bahamas. By landfall in south and Lee et al. (2000) demonstrated that TC wind Florida, at 1030 UTC on 25 August, Katrina had fields could be retrieved using the ground-based strengthened to a Category 1 Hurricane. Katrina velocity track display (GBVTD) method. Although spent only seven hours over land before it entered errors do exist, algorithm improvements of WSR- the warm waters of the Gulf of Mexico (GOM) 88D velocity interpretation are continuously in along a nearly due westward path. Due to the development (Harasti et al. 2004). Harasti et al. unusually warm upper ocean thermal structure of (2004) showed that GBVTD and tracking radar the GOM (referred to in detail in section 3b) echoes by correlation (TREC) wind estimates Katrina quickly regained hurricane strength with using improved WSR-88D algorithms have the maximum sustained winds of 65 kts at 0600 UTC ability to retrieve TC wind speeds with an accuracy on 26 August (Knabb et. al 2005). A strong, very of ~2 m s-1 or better. To skillfully forecast TC large, upper-level ridge dominated the entire GOM surface wind fields, observations, theory, and 26 August, resulting in a region of weak wind modeling must continually be improved. shear and efficient upper-level divergence (two Powell and Houston (1998) combined key ingredients for hurricane development). automated surface observations (ASOS) and These synoptic conditions, along with the The eyewall quickly filled in as Katrina thermal structure of the GOM, allowed Katrina to moved inland and the storm’s overall intensity embark upon two periods of rapid intensification decayed rapidly, losing hurricane-strength 160 km (RI) (defined by a 30 kt or greater intensity inland, near Jackson, Mississippi by 0000 UTC 30 increase within a 24-h period) between 26 and 28 August. Katrina accelerated as it merged with an August. The first period of RI involved an increase eastward-moving trough over the Great Lakes, in maximum sustained winds from 65 kts to 100 affecting the central U.S.