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A Comparison of Rawinsonde Data from the Southeastern United States During El Niño, La Niña, and Neutral Winters

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A Comparison of Rawinsonde Data from the Southeastern United States During El Niño, La Niña, and Neutral Winters P5.7 A COMPARISON OF RAWINSONDE DATA FROM THE SOUTHEASTERN UNITED STATES DURING EL NIÑO, LA NIÑA, AND NEUTRAL WINTERS Victoria Sankovich* Pennsylvania State University, University Park, Pennsylvania Joseph T. Schaefer and Jason J. Levit, NOAA/NWS Storm Prediction Center, Norman, Oklahoma 1. INTRODUCTION statistically significant. An interesting paradox was noted by Browning (1998) in a study relating Meteorologists, as well as the public, have ENSO phase with severe thunderstorm activity in speculated on possible relations between the El northwest Missouri. He found a tendency for Niño/ Southern Oscillation (ENSO) phases and fewer tornadoes during El Niño years and more severe weather events occurring in the United tornadoes during La Niña years. In contrast, hail States. Schaefer and Tatom (1998) considered and wind storms showed the opposite tendency. the number of tornadoes per year in the United Because of the conflicting results from the States and sea surface temperatures (SST) in various studies, it was decided to study various portions of the Pacific Ocean to try to meteorological conditions rather than historic discern an impact of ENSO on the occurrence of weather events to try to get a clearer tornadoes. Their statistical inquiries showed no understanding of whether tornado and severe major influence, but their data does show a signal thunderstorm activity favors a specific ENSO that more tornadoes tend to occur in the mid- phase. Kinematic and thermodynamic parameters eastern states during the La Niña phase. Agee calculated from rawinsonde data are examined in and Zurn-Birkhimer (1998) also use the annual order to identify differences in the typical total of United States tornadoes to attempt to stratification of the atmosphere during the El Niño, determine a rise or decline in tornadoes during the La Niña, and Neutral ENSO phases. This study El Niño phase. They conclude that tornado concentrates on the southeastern region of the occurrences do not favor one ENSO phase but United States (North Carolina, South Carolina, rather exhibit a shift in geographic location. For Georgia, Florida, Tennessee, Alabama, example, their results suggest that more Mississippi, Arkansas, Louisiana, Oklahoma, and tornadoes occur in the lower Midwest, Ohio Valley, Texas) during the winter months (January, Tennessee Valley, and mid-Atlantic region during February, and March). These were chosen in the La Niña phase than in any other phase. Bove agreement with Montroy (1997), who found that (1998), using the bootstrap method on tornado the months of November and January through segments, found increased tornado activity over March exhibit a connection between Pacific SST the Ohio and Tennessee River Valleys during the and precipitation in the southeastern region. cold La Niña phase. The Climate Prediction Center (CPC) has Marzban and Schaefer (2001) correlated categorized every season by ENSO phase for all various measures of tornado activity over various years dating back to 1950 by evaluating the SST parts of the United States with the sea surface of the area along the equator extending from 150 temperature in four different areas in the Pacific degrees west to the international dateline. The Ocean. The strongest correlation found was very data are available from the CPC Internet site small (r = -0.07), but there was a statistically (Climate Prediction Center 2003a). The seasons significant, correlation between sea surface classified as El Niño (EN) in this research are temperatures in the central tropical Pacific and the those categorized by the CPC as being either number of days with strong and violent tornadoes moderate EN or strong EN. La Niña (LN) was (F2 or greater on the Fujita Scale) in an area likewise classified. Weak EN and weak LN are running from Illinois to the Atlantic Coast and grouped with the Neutral winters to create the Kentucky to Canada, i.e., a La Niña effect. Other Neutral (N) ENSO phase. Table 1 presents each correlations were smaller and generally not winter with its corresponding ENSO phase. This paper is divided into two sections. The * Corresponding author: Victoria Sankovich, first section focuses on parameters calculated 3106 Algonquin Trail, Lower Burrell, PA 15068; from winter soundings associated with significant e-mail: [email protected] severe convective events from 1958 through 1996. In the second section, all 00 UTC soundings El Niño Neutral La Niña released from sites in the Southeastern United States during the winter months of 1958 through 1958 1957 1972 1988 1971 2003 are considered. In both sections, the years 1966 1959 1975 1990 1974 are stratified by ENSO phase, and the 1969 1960 1977 1991 1976 characteristics of each grouping are examined and 1973 1961 1978 1993 1989 compared to see if the typical atmospheric 1983 1962 1979 1994 1999 structure during any specific ENSO phase is more compatible with severe thunderstorm occurrence. 1987 1963 1980 1996 2000 1992 1964 1981 1997 2. METHODOLOGY 1995 1965 1982 2001 1998 1967 1984 2002 The following parameters are examined for 1968 1985 2003 each sounding: Lifted Index, Mixed Layer CAPE, Convective Inhibition, Surface–1 km Storm- 1970 1986 Relative Helicity, Surface–3 km Storm-Relative Table 1: Winters classified by ENSO phase (1957–2003) Helicity, and Surface–6 km Bulk Shear. Also, two composite indices, the Supercell Composite The parameters are categorized by ENSO Parameter and the Significant Tornado Parameter, phase (EN, N, and LN) and their distributions are are computed. Table 2 lists these parameters calculated to determine if there is a higher or lower along with their relation to severe weather. These propensity for severe weather during the ENSO parameters are commonly used in forecasting phases. Parameter distributions are analyzed via severe weather and, when used together, give a a box plot created with PSI Plot Version 6 software general overview of the configuration of the (Poly Software International 1999). The box plot atmosphere. CHARACTERISTIC FAVORING PARAMETER DESCRIPTION UNITS STORM DEVELOPMENT Thermodynamic: 100-hPa Mean Layer More Negative ML LI indicates Lifted Index (LI) oC Lifted Index at 300 hPa more instability 100-hPa Mean Layer CAPE J kg-1 High CAPE CAPE Higher CIN tends to prevent lifted 100-hPa Mean Layer Convective Inhibition (CIN ) J kg-1 parcels from reaching LFC, thus Convective Inhibition serving to limit deep convection Kinematic: Bulk Shear (BKSHR) Surface–6 km Bulk knots High SHR (magnitude of shear vector) Shear 1 km Storm-Relative Helicity SFC–1 km Storm- High Sfc–1 km SRH promotes m2s-2 (Sfc–1 km SRH) Relative Helicity rotation 3 km Storm-Relative Helicity SFC–3 km Storm- High Sfc–3 km SRH promotes m2s-2 (Sfc–3 km SRH) Relative Helicity rotation Composite: Combination of CAPE, Supercell Composite SRH, Bulk Richardson --- Supercells likely when SCP > 1 Parameter (SCP) Number Combination of CAPE, Significant Tornado Significant Tornadoes likely when SRH, BKSHR, and lifted --- Parameter (STP) STP > 1 condensation level Table 2. Parameters analyzed from soundings displays the minimum value, 10th percentile, 25th Storm-Relative Helicity (SRH) measures how percentile, mean (50th percentile), 75th percentile, rapidly storm inflow is bringing rotation into the 90th percentile, and maximum values of the storm. Mathematically, it is the inner product of distribution. Using Fig. 1A as an example, the the storm-relative winds and the streamwise maximum and minimum values appear at the ends vorticity. This parameter is generally related to the of the outer whiskers on the plot. The lower tendency for a supercell to rotate (Glickman 2000). trapezoidal area of the shaded region represents The 1 km Storm-Relative Helicity (Sfc–1 km SRH) the 10–25% portion of the data, and the upper is the integral of SRH over the layer stretching trapezoidal shaded region is the 75–90% from the surface to 1 km, and the 3 km Storm- distribution. The central rectangle displays the Relative Helicity (Sfc–3 km SRH) is the integral inner 50% of the data, and the middle horizontal over the lowest 3 km of the atmosphere. line is the mean. 2.3 Composite Parameters 2.1 Thermodynamic Parameters The two composite parameters considered The Lifted Index (LI) examined is the “100-hPa were first presented in Thompson et al. (2003). Mean Layer Lifted Index at 300 hPa.” It is the They provide a preliminary screen of rawinsonde difference between the observed 300-hPa data to identify environments most capable of temperature and the temperature that the lowest supporting severe convection. 100-hPa mean parcel would have after being lifted The Supercell Composite Parameter (SCP) is to 300 hPa. It is similar to Galway’s Lifted Index a combination of the CAPE of the most unstable (Galway 1956) except 300 hPa is used as the parcel in the lowest 300 mb (MUCAPE), the Sfc–3 reference level instead of 500 hPa. By using 300 km SRH, and the square of the difference between hPa, the index considers the lifted parcel the density weighted winds in the Surface to 6 km buoyancy in the upper-troposphere. layer and those in the Surface to 500 m layer (this Mean Layer Convective Available Potential wind is often called the Bulk Richardson Number Energy (CAPE) is the maximum amount of energy or BRN shear). SCP can be computed from the available to an ascending mean parcel from the formula: lowest 100 hPa of the atmosphere. CAPE is -1 computed using parcel theory and is the area on a SCP = (MUCAPE/1000 J kg ) 2 -2 thermodynamic diagram between the lifted parcel • (Sfc–3 km SRH/100 m s ) curve and the observed sounding. The more • (BRN shear/40 m2s-2). energy a lifted parcel has available, the greater the possibility of the atmosphere to evolve into a A study of RUC–2 soundings found that values of severe storm or tornado.
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