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Steven W. Lyons

Abstract

Interannual variations in U.S. tropical storm and hurricane landfalls are examined over the interval 1950– 2002. The 10 highest and 9 lowest U.S. storm and hurricane landfall years are highlighted. U.S. landfalls are 6 times more frequent during the 10 highest than during the 9 lowest years. Nine major hurricanes struck the United States during the 10 highest U.S. landfall years, one struck during the 9 lowest U.S. landfall years. There is a positive correlation between Atlantic basin storm and hurricane frequency and U.S. storm and hurricane landfall frequency, but U.S. landfall variability explained by that relationship is small. Years with high (low) U.S. landfalls have a high (low) frequency of storm and hurricane formation in the Gulf of Mexico and the western Caribbean Sea and a high (low) percentage of them landfall along the U.S. Gulf coast. U.S. landfall frequency of Atlantic storms and hurricanes is much higher (lower) during high (low) U.S. landfall years, implying that Atlantic steering currents are more (less) favorable for U.S. landfall. La Niña conditions occurred 19% more often during high U.S. landfall years than during remaining years. El Niño conditions occurred 10% more often during low U.S. landfall years than during remaining years. Skill of inferring how many storms and hurricanes will landfall in the United States from a forecast of the number of Atlantic basin storms and hurricanes explains an average of 18% or less of U.S. storm and hurricane variability in a hindcast setting. Results indicate that a large portion of U.S. storm and hurricane landfall variability is related to where the storms form and whether steering currents are favorable or unfavorable for bringing them to the United States.

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Mary Beth Whitfield and Steven W. Lyons

Abstract

National Meteorological Center 200-mb analyses are used to develop an abridged six-year climatology of the tropical upper-tropospheric trough (TUTT) over the Gulf of Mexico. The climatology reveals large intraseasonal and interannual variability in TUTT axis position. The summer of 1988 is identified as having had an active TUTT near Texas, and is examined in detail. Satellite imagery and 200-mb winds are used to identify and track TUTT lows.

A TUTT low that remained quasi-stationary over Texas is selected for detailed examination. Horizontal and vertical cross sections of wind, temperature, vorticity, and relative humidity illustrate that 1) the maximum circulation around the low occurs near 200-mb, 2) the cold anomaly is largest near 300 mb, and 3) the troposphere moistens as the TUTT low strengthens over Texas. The initial vorticity source for the TUTT low is attributed to positive vorticity advection from midlatitudes, conservation of absolute vorticity, and vorticity convergence along the TUTT axis.

Calculations reveal a net direct circulation in and around the TUTT low, indicating it is an energy-generating system. Vertical motion and temperature fields display warm air rising on the eastern side of the low and cool air sinking near the center and western side of the low in the middle and upper troposphere. Precipitation is maximum in the SE quadrant relative to the TUTT low center.

Examination of the temporal distribution of rainfall over Texas reveals that the TUTT low was one of six synoptic systems that resulted in significant rainfall over the state during July and August 1988. The amount of cold convective cloud as seen from satellite imagery associated with the TUTT low displays significant diurnal variation with a maximum (minimum) observed in afternoon and early evening (late night and early morning) hours, consistent with the diurnal heating cycle over land. It is shown that TUTT lows can be significant sources of summer precipitation in Texas on synoptic space and time scales.

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Stacy R. Stewart and Steven W. Lyons

Abstract

The Guam WSR-88D (Weather Surveillance Radar-1988 Doppler) Doppler radar collected reflectivity, Doppler radial velocity, and other information for Supertyphoon Ed as it traversed the northern sections of Guam as a minimal tropical storm on 30 September 1993. This was the first-ever recorded passage of a tropical cyclone over a Next Generation Weather Radar WSR-88D Doppler radar site. Reflectivity data provided valuable information about the location of a precipitation-free “eye,” while radial velocity data provided useful information about tropical cyclone wind center location and strength. The velocity data also provided a 3-h lead time to upgrade the tropical cyclone to tropical storm intensity prior to landfall. Forecasters at Andersen Air Force Base used this information to give what turned out to be a very accurate short-range forecast of a brief period of gales with maximum gusts to 26 m s−1. Land-based surface wind observations correlated extremely well with 75%–80% of the 1500-m radial velocity estimates, which is similar to findings made by Powell and Tanner et al. Additional radar signatures of interest include offsets between the reflectivity center and velocity circulation center, detection of tropical storm and typhoon/hurricane force winds, Doppler velocity maxima within the convective rainbands, and mesocyclonic circulations detected by the WSR-88D's mesocyclone algorithm.

Concerning mesocyclones, one was detected very close to the location of the actual wind center when Ed was developing an eye prior to landfall. Within approximately 1 h of initial mesocyclone occurrence, a cyclonic divergent velocity-couplet pattern formed, possibly due to the development of low-level supergradient winds. The observed Doppler velocity patterns were consistent with the lower-tropospheric horizontal wind and vertical motion patterns in and around the eye as described by Malkus, Kuo, and Gray and Shea. After clearing the west coast of Guam, two separate mesocyclones formed just inward of the eyewall and appeared to be ingested into the main eye circulation. Similar findings were obtained from airborne Doppler radar analyses made by Marks and Houze. Shortly thereafter, Ed underwent a period of rapid intensification. In both instances, the mesocyclones appeared to have played a role in eye development and intensification.

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