NOAA Technical Memorandum NWS SR-181

NOAA Technical Memorandum NWS SR-181


Charles H. Paxton
Rocco D. Pelatti
M. Elaine Powell
National Weather Service Office
Tampa Bay Area, Florida

Scientific Services Division Southern Region
Fort Worth, TX

December 1996

1. Abstract

Radiosonde parameters were assembled into a 36-year climatology. Monthly minima, maxima and averages of height, temperature, dewpoint, relative humidity, wind direction and wind speed were computed from 1200 UTC soundings. Average Skew T diagrams and stability indices were derived from the data. Tables of results are included in the Appendices.

2. Introduction

Radiosonde data from the Tampa Bay area from 1958 to 1992 comprising height, temperature, dewpoint, wind direction, and wind speed were used for this atmospheric climatology. Maximum, minimum, averages and wind direction frequencies were calculated for mandatory levels of 1000, 850, 700, 500, 400, 300, 250, 200, 150, and 100 mb. Average relative humidities were calculated from average temperature and dewpoint. For consistency and to eliminate contamination from afternoon thunderstorm activity, only 1200 UTC soundings were used. The soundings were obtained from the Radiosonde Data of North America CD-ROM (1993). Tables and graphs were computed using Quattro Pro spreadsheets. The radiosonde site originally at the Tampa International Airport was moved on June 1, 1975, to a new National Weather Service office in Ruskin about 17 nm south southeast of the airport.

In a 25-year upper air climatology for Cape Canaveral, Hagemeyer and Schmocker (1991) computed and processed data into graphs. Profiles of mean annual temperature and dewpoint, potential temperature, wet-bulb potential temperature, and mean wind U and V wind components were computed. Temperature, dew point, and pressure data were converted to potential temperature and wet bulb potential temperature so that information concerning moist and dry static stability could be evaluated directly from the monthly graphs. In contrast, this study uses conventional parameters from soundings so direct comparisons to current and forecast sounding data can be made by forecasters. Average Skew T diagrams and stability indices were produced from the compiled average data.

3. Data

a. Geopotential Height

Average geopotential heights (Fig.1) show only minor variations annually. Average heights are lowest during the cool month of February above 1000 mb. In a climatology of the upper troposphere, Sadler (1975) documented subtropical ridge migration from the Caribbean Sea through Mexico into the southern United States during summer. This results in height field bulges during warmer summer months, with average heights highest in July or August above 1000 mb.

b. Temperature

Average temperatures (Fig. 2) vary most in the lower levels. Cold fronts and associated upper-level systems pass through central Florida regularly from fall through spring. Fronts rarely move into central Florida during summer; therefore, temperatures show the least variability during summer. The temperature field exhibits cooler winter and warmer summer temperatures up to 200 mb. Higher in the atmosphere, the opposite is true. The lower winter tropopause results in warmer temperatures above 200 mb. Warmer temperatures occur at 150 mb during winter, with an average of -62.1 C in February, and cooler temperatures occur during the summer with an average -66.5 C in July. At 200 mb average temperatures vary by only by 1.8 C.

c. Dewpoint

Maximum dewpoints occur during August from 1000 mb to 300 mb. Minimum dewpoints occur during the colder winter months of January and February. At 1000 mb, average dewpoints undergo a gradual increase, while the minimum dewpoint readings have a significant increase into the summer months. At 850 mb, average dewpoints have sharp transitions more than 8.0 C from April through June and from September through November. These are periods when frontal activity transitions from frequent winter passages to nearly none during summer. Average annual dewpoint variability decreases with height from a range of 13.2 C at 1000 mb to 4.4 C at 300 mb.

d. Relative Humidity

Monthly average relative humidity was calculated from average temperature and dewpoint for levels up to 300 mb. An increase in moisture and instability leads to almost daily thunderstorms over Florida during summer. This moisture increase is reflected in Fig. 3. Relative humidity decreases from around 80 percent at 1000 mb to around 30 percent at 400 mb and 300 mb. At 1000 mb through 400 mb, relative humidity is highest during the summer; but at 300 mb, relative humidity is slightly higher during the winter. The soundings used in this climatology were taken during the morning around sunrise (1200 UTC); therefore, less annual variation in relative humidity is noted at 1000 mb.

e. Wind Speed

Figure 4 illustrates the transition of middle and upper atmospheric winds from winter to summer. Comparing wind speed with geopotential height and temperature, migration of the subtropical and polar jet streams may be inferred. During winter months, geopotential height values and the stratosphere lower, while upper-level westerly winds increase considerably. The opposite is true during the summer months of July and August when the polar jet stream is over northern latitudes and the subtropical ridge extends across Florida. This leads to a pronounced decrease in upper level winds and more directional variability. Wind speeds increase with height up to 200 mb then decrease at 150 and 100 mb. All levels have winter maxima and summer minima.

f. Wind Direction

Individual wind directions were related to eight compass points. For example, all directions within 12.5o of 270o (west) were assigned as westerly winds. In the lowest levels, wind direction is much more variable during winter, but in middle levels wind direction is more variable during summer. During winter (Figs. 5a-c), westerly 1000 mb winds are least common, but at 850 mb and above, west winds occur most frequently. At 200 mb, winter wind directions show a consistent westerly flow associated with the subtropical jet stream. During spring and fall, winds aloft become more variable in direction, while lower level winds are primarily from the northeast and southeast quadrants. During summer (Figs. 5d-f), southeast winds are most common in the lower levels. Winds are evenly distributed at 500 mb, and the mean subtropical ridge position produces northeast to east winds above 500 mb during summer.

g. Average Skew T Diagrams

Average monthly Skew T diagrams and derived parameters (Figs. 6a-l) were produced from the SHARP - Skew T/Hodograph Analysis and Research Program (Hart and Korotky, 1991). Average 1200 UTC mandatory level temperature, wind, and dewpoint data were plotted from 1000 mb to 100 mb. Since surface data are excluded, low level inversions typically on 1200 UTC soundings are not present. Several stability indices were calculated. Convective available potential energy (CAPE) was calculated using the PMAX method that uses the most unstable parcel in the lowest 150 mb. CAPE values of zero and lifted indices between 7 and 12 are observed during the fall and winter. CAPE values above zero and negative lifted indices, occur during late spring and summer. A maximum average CAPE value of 1603 Jkg-1 and -4C lifted index occurs during August when warm, moist, conditionally unstable air is most prevalent. Calculated precipitable water values range from a relatively dry 0.71 inches during February to a peak value of 1.66 inches during August.

4. Conclusion

This data set provides insight into the typical atmosphere over west-central Florida. Skew T diagrams and tables of maxima, minima, and averages will allow forecasters to assess atmospheric deviations from the average. This may be helpful for forecasting atypical events. The data provide a baseline and may also be used to determine maximum and minimum record events in the atmosphere.

5. Acknowledgments

Hearty thanks to the WSO Tampa Bay staff who provided important front line ideas. Also thanks to NWS Southern Region SSD and to the NWS Office of Meteorology for partial funding of work done on this paper.

6. References

Hagemeyer, B. C. and G. K. Schmocker, 1991: Mean Atmosphere over Cape Canaveral. NOAA Tech. Attach. SR/SSD 91-11, 7p.

Hart, J. A. and W. D. Korotky, 1991: The SHARP Workstation - v1.50. A Skew T/Hodograph Analysis and Research Program for the IBM and Compatible PC. Users Manual. NOAA/NWS Forecast Office, Charleston WV., 62 pp.

Forecast Systems Laboratory, Boulder, Colorado, National Climatic Data Center, Asheville, North Carolina, 1993: Radiosonde Data of North America 1946-1992. Version 1.0, CD disc 1-4.

Sadler, J. C., 1975: The upper Tropospheric Circulation over the Global Tropics. Dept of Meteorology, University of Hawaii, Rep. No. UHMET-75-05, 35 pp.

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