SOME SURFACE DRY BULB AND DEW POINT TEMPERATURES

ASSOCIATED WITH CONVECTIVE THUNDERSTORMS IN EL PASO, TEXAS

James A. Reynolds

NWSO El Paso, TX (Santa Teresa, NM)

Introduction

By definition, convective thunderstorms are dependent on intense surface heating coupled with sufficient moisture. Because of this, the analysis of surface dry bulb and dew point temperatures associated with convective activity is highly appropriate. To date, little research in this regard is known to have been completed for the El Paso, Texas area. This study seeks to develop aids in forecasting convective development in El Paso, Texas during the summer convective season through the analysis of certain surface dry bulb and dew point temperatures. In total, the purpose of this paper is threefold:

1. To determine, on a monthly basis, if there is an optimum range of maximum daytime surface dry bulb temperatures, and possibly singular maximum daytime surface dry bulb temperatures, most often associated with summer convective thunderstorms in El Paso,

2. To determine, on a monthly basis, if there is an optimum range of 10 a.m. surface dry bulb and dew point temperatures, and possibly singular 10 a.m. surface dry bulb and dew point temperatures most often associated with summer convective thunderstorms in El Paso, and,

3. To develop a substantial database of summer convective events in El Paso to be referenced in this and future studies.

The idea to explore maximum daytime dry bulb surface temperatures associated with convective activity in El Paso comes from the Arizona Thunderstorm Chasers (AZTC) Safety Guide and Training Manual (1993). In this publication, it is stated that for the production of summer "monsoon" thunderstorms in Phoenix, maximum daytime surface temperatures most often lie between 100 to 108F with the optimum temperature being about 105F. Considering elevation differences between Phoenix and El Paso, temperatures needed to produce thunderstorms in El Paso are undoubtedly lower.

As previously stated, sufficient moisture must also be available for the production of convective storms. The AZTC Safety Guide refers to the notion of 'bursts', which is the transport of moisture into Arizona via strong southerly or southeasterly low level flows. The operational criterion for the onset of "monsoon" conditions, thereby leading to convective activity in Arizona, is a prolonged period (3 consecutive days or more) of surface dew points averaging 55F or higher as a result of the aforementioned moisture transport. This value was originally linked to a precipitable water amount present in the local atmosphere equal to about 1.00". Since New Mexico and far west Texas are also subject to the same moisture flow regime during the summer months as Arizona, comparable results in regard to dew point temperatures for El Paso should be obtainable.

Methodology

The convective season in El Paso generally runs from mid-July to mid-September. However, in an effort to gather as many convective cases as possible for this study, the time span of this study was expanded to the period from May 1 to September 30 of each year inclusive in this study. Data analysis was performed for the years 1978 through 1997 thereby providing a 20 year database record.

To determine the dates between May and September in which El Paso had occurrences of thunderstorms, Local Climatological Data summaries for the El Paso International Airport were reviewed. While establishing the dates of thunderstorm occurrences, denoted as "convective days" in table 1, the maximum daytime dry bulb temperature associated with these occurrences was also noted. These same dates were also checked to see if .01 inch or greater of precipitation had been received at the airport or not. Once this was completed, the maximum daytime dry bulb temperature data were analyzed to determine if there is an optimum range of maximum daytime surface dry bulb temperatures, and possibly singular maximum daytime surface dry bulb temperatures, most often associated with summer convective thunderstorms in El Paso, Texas. For greater definition, this was done on a monthly basis. The percentage of days receiving .01 inch of precipitation or greater during dates of thunderstorm occurrence in El Paso was also calculated.

By request, El Paso hourly surface dry bulb and dew point temperatures for the months and years inclusive of this study were received from the National Climatic Data Center. The 10 a.m. dry bulb and dew point temperatures were noted for days with thunderstorm occurrences. These data were then also analyzed to determine if there is an optimum range of 10 a.m. surface dry bulb and dew point temperatures, and possibly singular 10 a.m. surface dry bulb and dew point temperatures, most often associated with summer convective thunderstorms in El Paso. Surface dew point depressions at 10 a.m. were also calculated. Again, for greater definition, this was done separately for the individual months of this study. The 10 a.m. hour was chosen in an attempt to minimize possible effects related to the nightly temperature inversion, while maintaining some predictive use.

Results

The number of days in this study with an occurrence of thunderstorms at the El Paso airport was 648, with 434 of these occurrences (67%) producing .01 inch or greater of precipitation (table 1.). While the months of May and September had the fewest number of days with occurrences of thunderstorms, it was more likely during these months to receive .01 inch or greater of precipitation on days with thunderstorm occurrences than the other months in this study.

Analysis of maximum daytime surface dry bulb temperatures did indeed indicate optimum ranges of values most often associated with thunderstorms in El Paso. These ranges differed from month to month and generally ran between 8-10 F (table 1). Singular values with the highest frequency of thunderstorm occurrence were observed on a monthly basis in May, August, and September. June and July had two values. September showed the greatest range of maximum daytime surface dry bulb temperatures associated with thunderstorm occurrences. May and June had smaller ranges than September, and were nearly identical in their degree of variability. July and August showed the smallest ranges. These results were not unexpected as larger temperature ranges would be more typical of transitional months. Also not unexpected, was the strongly positive skewed normal distribution of maximum daytime surface dry bulb temperatures during the months of May, June, and September which was a result of the lower frequency of thunderstorm events during those months.

Analysis of the 10 a.m. surface dry bulb temperatures also indicated optimum ranges of values most often associated with thunderstorms in El Paso (table 1.) Again, individual months exhibited different ranges, with the limits of these ranges falling between 6-11F. Each month rendered only one 10 a.m. surface dry bulb temperature most often associated with convection. Much like with maximum daytime temperatures, September showed the greatest range of possible 10 a.m. surface dry bulb temperatures, while May and June had smaller ranges that were equal in their degree of variability to each other. July and August had the smallest ranges. The 10 a.m. surface dry bulb temperatures exhibited normal distributions for each month of this study.

Optimum ranges were also identified with 10 a.m. surface dew point temperatures on a monthly basis and these went from 9-11F (table 1.). May, June, and July had a singular optimum 10 a.m. surface dew point temperature most often associated with convection while August and September had two. June had the greatest range of possible 10 a.m. surface dew point temperatures. May and July had smaller ranges and these were nearly equal in their degree of variability. August and September had the smallest ranges. Ten a.m. surface dew point temperatures showed a strongly positive skewed normal distribution pattern for every month inclusive of this study. As expected, singular optimum surface dew point temperatures most often associated with convection in El Paso, at least at 10 a.m., were overall quite similar to those necessary for the onset of convection in Phoenix, Arizona.

The range of surface dew point depressions at 10 a.m. (table 1.) showed a high degree of variability in every month with the highest observed in May and June. The degree of variability was less in July, with August and September showing the least variability. The distribution of 10 a.m. surface dew point depressions on a monthly basis showed a bi-modal pattern. With this distribution,

it was basically impossible to determine an optimum range of dew point depression values that would be useful in the forecasting of convective activity. In an effort to obtain more useful information from these data, the data were broken into classes with 5 spreads that were within the linear distributions of each month. From this, ranges of dew point depressions most often associated with convective activity were identifiable, though these ranges were less convincing than those derived from the other variables analyzed in this study.

Conclusions

Analysis of maximum daytime surface dry bulb temperatures, as well as 10 a.m. surface dry bulb and dew point temperatures, indicates that on a monthly basis, there are optimum ranges and singular values of these parameters that are most often associated with convective thunderstorms in El Paso, Texas. Surface dew point depressions observed at 10 a.m. on days of convective activity are highly variable, and despite breaking these data into classes, these data do not appear to be highly useful as a forecasting tool.

While no forecast of convection will ever be solely dependent on the variables discussed here, the results of this study can be used as a way to provide forecasters an additional "heads-up" in regard to the formation of convective activity in El Paso. To use these data as a true forecasting tool, it would first be necessary to review the surface dry bulb and dew point temperature parameters of the 3,060 possible days within the time frame of this study, to determine the number of days that had parameter values similar to the results of this study but did not produce convective activity in El Paso (so called "null" events.).

Acknowledgments |

Thanks to Val J. MacBlain, NWSO El Paso, and Dan Smith of Scientific Services Division, NWS Southern Region Headquarters, for providing suggestions that led to the development and improvement of this paper. Maximum daytime temperature and precipitation data were found in on-station El Paso, Texas Local Climatological Data sheets from the National Climatic Data Center (NCDC). Ten a.m. surface dry bulb and dewpoint temperature data supplied on request from NCDC.

Reference

Cerveny, R.S. and J.A. Reynolds, 1993, Arizona Thunderstorm Chasers Safety Guide and Training Manual.

Table 1.