National Aeronautics and Space Administration

Research Results

Storms in the Future: Changes in Intensity, Cloudiness, Rainfall and Economic Costs

Title | Introduction | Methods | Results 1 | Results 2 | Results 3 | Discussion

Results, Part 3

Team 3: How are the properties of the storm clouds over the United States changing with time?

The average, minimum and maximum values of cloud properties over the United States for the period 1985 to 1993 were examined. Of these properties, only three showed any trends that were of interest. There is a trend towards increasing maximum and average optical thickness of these clouds (Figure 14) as well as a trend in decreasing average cloud top temperature of these clouds (Figure 15). Taken together, these two trends indicate that thicker and higher clouds are being produced in the later years, and these are characteristics of the rainmaking clouds in storms. However, when the amount of nimbostratus and deep convective clouds is examined over this period, there is no noticeable trend, the amount of these clouds remains relatively constant for the entire period. More rain clouds are not being produced. A possible explanation for this apparent contradiction is that while the amount of rain clouds is not increasing, the properties of these rain clouds are intensifying; they are becoming more optically thick and higher in the atmosphere. The possibility exists that the clouds of storms over the United States are at the end of this period are holding more water than the clouds at the beginning of the period.

In order to more closely examine these changes, the storm and cloud properties of two specific storms were compared. The storms were selected from the winters of high and low damage years so that the cloud properties of the storms could also be compared to the amount of damages. The tracks of these storms are shown in Figure 16. The storm and cloud properties for each storm were compared over the lifetimes of the storms. Once again, only a few of these properties showed any interesting trends.

The storm strength of the low damage year (1985) was greater than that of the storm from the high damage year for the first day, smaller on the second day, and approximately the same for the third day (Figure 17). This contradicted the initial expectation and instigated examinations of other factors that are related to the strength of storms and the damages they can produce. The average optical thickness of the clouds for each day of each storm is shown in Figure 18. Once again the storm from 1985 possesses the characteristic of a stronger storm optically thicker clouds on the first two days. A clue to the source of these optically thicker clouds, which hold more water and have the potential of causing more damage, came when the tracks of the storms were examined. The 1985 storm formed over Texas, close to the Gulf of Mexico; the 1993 formed over the Washington State area. Warmer, more humid conditions in Texas could account for the thicker clouds in the 1985 storm.

The relative humidities on the first day of both storms were approximately equal as can be seen in Figure 19, however, the 1985 storm formed under much warmer surface temperatures as shown in Figure 20. The amount of water that a parcel of air can contain depends on the relative humidity and the temperature. The water content of a kilogram of air was calculated for the first day of each storm. As can be seen in Table 1, the 1985 storm had more water in each kilogram of air. What is even more interesting is that when the ratio of the mass of water per kilogram of air for the two storms is compared to the ratio of the optical thickness of the clouds of each storm, the results are approximately the same (Table 1). This similarity indicates that the humidity and the temperature of the air where the storm forms may have a major influence on the optical thickness of the clouds of that storm.

Title | Introduction | Methods | Results 1 | Results 2 | Results 3 | Discussion