Diurnal Variations in Convective Activity and Precipitation During Phases II and III of GATE

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  • 1 Department of Atmospheric Science, University of Washington, Seattle 98195
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Abstract

Harmonic analysis is used to determine the phase and normalized amplitude of the diurnal variations in convective activity and precipitation during phases II and III of GATE. Satellite results are based on the percent coverage of squares, 3° of latitude and longitude in dimensions, by convective clouds, as estimated subjectively from SMS-1 infrared images. Rainfall results are based on hourly precipitation records for 33 land stations and hourly or 3-hourly rainfall amounts from 13 ships in the A/B-scale networks. The rainfall data for the land stations were combined into four geographically distinct groups and the ship data into a single group before making the harmonic analyses. The long-term representativeness of the results for the land areas is judged by comparing them with the results obtained from analysis of data presented by Burpee (1976) on the frequencies of occurrence of thunder, moderate to heavy precipitation and light precipitation during the period June-September 1966–69.

The main findings are the following:

1) Over the northern part of the land area (∼15–20°N) thunderstorms were most frequent in the late afternoon or early evening, rain amounts were greatest shortly before midnight, convective cloud cover was most extensive shortly after midnight and light rainfall most common near dawn. This behavior is attributed to the fact that much of the rainfall in this region is produced by long-lived squall lines that typically form in the afternoon as a result of the daily beating cycle and then take several hours to reach the stage of maximum rain production.

2) For the ship array as a whole the rainfall maximum occurred in the early afternoon (about 1400 local time). The diurnal range was about half the daily mean. The satellite results also yielded an afternoon maximum in convective cloud cover and further revealed that the diurnal oscillation was stronger in the eastern part of the network than in the western. No explanation is offered for the afternoon maximum over the ocean. However, it is shown to be associated with a similar maximum in the large-scale convergence field.

Abstract

Harmonic analysis is used to determine the phase and normalized amplitude of the diurnal variations in convective activity and precipitation during phases II and III of GATE. Satellite results are based on the percent coverage of squares, 3° of latitude and longitude in dimensions, by convective clouds, as estimated subjectively from SMS-1 infrared images. Rainfall results are based on hourly precipitation records for 33 land stations and hourly or 3-hourly rainfall amounts from 13 ships in the A/B-scale networks. The rainfall data for the land stations were combined into four geographically distinct groups and the ship data into a single group before making the harmonic analyses. The long-term representativeness of the results for the land areas is judged by comparing them with the results obtained from analysis of data presented by Burpee (1976) on the frequencies of occurrence of thunder, moderate to heavy precipitation and light precipitation during the period June-September 1966–69.

The main findings are the following:

1) Over the northern part of the land area (∼15–20°N) thunderstorms were most frequent in the late afternoon or early evening, rain amounts were greatest shortly before midnight, convective cloud cover was most extensive shortly after midnight and light rainfall most common near dawn. This behavior is attributed to the fact that much of the rainfall in this region is produced by long-lived squall lines that typically form in the afternoon as a result of the daily beating cycle and then take several hours to reach the stage of maximum rain production.

2) For the ship array as a whole the rainfall maximum occurred in the early afternoon (about 1400 local time). The diurnal range was about half the daily mean. The satellite results also yielded an afternoon maximum in convective cloud cover and further revealed that the diurnal oscillation was stronger in the eastern part of the network than in the western. No explanation is offered for the afternoon maximum over the ocean. However, it is shown to be associated with a similar maximum in the large-scale convergence field.

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