An Analysis Of The Relationship Between The Position Of The Major Wind Discontinuity And The Position Of Areas Of Rainfall Over The Philippines

Gaudioso R. Tabamo

The intertropical convergence zone is defined by different writers in different ways and it has been discussed under many different names. There is also difference of opinion as to whether rainfall occurs in separate patches or as a continuous belt along the major wind discontinuity or whether it is dispersed over a narrow zone or a wider zone in the general vicinity of the wind discontinuity. In general, it is implied from the literature that the more prominent wind discontinuity at some level or levels in the tropical atmosphere is sometimes associated with close or distant precipitation.

The association of precipitation with wind discontinuity has been implicitly accepted without proof, and the lines or zones which have been drawn on published synoptic charts appear to be a composite representation of rainfall and wind-field. To determine whether the major wind discontinuity is associated in some way with areas of rainfall and whether there is a zone of rainfall along the wind discontinuity or whether rainfall is dispersed over a wide zone, completely separate analyses of wind-fields and of the rainfall charts over the Philippines have been made in the present study and these have been related graphically. For determination of the wind field there are 11 pilot balloon stations and 46 synoptic observing stations, approximately 70 miles apart, which regularly report the direction of movement of the lowest and middle clouds at various levels. The scarcity of observations of wind-speed make computations of convergence impracticable. Therefore, the wind discontinuities which emerged from the analyses were really the assymptote of confluence in the streamlines in the lower 2000 to 4500 feet of the atmopshere, so that it was not practicable to take into account any slope in the surface of separation between two airstreams in the present study. Since the analyses show that the wind discontinuity can undergo considerable displacement in one day and because near the coast its position may also be affected by the difference between land and sea temperatures, analyses of the wind field have been made twice daily, at 0000 GMT and 1200 GMT, respectively.

For the determination of the rainfall patterns, rainfall observations are available at 46 synoptic stations in the Philippines, representing an average spacing of approximately 70 miles between stations. It was realized that, with an observing network over land, the diurnal temperature variation might have an influence on rainfall, and therefore separate precipitation charts for the periods 1800-0600 GMT and 0600-1800 GMT (each of which includes a time of wind observations) have been analyzed so that any diurnal effect would be discernible and, if appreciable, could be investigated. The analyses have been extended over the surrounding seas with the aid of cloud pictures from weather satellites.

The period examined in this study is June to October, 1964-1967, and the major wind discontinuity is discernible over the Philippines on 473 of the 12-hourly wind charts drawn during this period.

It was found that, in the majority of cases, rainfall was greater at one or both sides of the wind discontinuity than at the discontinuity itself. Since the most likely explanation of this is that the surface of wind-discontinuity possesses slope, a new investigation requiring special observations over a long period has been proposed. Another proposal for further investigation has been made in relation to the diurnal variation of rainfall.