A Study Of Long Duration Rainfall Episode In The Philippines During The Southwest Monsoon

Esperanza O. Cayanan

A long duration rainfall episode which happened during the last two weeks of August 1990 was investigated in order to establish some indicators to predict the future occurrence of a similar phenomenon. The synoptic situations before, during and after the rainfall episode were studied and described. The investigation involved analyses of surface pressure charts, wind profile from surface to 200 hPa levels, important weather systems like tropical cyclones and satellite imageries. Composite maps of surface pressure and geopotential heights at 850 hPa level were preparted. Results showed that the rain episode was caused by the Intertropical Convergence Zone (ITCZ) which passed thru the islands of Batanes connecting with the monsoon trough over China Mainland. The passage of three tropical cyclones during the period activated the ITCZ and enhanced the southwest monsoon flow over the western sections of Luzon, thereby increasing the intensity of rainfall over the area.

The distribution of rainfall in time and space was also examined. It was noted that the long duration rainfall episode started over Southwestern Luzon (Palawan) moving northward and westward. The intensity of rainfall also increases in the same direction. A preliminary criterion for defining a rainfall episode as a SW wet spell was proposed after the investigation.

Numerical Model For Storm Surge Prediction Incorporating Overland Flooding

Vicente B. Malano

A numerical model is developed for storm surge prediction incorporating overland surges. The model is formulated using the depth-integrated form of the transport equations applicable for storm surges and is applied over an idealized basin with straight coast. Movement of land/sea boundary is one dimensional. The local water elevation and the sea surface slope are examined first to regulate the movement of the boundary. The forcing function of the model consists of surface wind generated by a symmetric cyclone model of Jelesnianski with inflow angle of 15 degrees. The effects of inland inundation of sea water are investigated by varying the slope of inland elevation as well as the basin size and shape. The model is simulated with observed surges in the Bay of Baler and Tandag, Surigao del Sur Basin with considerable success.

The Dependence Of Tropical Cyclone Motion On Environmental Conditions, Vortex Characteristics and Beta Effect

Nathaniel T. Servando

A set of idealized experiments using a nondivergent barotrophic model are used to investigate the influence of different environmental conditions, vortex characteristics and beta effect on tropical cyclone motion. The environmental flow is found to be essential in moving the cyclone along the direction of the flow by advecting the vorticity of the vortex along. The differential advection of earth vorticity by the cyclone circulation or B-effect distorts the cyclone which causes deviation poleward and westward from the imposed flow. The imposition of a more complicated environmental flow and distinctive change in motion is explained by the local changes of relative vorticity brought about by the interaction between the vortex and the environmental flow with their associated vorticity. The sensitivity of tropical cyclone motion to changes on the characteristics of the vortex with respect to maximum wind, radius of maximum wind, decay parameter of the vortex and asymmetry is discussed. Results show that increasing the maximum wind speed and radius of maximum wind increases the poleward component of motion while no significant change in motion when the decay parameter is changed. The initially prescribed asymmetry on the vortex found to have small effect on motion except when the flow is relatively calm. Experiments on the sensitivity of vortex motion on model resolution reveal the importance of better resolution in modelling the tropical cyclone motion in order to better resolve the maximum wind and the inner wind profile and to determine the vortex center properly.

Application Of HEC-I Model In The Sipocot River Basin

Susan R. Espinueva

The need for a timely and accurate flood forecast has always been the primary concern of the hydrologist or flood forecaster. This need has been evident during the past decades up to the present due to frequent flooding resulting from environmental and geomorphological factors. One (1) crucial factor in the preparation of a flood forecast is the availability of an objective tool i.e. flood forecasting model to aid in finding out the relation between rainfall and runoff to minimize the time required in the formulation of the forecast. The need to augment the forecasting scheme of the Flood Forecasting Branch prompted the study to tackle application of an existing model (HEC-1) to one (1) of the monitored river basins in the Philippines. HEC-I was developed by the US Army Corps of Engineering Hydrologic Engineering Center (HEC). HEC-I is an event based model capable of simulating multiple floods for multiple basin development plans (Flood Hydrograph Package User's Manual, 1990). The model performs various computations in one single storm analysis because there is no provision for recovery of precipitation loss rate during periods of little or no precipitation. The input is the basin hyetograph and the output is the streamflow hydrograph at desired locations. The model wa applied in the Sipocot river basin using four (4) flood events for simulation and two (2) independent events for verification. Sensitivity of parameters was conducted and the forecasting capability of the model was tested.

Heat, Moisture, Momentum And Kinetic Energy Budgets Over South China Sea

Eugenio M. Aquino

In this study, the quantitative calculations of heat, moisture, momentum and kinetic energy budgets are pursued over the South China Sea during the prevailing northeast monsoon. These terms are expressed as fluxes. The calculations are carried out using thermodynamic and moisture equations as well as momentum equations averaged over space and time with the radiational heating values taken from climatological results of Dopplick (1970). The distribution of these terms yield pattern of the apparent heat source and apparent heat sink together with kinetic and momentum fluxes. As a result of this study, a noticeable trend of the calculated values of the budgets and fluxes is similar to the results of other experiments carried during Northern winter periods and almost within the same area of concern. It may be added too that since the atmosphere is a large and uncontrollable laboratory, reasonable estimate of the quantified parameters are conclusive enough to merit this study.