An Observational And Theoretical Case Study Of Rainy Season Wet Spell Or "Siyam-Siyam"

Adelaida T. Castillo

A case study has been made to describe the synoptic and sub-synoptic conditions of an extreme example of rainy season wet spell that occurred in Central Luzon on July 2002 using both observations and a numerical model. Heavy monsoon rains inundated Central luzon starting 5 July. Observations showed that an unusual disturbance developed over the southwestern section of Luzon which extends from northern Palawan to eastern Mindoro. This convective cell moved northwestward and developed into a deep convection producing heavy rainfall over the coast and near the mountains. Strong convergence of Indian Southwesterlies and South Pacific Trades is evident. The mesoscale systems actively interacted with large scale conditions providing a favorable environment for the development of heavy rainfall during this particular rainy season wet spell. The simulated convection using NCAR MM5 shows consistency with observations near coastal areas although there is a notable difference from observations near the mountains. Simulated results suggest two types of band formation: one initiated by convergence of large scale flows and another by convective cell existing in a strong southwesterly air stream.

Characteristics Of Thunderstorm Activity Using Cloud-To-Ground Lightning Data

Nikos Viktor B. Pe├▒aranda

This paper is an initial exploration of cloud-to-ground (CG) lightning occurrences in the Philippines using the untapped lightning data generated by the subset of the WSI Global Lightning Network® operating in the Southeast Asian region. Using thunderstorm (PAGASA) and CG lightning (WSI) data, the use of lightning detection network as an alternative or supplemental tool in detecting and analyzing thunderstorms events has been examined by analyzing the seasonal pattern of thunderstorms activity as revealed by Cloud-to-Ground (CG) lightning strokes occurring in the Philippine domain. Results reveal the profile of thunderstorm activity during the study period that includes information on spatio-temporal hazard related to CG lightning and indirectly, to thunderstorm severity. Additionally, the use of lightning data refined the analyses of thunderstorm events by providing information that relate thunderstorm dynamics and microphysical structure, which is unavailable in the traditional thunderstorm data.

Diurnal Variation Of Rainfall In Northern Luzon

Ariel R. Zamudio

The diurnal variation of rainfall in Northern Luzon is described by discussing the role of topography and local wind systems during three different monsoon periods in three different years using observation data from PAGASA-DOST. Baguio, which is a mountainous area, is found to experience maximum rainfall activity during the day in all the different monsoon seasons of three different years. This is attributed to the upslope winds. The daytime and nightime maximum in coastal areas is caused by the convergence between sea/land breeze and prevailing flow of by sea breeze and land breeze itself. During the northeast monsoon period, coastal stations in the eastern part of the region received more rainfall compared to the coastal stations in the western part. While during southwest monsoon period, coastal stations in the western part received more rainfall than coastal stations in the eastern part. Furthermore, this study will evaluate if the numerical model PSA/NCAR MM5 can simulate the diurnal variation of rainfall in Northern Luzon by adapting parameterization scheme from previous numerical studies. The result shows that although the model captured the accumulated rainfall pattern at some coastal stations, it did not capture the pattern in the mountainous and valley areas. In general, the model results do not capture the observed diurnal variation of rainfall. This shows that simulating diurnal variation of rainfall is sensitive in terms of topography since topography has a significant effect on the daily variation of rainfall.

Numerical Simulation Of Storm Surge For Landfalling Tropical Cyclone

Julie M. Nimes

The inaccurate prediction of storm surge accounts for the uncertainty of the landfall point forecast. This study describes the method for providing relatively accurate prediction of storm surges using numerical modelling techniques. Two cases of landfalling tropical cyclones were examined focusing on the storm surge response of the basin, using observational and numerical modelling techniques. Observational data analyses include the study area, meteorological and synoptic weather features of two cyclones affecting the area, observed surges and astronomical tide height. For modelling studies, the depth integrated hydro-dynamical storm surge model for shallow water was employed. Maximum sustained wind was used as the atmospheric driving force to generate storm surges. Other input parameters are the radius of maximum  winds, the forward speeds, the angles of approach to the basin and the initial cyclone position. Fourteen datasets of simulated and observed surge heights were used in the calibration and validation of the model. The calibration involves the selection of the best surface wind coefficient for the basin of Dingalan and adjacent coast of San Luis. The selection was done by conducting sensitivity tests. The results showed that the model with a 0.000658 surface wind coefficient and with 0.00005 constant bottom coefficient was found to be the best values. The calibrated storm surge model performed satisfactorily. The calibrated model was used in the generation of the maximum envelope of water (MEOW). Three model runs were performed from three different landfall points of TY Kading (1978) along the coast of Dingalan and adjacent coast of San Luis. The resulting composite of peak surges make up a map of MEOW. The developed technique of the composite of pre-computed MEOW from the equally spaced distance of parallel points of landfall can provides a summary of the worst case surge scenario given the uncertain in the current forecast situation. Compilation of historical pre-computed MEOWs has been found by other countries as a practical warning tool in the formulation of advisories for disaster mitigation.