Abstract/Summary

In this thesis, I investigate different factors that affect Tropical Cyclone (TC) precipitation and characteristics in Luzon, the northern regions of the Philippines. This thesis is is divided into two parts: an observational study to establish relationships between TC characteristics and rainfall in Luzon, and modeling experiments to understand and explain the processes that link TC rainfall with the topography of Luzon. In the first part of this thesis, I investigate the relationship between TC extreme precipitation, TC characteristics, and the orography of the Philippines. I first introduced the Weighted Mean Precipitation Exceedance (WPE), a measure of extreme precipitation. WPE is adapted from different studies that calculate extreme precipitation based on extreme thresholds for specific regions, as well as the spatial coverage of precipitation in the said region. As the WPE calculates both intensity and extent in a single value, we are easily able to compare extreme precipitation for different TCs. For all TCs that cross Luzon between 1978-2015, we find that when comparing between intensity or movement speed categories, stronger or slower TCs that make landfall significantly yield higher WPE. The proportion of landfalling strong TCs, or Typhoons, is lower during June to September (29.58%) compared to the proportion of Typhoons making landfall in October to December (60.71%). However, the proportion of TCs that exceed the median WPE is higher during June to September (71.43%) compared to those making landfall during October to December (61.76%). This shows that the relationship between TC intensity and WPE is more pronounced during the months June to September — the Southwest Monsoon season in the Philippines. These results suggest that it is important to consider the pre-landfall cyclone movement speed, intensity, and season to anticipate extreme precipitation of incoming TCs. To explain the underlying processes that link TC winds, precipitation, and orography, for the second part of this thesis I conduct Numerical Weather Prediction experiments for eight different TCs for different heights of the Cordillera Mountain Range (CMR): Control, Flat, Reduced (0.5x height), and Enhanced (2x height). From the results, we find that precipitation along the mountain range increases for increasing CMR height on average. Comparing precipitation rates for the Flat and Reduced terrain profiles with the Control terrain shows an average increase of 5 mm/hr rain rates, with precipitation rates similarly increasing by 5 mm/hr between the Control and Enhanced terrain. There are no significant changes for TC movement speeds and TC position for different terrain profiles. Regarding TC intensity, TCs weaken as early as 21 hours prior to landfall for higher CMR elevation when comparing the Enhanced and Control experiments, while the TC strength is similar between the Flat/Reduced and Control experiments. It is possible that TC intensity is less sensitive to mountain ranges at or below the height of the actual CMR. We also find that mechanical uplift caused by stronger winds blowing up steeper slopes results in higher amounts of precipitation along the CMR. The results of this two-part study highlight the complexity of the relationships between TC characteristics, orography, environmental wind, moisture, and TC rainfall. Nevertheless, this study shows that features such as more intense TCs, the June to September monsoon season, and steeper mountain range slopes, are fundamentally linked to higher amounts of TC precipitation in the Philippines. The findings of this study can be used to fill gaps in current forecasting limitations and may help improve our response to potential hazards associated with TCs.