title: Understanding tropical cyclone precipitation in the Northern Philippines
Creators: name: Racoma, Bernard Alan B.
Publisher: name: University of Reading
Year: 2024

description: In this thesis, I investigate different factors that affect Tropical Cyclone (TC) precipitation and character�istics 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 charac�teristics, 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 pre�cipitation 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 fundamen�tally 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.