Publication Date

2018-08-03

Availability

Open access

Embargo Period

2018-08-03

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2018-07-06

First Committee Member

Brian Mapes

Second Committee Member

David Nolan

Third Committee Member

Russ Schumacher

Abstract

Rainfall extremes are caused by a complicated interplay between moisture and dynamics with many challenges and ideas that have yet to be fully understood in the literature. This study examines the dynamics and thermodynamics of extreme precipitation events through a combination of observational data and reanalysis model outputs. Data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 product, version 7, were retrieved for the time period 1998-2016. The greatest single-day and three-day rainfall accumulations for each grid pixel in several different spatial grid resolutions were identified along with its associated date and time of occurrence. Global maps of these record events were created and statistics were computed over their worldwide occurrence. The mode of precipitation events occur for records of ~100 mm over a single day and ~160 mm over 3-day accumulations, a ratio that is much smaller than 1:3 even for the largest-scale events (4-degree grid cells). Much higher accumulations are observed but are quite rare as expected. The densest concentration of high precipitation accumulation events occurred over east Asia and the Caribbean. Rainfall extremes were also found to occur primarily during the local summer months of the northern and southern hemispheres. Three distinct case studies over the Indian subcontinent were selected based on the global map of 4-degree, 75-hour record rainfall events. These events were selected such that no two events on the map coincided with one another. For each case, an intercomparison of 14 various daily rainfall products (both observation and reanalysis-based) assembled as part of the NASA-NEWS program shows challenges in quantitative estimation. However, the MERRA-2 reanalysis analyzed rainfall totals were comparable to the event-defining TRMM 3B42 data. Various two and three-dimensional dynamic and thermodynamic variables from MERRA-2 were visualized to explore the meteorological story behind these extreme rainfall cases. Two cases involved deep depressions that formed in the Bay of Bengal and subsequently made landfall over India during the 2006 North Indian Ocean cyclone season, while a third case was a quasi-stationary low-pressure system over central India in 2005. In all three cases, a mid-level vortex, defined by its potential vorticity (PV), was embedded in the monsoon shear flow. These sheared cyclones exhibited induced ascent on the downshear-left quadrant (southwest of the center in monsoon depressions). Because PV is centered in the mid-levels between the low-level westerlies and upper-level easterlies, these systems moved slowly which contributed to large rainfall accumulations at fixed locations.

Keywords

Extreme Precipitation; Monsoon Cyclone; Floods; Tropical Rain; TRMM 3B42; Integrated Data Viewer

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