Title: The Heavy Rainfall Mechanism Revealed by a Terrain-Resolving 4DVar Data Assimilation System—A Case Study

Abstract

In this study, a newly developed terrain-resolving four-dimensional variational (4DVar)-based data assimilation system, Immersed Boundary Method_Variational Doppler Radar Analysis System (IBM_VDRAS), is applied to investigate the mechanisms leading to a heavy precipitation event that occurred in Taiwan during the Southwesterly Monsoon Experiment (SoWMEX) conducted in 2008. The multivariate analyses using IBM_VDRAS and surface observations reveal that the warm and moist southwesterly flow from the ocean decelerates after making landfall, forming a surface convergence zone along the coast, which is further strengthened during the passage of a prefrontal rainband. The flow ascends as it advances inland until reaching the mountains, producing persistent precipitation and the enhancement of evaporative cooling as well as a widespread high pressure zone. A very shallow (<0.4 km) layer of offshore flow can be identified over the southwestern plain, which helps to generate a quasi-stationary convergence zone near the coast. Furthermore, sensitivity studies are carried out to quantify the relative importance of the contributions made by topographic blockage, evaporative cooling, and their nonlinear interaction, to the evolution of this type of convective system. The influence of the topography is identified as the dominant factor in modulating the flow structure of the rainfall system. However, it is themore » nonlinear interaction between terrain and evaporation that determines the distribution of the temperature and pressure fields.« less

Authors:

Tai, Sheng-Lun ^[1]; Liou, Yu-Chieng ^[2]; Chang, Shao-Fan ^[2]; Sun, Juanzhen ^[3]

---

+ Show Author Affiliations

1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States); National Central Univ., Taoyuan (Taiwan)
2. National Central Univ., Taoyuan (Taiwan)
3. National Center for Atmospheric Research, Boulder, CO (United States)

Publication Date:

Thu May 07 00:00:00 EDT 2020

Research Org.:

Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Sponsoring Org.:

USDOE; Ministry of Science and Technology of Taiwan; Central Weather Bureau

OSTI Identifier:

1638962

Report Number(s):

PNNL-SA-152383
Journal ID: ISSN 0027-0644

Grant/Contract Number:  

AC05-76RL01830; MOST107-2111-M-008-040; MOST107-2625-M-008-008; MOTC-CWB-107-M-02

Resource Type:

Accepted Manuscript

Journal Name:

Monthly Weather Review

Additional Journal Information:

Journal Volume: 148; Journal Issue: 6; Journal ID: ISSN 0027-0644

Publisher:

American Meteorological Society

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; Orographic effects; precipitation; squall lines; radars; radar observations; data assimilation