Large-Scale Conditions Favorable for the Development of Heavy Rainfall during TAMEX IOP 3

Yi-Leng Chen Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii

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Jun Li Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii

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Abstract

The large-scale processes responsible for development of heavy precipitation during 20–23 May 1987 along the southeastern China coast are studied. There are two distinct rainfall peaks around 0000 UTC 20 May and 0000 UTC 22 May. Prior to the heavy rain event, strong low-level southwesterly flow developed ahead of a 850-hPa trough and transported warm, moist air from the south into southeastern China to generate the convective instability. For the first rainfall period, positive vorticity advection by thermal winds was observed ahead of the 850-hPa trough. Upper-level divergence as a result of the imbalance between winds and geopotential height fields was found in the different airflow region on the northeastern quadrant of the south. Asian anticyclone prior to the first rainfall period. For the second rainfall period, an upper-level trough deepened in the lee side of the Tibetan Plateau and moved to the southeastern China coast. Strong upper-level frontogenesis caused by horizontal deformation was found along the trough axis. The heaviest precipitation occurred along the low-level warm, moist tongue when the low-level baroclinic forcing was coupled with the upper-level one as the upper-level trough approached. These results are in contrast to a CISK (conditional instability of the second kind) process proposed by Chen and Chang for a Mei-Yu case in mid-June over southern China.

Analyses of the large-scale heat and moisture budgets show a vertical separation of Q1 and Q2 peaks during the first rain period, suggesting that the rainfall is convective in nature. For the heaviest rain period, the Q1 peak increases and shifts upward. The Q2 profile shows a double-peak structure with the upper-level peak coinciding with the Q1 peak but with a much smaller magnitude, indicating that the precipitation mainly falls from convective clouds and the anvils associated with them.

Over the Taiwan area, heavy rain did not develop as forecasted, because of the weakening of both the upper-level and low-level troughs and the lack of coupling between the upper-level and low-level forcings. When the low-level southwesterly flow interacted with the island of Taiwan, the low-level flow moved around the island with a windward ridge-leeside trough pressure pattern. Along the western coast, the wind component parallel to the central mountain range increased northward down the pressure gradient and had a maximum value (≥14 m s−1) at approximately 1.5 km above sea level over the northwestern coast. With the turning of the large-scale low-level flow from south-southwest to west-southwest and the strengthening of the southwesterly flow, the surface pressure ridge became stronger and extended northward along the western coast of Taiwan. The deflection and flow deceleration upstream were also more significant.

Abstract

The large-scale processes responsible for development of heavy precipitation during 20–23 May 1987 along the southeastern China coast are studied. There are two distinct rainfall peaks around 0000 UTC 20 May and 0000 UTC 22 May. Prior to the heavy rain event, strong low-level southwesterly flow developed ahead of a 850-hPa trough and transported warm, moist air from the south into southeastern China to generate the convective instability. For the first rainfall period, positive vorticity advection by thermal winds was observed ahead of the 850-hPa trough. Upper-level divergence as a result of the imbalance between winds and geopotential height fields was found in the different airflow region on the northeastern quadrant of the south. Asian anticyclone prior to the first rainfall period. For the second rainfall period, an upper-level trough deepened in the lee side of the Tibetan Plateau and moved to the southeastern China coast. Strong upper-level frontogenesis caused by horizontal deformation was found along the trough axis. The heaviest precipitation occurred along the low-level warm, moist tongue when the low-level baroclinic forcing was coupled with the upper-level one as the upper-level trough approached. These results are in contrast to a CISK (conditional instability of the second kind) process proposed by Chen and Chang for a Mei-Yu case in mid-June over southern China.

Analyses of the large-scale heat and moisture budgets show a vertical separation of Q1 and Q2 peaks during the first rain period, suggesting that the rainfall is convective in nature. For the heaviest rain period, the Q1 peak increases and shifts upward. The Q2 profile shows a double-peak structure with the upper-level peak coinciding with the Q1 peak but with a much smaller magnitude, indicating that the precipitation mainly falls from convective clouds and the anvils associated with them.

Over the Taiwan area, heavy rain did not develop as forecasted, because of the weakening of both the upper-level and low-level troughs and the lack of coupling between the upper-level and low-level forcings. When the low-level southwesterly flow interacted with the island of Taiwan, the low-level flow moved around the island with a windward ridge-leeside trough pressure pattern. Along the western coast, the wind component parallel to the central mountain range increased northward down the pressure gradient and had a maximum value (≥14 m s−1) at approximately 1.5 km above sea level over the northwestern coast. With the turning of the large-scale low-level flow from south-southwest to west-southwest and the strengthening of the southwesterly flow, the surface pressure ridge became stronger and extended northward along the western coast of Taiwan. The deflection and flow deceleration upstream were also more significant.

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