Analysis of the diurnal cycle in Taiwan during the terrain-influenced monsoon rainfall experiment

Abstract

The diurnal cycle is investigated in Taiwan during the summer monsoon ("Mei- yu" or plum rain) season using enhanced observations from the 2008 Terrain-influenced Monsoon Rainfall Experiment (TiMREX). The diurnal cycle of an undisturbed period is compared with that of a disturbed period in an aim to 1) better understand the variability of the diurnal cycle as a function of large-scale forcing, 2) describe the complex relationships between rainfall and orographically modified flow, and 3) determine the governing environmental characteristics that distinguish disturbed and undisturbed periods. The study is performed using a regional reanalysis generated by employing three-dimensional variational data assimilation techniques, 0.5° 6-h forecasts from the NCEP GFS (National Centers for Environmental Prediction Global Forecast System), and multiple observation platforms (from TiMREX datasets and others). The undisturbed period (UNDIST) was characterized by southwesterly monsoon flow at low levels, zonal flow in the upper troposphere, suppressed daily-mean rainfall, and unimpeded insolation. Accordingly, pronounced diurnal land-sea breeze (LSB) and mountain-valley (MV) circulations strongly controlled rainfall patterns, which exhibited patterns consistent with low-Froude-number (Fr) flow diverting around the mountainous island of Taiwan. Maximum daytime onshore/upslope flows were associated with enhanced rainfall along the coastal plains and foothills of Taiwan (as opposed to the high peaks), until the nighttime transition brought offshore/downslope flows and development of offshore rainfall where nocturnal density currents converged with the impinging southwesterly monsoon flow. During the disturbed period (DIST), the positioning of a prominent upper- tropospheric trough put Taiwan in a favorable area for large-scale ascent and convective organization, while a shallow, northerly cold intrusion (the Mei-yu front) provided a low- level triggering mechanism for vigorous deep convection. Although the amplitude of diurnal LSB/MV circulations was suppressed during this period (in association with reduced insolation), rainfall diurnal variability was noteworthy, suggesting heightened sensitivity of rainfall to diurnal flows. Consistent with moist conditions and higher-Fr flow, rainfall during this period was maximized over the high mountain peaks. Analysis of vertical profiles of vertical motion and apparent heat sources and moisture sinks for UNDIST demonstrates a predominance of shallow vertical circulations and bottom-heavy convection. In contrast, vigorous deep convection was the dominant rainfall mode during DIST. That the environment was more conducive for vigorous deep convection during DIST explains the increased sensitivity of rainfall to diurnal flows. Common to both periods was an afternoon transition from shallow to deep convection to stratiform rainfall (heating above the freezing level and cooling below; consistent with previous studies). The evolution of rainfall prior to, during, and following DIST exhibited a similar transition. This reflects the "self-similar" nature of tropical convective rainfall systems across spatial and temporal scales.