Chung-Hsiung Sui

Heating, moisture, and water budgets of tropical and midlatitude squall lines : comparisons and sensitivity to longwave radiation

Abstract A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to estimate the heating (Q1, moisture (Q2), and water budgets in the convective and stratiform regions for a tropical and a midlatitude squall line (EMEX and PRE-STORM). The model is anelastic and includes a parameterized three-class ice-phase microphysical scheme and longwave radiative transfer processes. A quantitative estimate of the impact of the longwave radiative cooling on the total surface precipitation as well as on the development and structure of these two squall lines is presented. It was found that the vertical eddy moisture fluxes are a major contribution to the model-derived Q2 budgets in both squall cases. A distinct midlevel minimum in the Q2 profile for the EMEX case is due to vertical eddy transport in the convective region. On the other hand, the contribution to the Q1 budget by the cloud-scale fluxes is minor for the EMEX case. In contrast, the vertical eddy heat flux is relatively important f...

Interdecadal Changes in Summertime Typhoon Tracks

Abstract The present work examines interdecadal variations of typhoon tracks in the western North Pacific (WNP) during the boreal summer (June–September) for the period 1951–2001. Typhoon tracks are expressed as percentage values of the total number of typhoon passages into a 5° × 5° latitude–longitude grid box with respect to the total number of typhoons formed in the WNP. The analysis period is divided into two interdecadal periods: ID1 (1951–79) and ID2 (1980–2001). From ID1 to ID2, typhoon passage frequency decreased significantly in the East China Sea and Philippine Sea, but increased slightly in the South China Sea. The time series of typhoon passage frequency over the East China Sea and South China Sea further reveal a regime shift in the late 1970s, while those over the Philippine Sea indicate a continuous downward trend of −9% decade−1. The interdecadal changes in typhoon tracks are associated with the westward expansion of the subtropical northwestern Pacific high (SNPH) in the late 1970s. The e...

Diurnal Variations in Tropical Oceanic Cumulus Convection during TOGA COARE

Diurnal variations in atmospheric convection, dynamic/thermodynamic fields, and heat/moisture budgets over the equatorial Pacific warm pool region are analyzed based on data collected from different observation platforms during the Intensive Observation Period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Results reveal that the diurnal variations in rainfall/convection over the TOGA COARE region can be classified into three distinct stages: warm morning cumulus, afternoon convective showers, and nocturnal convective systems. Afternoon rainfall comes mostly from convective cells, but the nocturnal rainfall is derived from deeper convective cells and large areas of stratiform clouds. Results further show that afternoon convective showers are more evident in the large-scale undisturbed periods when the diurnal SST cycle is strong, but the nocturnal convective systems and morning cumulus are more enhanced in the disturbed periods when more moisture is available. The primary cause of the nocturnal rainfall maximum is suggested to be associated with more (less) available precipitable water in the night (day) due to the diurnal radiative cooling/heating cycle and the resultant change in tropospheric relative humidity.

The Tropical Water and Energy Cycles in a Cumulus Ensemble Model. Part I: Equilibrium Climate

Abstract A cumulus ensemble model is used to study the tropical water and energy cycles and their role in the climate system. The model includes cloud dynamics, radiative processes, and microphysics that incorporate all important production and conversion processes among water vapor and five species of hydrometeors. Radiative transfer in clouds is parameterized based on cloud contents and size distributions of each bulk hydrometeor. Several model integrations have been carried out under a variety of imposed boundary and large-scale conditions. In Part I of this paper, the primary focus is on the water and heat budgets of the control experiment, which is designed to simulate the convective–radiative equilibrium response of the model to an imposed vertical velocity and a fixed sea surface temperature at 28°C. The simulated atmosphere is conditionally unstable below the freezing level and close to neutral above the freezing level. The equilibrium water budget shows that the total moisture source, Ms, which i...

Mechanisms of Cloud-Radiation Interaction in the Tropics and Midlatitudes

Abstract Radiative forcing and latent heat associated with precipitation are the two most important diabatic processes that drive the circulation of the atmosphere. Clouds can affect radiation and vice versa. It is known that longwave radiative processes can enhance precipitation in cloud systems. This paper concentrates on determining the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes. Three of the ways that longwave radiation is thought to interact with clouds are as follows: 1) cloud-top cooling and cloud-base warming may alter the thermal stratification of cloud layers, 2) differential cooling between clear and cloudy regions might enhance convergence into the cloud system, and 3) large-scale cooling could change the environment. A two-dimensional version of the Goddard Cumulus Ensemble model has been used to perform a series of sensitivity tests to identify which is the dominant cloud-radiative fo...

Mechanisms of short-term sea surface temperature regulation : Observations during TOGA COARE

Abstract Analyses of ocean–atmosphere data from Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment indicate that short-term (weekly to monthly) fluctuations of SST in the western Pacific warm pool are closely linked to the alternation of wet and dry spells driven by the Madden–Julian oscillation (MJO). The dry phase is characterized by increased convection over the Indian Ocean, a prolonged period of atmospheric subsidence, and surface easterlies over the western Pacific warm pool. During this phase, increased surface shortwave radiation and reduced evaporation contribute about equally to the warming of the warm pool. Pronounced diurnal variations in SST observed during the dry phase may be instrumental in leading to the prolonged warming. The dry phase is followed by the wet phase, in which the SST warming trend is arrested and a cooling trend initiated by a reduction in surface shortwave radiation accompanying the buildup of organized convection. Subsequently, the continued c...

Multiscale Phenomena in the Tropical Atmosphere over the Western Pacific

Abstract Multiscale variabilities in the atmosphere over the tropical western Pacific during the 1979 Northern Hemisphere winter are studied with an aim at identifying possible interactions between phenomena of different spatial and temporal scales. Based on the convection-index information derived from satellite measurements, two intra-seasonal oscillations (ISOs) are identified within the equatorial belt between 0° and 10°S in the analyzed period. The two ISOs, accompanied by both rotational and divergent circulations, propagate eastward from the Indian Ocean to the western Pacific. Over the warm pool in the western Pacific, the ISOs develop into quasi-stationary systems with an enhanced rotational circulation characterized by a strong westerly jet in the lower troposphere. The ISOs appear to interact with a number of regional- and synoptic-scale phenomena in the maritime continent and western Pacific region. For example, the onset of the monsoon coincides with the arrival of the first ISO at northern A...

Systematic Variation of Summertime Tropical Cyclone Activity in the Western North Pacific in Relation to the Madden–Julian Oscillation

Abstract The variability of observed tropical cyclone (TC) activity (i.e., genesis, track, and landfall) in the western North Pacific (WNP) is examined in relation to the various categories of the Madden–Julian oscillation (MJO) during summer (June–September) for the period 1979–2004. The MJO categories are defined based on the empirical orthogonal function analysis of outgoing longwave radiation data. The number of TCs increases when the MJO-related convection center is located in the WNP. The axis of a preferable genesis region systematically shifts like a seesaw in response to changes in the large-scale environments associated with both the eastward and northward propagation of the MJO and the intraseasonal variability of the WNP subtropical high. Furthermore, the authors show that the density of TC tracks in each MJO category depends on the systematic shift in the main genesis regions at first order. Also, the shift is affected by the prevailing large-scale steering flows in each MJO category. When th...

Dominant Cloud Microphysical Processes in a Tropical Oceanic Convective System: A 2D Cloud Resolving Modeling Study

Abstract Dominant cloud microphysical processes associated with a tropical oceanic convective system are investigated based on a 2D cloud resolving simulation. The model is forced by the zonal-mean vertical velocity, zonal wind, sea surface temperature, and horizontal temperature and moisture advections measured and derived from the TOGA COARE period. The analysis of cloud microphysics budgets shows that cloud water forms due to vapor condensation, but most of the conversion of cloud water to precipitation occurs primarily through two mechanisms, depending on the temperature when they occur: through riming of cloud water onto precipitation ice (snow or graupel) at colder than 0°C and collection of cloud water by rain at warmer temperatures. Processes involving the liquid phase are dominant during the early stages of convection development. The collection process produces rain, and the riming process enhances ice clouds. Ice processes are more dominant during the later stages. The melting of precipitation ...

Multiscale Air–Sea Interactions during TOGA COARE

Abstract Two distinct intraseasonal oscillations (ISO) are found in the tropical ocean atmosphere in the western Pacific region during Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The ISO is characterized by cycles of dry–wet phases in the atmosphere due to the passage of Madden–Julian oscillations, and corresponding warming/shoaling–cooling/deepening cycles in the ocean mixed layer (OML). During the wet phase, 2–3-day disturbances and diurnal variations in the atmosphere are pronounced. During the dry phase, diurnal cycles in sea surface temperature (SST) is much enhanced while the OML is shallow. These multiscale coupled air–sea variations are further investigated with an ocean mixed-layer model forced by the observed surface heat, water, and momentum fluxes. The variations of ocean mixed layer are shown to be crucially dependent on the vertical distribution of solar radiation, that is, diurnal SST variability primarily determined by the absorbed solar radi...