K.-M. Lau

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.

Radiative–Convective Processes in Simulated Diurnal Variations ofTropical Oceanic Convection

Abstract This paper presents an analysis of the diurnal variation of tropical oceanic convection and its associated energy cycle as simulated by an anelastic cumulus ensemble model. The model includes subgrid turbulence, cloud microphysics, and radiative transfer processes. In two experiments designed to simulate the diurnal cycles in large-scale disturbed (A1) and undisturbed conditions (A4) over the tropical western Pacific warm pool, the model produces diurnal variations that are in general agreement with observations. In A1, a time-independent SST and mean ascending motion are prescribed in the model. The model generates a diurnal cycle with positive (negative) rainfall anomalies during the night (day), and the maximum (minimum) rainfall near 0200 (1300–1400) local time. In A4, a diurnally varying SST is prescribed in the model and the domain-averaged vertical velocity is constrained to be zero. The simulated diurnal variations still have a nocturnal rainfall maximum but with a weaker magnitude and a ...

Large-Scale Forcing and Cloud–Radiation Interaction in the Tropical Deep Convective Regime

Abstract The simulations of tropical convection and thermodynamic states in response to different imposed large-scale forcing are carried out by using a cloud-resolving model and are evaluated with the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment observation. The model is forced either with imposed large-scale vertical velocity and horizontal temperature and moisture advections (model 1) or with imposed total temperature and moisture advections (model 2). The comparison of simulations with observations shows that bias in temperature and moisture simulations by model 1 is smaller than that by model 2. This indicates that the adjustment of the mean thermodynamic stability distribution by vertical advection in model 1 is responsible for better simulations. Model 1 is used to examine effects of different parameterized solar radiative and cloud microphysical processes. A revised parameterization scheme for cloud single scattering properties in solar radiation calculations is fo...

Observation of a Quasi-2-Day Wave during TOGA COARE

Abstract Detailed structure of the quasi-2-day oscillation observed in the active phase of the Madden–Julian oscillations during the intensive observation period of Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE IOP) was described. A variety of observational platforms is used including high-resolution GMS infrared histogram, rain-rate estimate from TOGA and MIT radar measurements, upper-air soundings, and boundary layer profiler winds from the Integrated Sounding System and surface data from the IMET buoy. The quasi-2-day mode had a westward propagation speed of 12°–15° day −1, a horizontal wavelength of 25°–30° longitude. A coupling with the westward-propagating n = 1 inertio–gravity waves was hypothesized from the space–time power spectral distribution of the cloud field. The wind disturbance structure was consistent with the hypothesis. The vertical wave structure had an eastward phase tilt with height below 175 hPa and vice versa above, indicating the wav...

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...

Equilibrium States Simulated by Cloud-Resolving Models

Abstract Recently, several cloud-resolving models (CRMs) were used to study the tropical water and energy cycles and their role in the climate system. They are typically run for several weeks until modeled temperature and water vapor fields reach a quasi-equilibrium state. However, two CRMs produced different quasi-equilibrium states (warm and humid versus cold and dry) even though both used similar initial thermodynamic profiles, horizontal wind, prescribed large-scale vertical velocity, and fixed sea surface temperature. Sensitivity tests were designed to identify the major physical processes that determine the equilibrium states for the different CRM simulations. Differences in the CRM simulated quasi-equilibrium state can be attributed to how the atmospheric horizontal wind was treated throughout the integration. The model that had stronger surface wind produced a warmer and more humid thermodynamic equilibrium state. The physical processes responsible for determining the modeled equilibrium states ca...

Interactions between Tropical Convection and Its Environment: An Energetics Analysis of a 2D Cloud Resolving Simulation

The phase relation between the perturbation kinetic energy (K9) associated with the tropical convection and the horizontal-mean moist available potential energy ( ) associated with environmental conditions is investigated P by an energetics analysis of a numerical experiment. This experiment is performed using a 2D cloud resolving model forced by the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) derived vertical velocity. The imposed upward motion leads to a decrease of through the P associated vertical advective cooling, and to an increase of K9 through cloud-related processes, feeding the convection. The maximum K9 and its maximum growth rate lags and leads, respectively, the maximum imposed large-scale upward motion by about 1‐2 h, indicating that convection is phase locked with large-scale forcing. The dominant life cycle of the simulated convection is about 9 h, whereas the timescales of the imposed largescale forcing are longer than the diurnal cycle. In the convective events, the maximum growth of K9 leads the maximum decay of the perturbation moist available potential energy (P9) by about 3 h through vertical heat transport by perturbation circulation, and perturbation cloud heating. The maximum decay of P9 leads the maximum decay of by about 1 h through P the perturbation radiative processes, the horizontal-mean cloud heating, and the large-scale vertical advective cooling. Therefore, maximum gain of K9 occurs about 4‐5 h before maximum decay of . P

Impacts of precipitation in the upper ocean in the western Pacific warm pool during TOGA-COARE

An ocean mixed-layer model with a modified Kraus-Turner parameterization scheme is used to investigate the impacts of precipitation in the upper ocean in the western Pacific warm pool during Tropical Ocean Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE). Heat and salt budgets calculated in the upper ocean indicate local balance between surface forcing and the ocean response. Thus the mixed-layer model captures the dominant processes governing heat and salt variability. The model responses are analyzed and compared with the observed upper ocean in three distinctive layers determined by Monin-Obukhov length scales. In the top layer (the top 5 m), about 90% of the surface buoyancy flux is absorbed, and strong diurnal and intraseasonal variations are excited. The second layer, 5–20 m, contains intraseasonal variability that is characterized by nearly neutral stratification during strong westerly wind events, strong thermal stratification during clear-sky days, and strong saline stratification during heavy precipitation. The dominant effect of precipitation is to generate a stable stratification and to form a barrier layer. The third layer, 20–50 m, has intraseasonal variations due to mixing during westerly wind events. Heavy precipitation amplifies mixed-layer temperature fluctuations by a cycle of strong surface cooling and entrainment warming through the following processes. Heavy precipitation causes a shallower mixed layer and a larger cooling rate. Surface temperature drops rapidly, and the upper ocean becomes thermally unstable. The salinity maintains a weak density stability, which causes strong entrainment warming. Surface freshwater flux is the key factor controlling saline structure when advection is excluded. However, experiments without the entrainment process show a significant bias toward a lower salinity, indicating that the entrainment process must be properly treated in the model to prevent a biased trend.

Genesis and Evolution of Hierarchical Cloud Clusters in a Two-Dimensional Cumulus-Resolving Model

Abstract A two-dimensional cloud ensemble model is integrated over a basin-scale domain with prescribed sea surface temperature (SST), to study the formation and evolution of cloud clusters over a large-scale warm pool. Neither a basic zonal flow is prescribed nor is a single perturbation initially given. The results show that deep convective clouds appear in hierarchical clustered patterns and are limited to the area of warm SST above 28°C. The most fundamental cloud cluster in the model has a horizontal scale of a few hundred kilometers, in which new cumulus clouds are generated at the leading edge of a propagating surface cold-air pool—the “gust front.” It may last for days and propagate for a long distance if the background flow is broad and persistent as is the case in the low-level convergence zone of the SST-induced background flow. The largest hierarchical propagating cloud systems in the model have horizontal scales up to 3000 km and consist of up to four cloud clusters that are generally of gust...