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C-H. Sui and K-M. Lau

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 Australian region (140°E) in late December. The passage of ISOs in the monsoon flow are also associated with surface westerly wind outbreaks. On shorter lime scales (<10 days), the ISOs appear to provide a favorable condition over the warm ocean for the development of 2–4-day disturbances that further organize mesoscale cloud clusters. In addition, the diurnal cycle provides another important forcing mechanism modulating cloud clusters, particularly over the maritime continents. There appears to be an inverse relationship between diurnal cycle and intraseasonal disturbances (⩾10 days) over the maritime continent; that is, periods of active intraseasonal variabilities are characterized by diminished diurnal cycles and vice versa. The possible role of these multiscale processes in the coupled ocean-atmosphere system is discussed.

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K-M. Lau and C-H. Sui

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 cooling of the upper ocean is accelerated by increased westerly surface wind leading to enhanced surface evaporation and increased entrainment of cold water from below the thermocline. At this stage, the increased surface shortwave radiation due to the diminished cloud cover from reduced convection opposes the cooling by evaporation. The cooling trend is reversed as soon as the westerly phase terminates and the dry phase is reinitiated by the establishment of new organized convection over the Indian Ocean.

The authors’ results suggest that short-term SST variability in the western Pacific warm pool is closely linked to surface fluxes, which are strongly modulated by atmospheric low-frequency variability associated with the MJO. The implications of the present results on the dynamics of the MJO and the possible role of coupled SST in influencing the MJO variability are also discussed.

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Yukari N. Takayabu, K-M. Lau, and C-H. Sui

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 wave energy emanating from the upper troposphere.

Four stages in the life cycle of the oscillating cloud–circulation system were identified:. 1) the shallow convection stage with a duration time of 12 h, 2) the initial tower stage (9 h), 3) the mature stage (12 h), and 4) the decaying stage (15 h). Surface and boundary layer observations also showed substantial variation associated with the different stages in the life cycle. Results suggest that the timescale of quasi-2-day oscillation is determined by the time required by the lower-tropospheric moisture field to recover from the drying caused by deep convection.

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C-H. Sui, X. Li, K-M. Lau, and D. Adamec

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 radiation in the surface layer (∼1 m), and intraseasonal variations determined by penetrating solar radiation below the surface layer. Results further reveal that the accumulative effect of diurnal mixing cycles (solar heating/nocturnal deepening) is essential to maintain a stable temperature stratification and a realistic evolution of mixed-layer depth and temperature at the intraseasonal scale. The nonlinear response of the ocean mixed layer to the surface heat and momentum fluxes indicates the need to resolve the high-frequency response including diurnal atmospheric radiative–convective processes and ocean mixing processes in a coupled model to simulate the whole spectrum of multiscale variations within ISOs.

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Xiaofan Li, C-H. Sui, and K-M. Lau

Abstract

The phase relation between the perturbation kinetic energy (K′) associated with the tropical convection and the horizontal-mean moist available potential energy (P) associated with environmental conditions is investigated 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 P through the associated vertical advective cooling, and to an increase of K′ through cloud-related processes, feeding the convection. The maximum K′ 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 large-scale forcing are longer than the diurnal cycle.

In the convective events, the maximum growth of K′ leads the maximum decay of the perturbation moist available potential energy (P′) by about 3 h through vertical heat transport by perturbation circulation, and perturbation cloud heating. The maximum decay of P′ leads the maximum decay of P by about 1 h through the perturbation radiative processes, the horizontal-mean cloud heating, and the large-scale vertical advective cooling. Therefore, maximum gain of K′ occurs about 4–5 h before maximum decay of P.

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K. M. Lau, C. H. Sui, and W. K. Tao

This paper presents the preliminary findings of an investigation of the water budget of tropical cumulus convection using the Goddard Cumulus Ensemble Model (GCEM). Results of an experiment designed to obtain a “fingerprint” in the tropical hydrologic cycle in response to surface warming are also presented. The ensemble mean water budget shows that the distribution of water vapor and cloud water in the tropical atmosphere is maintained as a result of a balance between moisture convergence (including cloud scale and large scale) and condensation and reevaporation by various microphysical species within the cumulus clusters. Under radiative convective equilibrium conditions, 66% of the precipitation reaching the ground comes from the convective region and 34% from the stratiform region. In a climate with above-normal sea surface temperature but fixed large-scale vertical velocity, tropical convection is enhanced with more abundant moisture sources. Water vapor is increased throughout the troposphere with the surplus largest near the surface and decreases monotonically up to 10 km. However, the percentage increase in water vapor is largest near 8 to 16 km. As a result of the warming, the freezing level in clouds is elevated resulting in a large increase (decrease) in cloud water just above (below) 5 km. As with water vapor, the fractional increase in cloud water and cloudiness amount is largest at the upper troposphere.

In spite of the detailed microphysics and cloud-scale dynamical processes included in the GCEM, the results on changes in temperature and water vapor induced by surface warming are in agreement with those from general circulation models that use crude cumulus parameterization. This is consistent with previous findings that equilibrium water vapor distribution is a strong function of temperature. In an open domain such as the tropical convective environment, with a specified climatological vertical velocity, the ratio of increased precipitation to increased surface evaporation due to a 2°C surface warming is approximately 5. The increase is mostly found for convective rain and is negligible for stratiform rain. The climate implication of these changes is also discussed.

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C-H. Sui, X. Li, and K-M. Lau

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 secondary peak in the afternoon.

The model is then used to investigate the radiative effects of clouds that has been suggested as the cause of the nocturnal rainfall maximum by many studies. Two experiments (A2 and A3) are performed with the model, in which a time-independent SST is prescribed and the domain-averaged vertical velocity is constrained to be zero. The only difference between the two experiments is that the cloud–radiation interaction is suppressed in A3. The results show that despite the significant difference in total cloudiness/rainfall due to the difference in mean vertical motion between A2 and A1, and the difference in cloud–radiative forcing between A2 and A3, the simulated diurnal cycles in all three experiments show a dominant nocturnal rainfall maximum. The results support the suggestion by Sui et al. that the nocturnal rainfall maximum is related to more (less) available precipitable water in the night (day) due to the diurnal radiative cooling/heating cycle.

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Xiaofan Li, C-H. Sui, and K-M. Lau

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 ice and vapor deposition become important in producing rain and ice clouds, respectively.

Based on the analysis of dominant cloud microphysical processes, a simplified set of cloud microphysics parameterization schemes are proposed. Simulations with the simplified and original sets show similar thermodynamic evolution and cloud properties.

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C-L. Shie, W-K. Tao, J. Simpson, and C-H. Sui

Abstract

A series of long-term integrations using the two-dimensional Goddard Cumulus Ensemble (GCE) model were performed with various imposed environmental components. Vertical wind shear, minimum surface wind speed (only used for computing surface fluxes), and radiation are found to be the three major components that determine the quasi-equilibrium temperature and water vapor fields simulated in this study. The genesis of a warm/wet quasi-equilibrium state is mainly due to either strong vertical wind shear along with strong surface winds or large surface fluxes, while a colder/drier quasi-equilibrium state is due to weak (mixed wind) shear along with weak surface winds. Latent heat flux and net large-scale temperature forcing dominate the beginning stages of the simulated convective systems, then considerably weaken in the final stages leading to quasi-equilibrium states. Radiation is necessary in establishing the quasi-equilibrium states but is not crucial to the considerable variation between them.

A warmer/wetter thermodynamic state is found to produce more rainfall, as convective clouds are the leading source of rainfall over stratiform clouds even though they occupy much less area. Convective clouds are more likely to occur in the presence of strong surface winds (latent heat flux), while stratiform clouds (especially the well-organized type) are favored in conditions with strong wind shear (net large-scale forcing). The convective systems, which consist of distinct cloud types due to the variation in horizontal winds, are also found to propagate differently. Convective systems with mixed-wind shear generally propagate in the direction of shear, while systems with strong, multidirectional wind shear propagate in a more complex way. Cloud-scale eddies are found to transfer the heat and moisture vertically and assist in balancing the heat (Q1) and moisture (Q2) budgets and in reaching a quasi-equilibrium state. Atmospheric stability, CAPE, and mass fluxes are also investigated and compared between the various quasi-equilibrium states.

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Xiaofan Li, C-H. Sui, K-M. Lau, and M-D. Chou

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 found to generate more solar heating in the upper troposphere and less heating in the middle and lower troposphere. The change in the vertical heating distribution is suggested to stabilize the environment and to cause less stratiform cloud that further induces stabilization through cloud–IR interaction. The revised scheme also causes a drier middle and lower troposphere by weakening vertical moisture flux convergence. Also tested is the effect of a revised parameterization scheme for cloud microphysical processes that tends to generate more ice clouds. The cloud-induced thermal effect in which less ice cloud leads to less infrared cooling at cloud top and more heating below cloud top is similar to the effect of no cloud–radiation interaction shown in a sensitivity experiment. However, the exclusion of cloud–radiation interaction causes drying by enhancing condensation, and the reduction of ice clouds by the microphysics scheme induces moistening by suppressing condensation.

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