The surface rainfall processes and diurnal variations associated with tropical oceanic convection are examined by analyzing a surface rainfall equation and thermal budget based on hourly zonal-mean data from a series of two-dimensional cloud-resolving simulations. The model is integrated for 21 days with imposed large-scale vertical velocity, zonal wind, and horizontal advection obtained from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment （TOGA COARE） in the control experiment. Diurnal analysis shows that the infrared radiative cooling after sunset, as well as the advective cooling associated with imposed large-scale ascending motion, destabilize the atmosphere and release convective available potential energy to energize nocturnal convective development. Substantial local atmospheric drying is associated with the nocturnal rainfall peak in early morning, which is a result of the large condensation and deposition rates in the vapor budget. Sensitivity experiments show that diurnal variations of radiation and large-scale forcing can produce a nocturnal rainfall peak through infrared and advective cooling, respectively.
With a hybrid atmosphere-ocean coupled model we carried out an experimental forecast of a well documented Madden-Julian Oscillation (MJO) event that was observed during the period of Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). The observed event, originated in the western Indian Ocean around 6 January 1993, moved eastward with a phase speed of about 6.2 m s 1 and reached the dateline around February 1. The hybrid coupled model reasonably forecasts the MJO initiation in the western Indian Ocean, but the predicted MJO event propagates too slow (～ 4.4 m s 1 ). Results from previous observational studies using unprecedented humidity profiles obtained by NASA Aqua/AIRS satellite suggested that two potential physical processes may be responsible for this model caveat. After improving the cumulus parameterization scheme based on the observations, the model is able to forecast the same event one month ahead. Further sensitivity experiment confirms that the speed-up of model MJO propagation is primarily due to the improved convective scheme. Further, air-sea coupling plays an important role in maintaining the intensity of the predicted MJO. The results here suggest that MJO prediction skill is sensitive to model cumulus parameterization and air-sea coupling.
The Louis scheme and the COARE algorithm （version 3.0） are tested against eddy covariance and inertial dissipation methods for friction velocity estimates in different wind-sea/swell regimes. Atmospheric forcing data, tabulated by Donelan et al. （1997.J Phys Oceanog, 27：2087-2099）, were collected from a mast on the foredeck of a SWATH （small water-plane area, twin hull） ship in deep sea off the State of Virginia during the surface wave dynamics experiment. These data are representative of low to moderate wind regimes. The aerodynamic roughness length is determined by using the Charnock relationship. The intercomparison shows that the Louis scheme and the COARE algorithm underestimate the friction velocity by 6% and 3% respectively under pure wind sea conditions, 15% and 13% respectively under cross swell conditions, and 21% and 17% respectively under counter swell conditions. The analysis shows that these underestimations were caused by the method chosen to determine the aerodynamic roughness length because it significantly underestimates the aerodynamic roughness length. It is especially true under the cross swell and counter swell conditions.
Diurnal variation of tropical convection and kinematic and thermodynamic conditions was investigated for different large-scale environments of the convectively active and inactive periods by using satellite observations and surface measurements during the Intensive Observation Period （IOP） of the Tropical Ocean Global Atmosphere/Coupled Ocean-Atmosphere Response Experiment （TOGA/COARE）. During the convectively active period, the features of nocturnal convection appear in vertical profiles of convergence, vertical velocity, heat source, and moisture sink. The specific humidity increases remarkably in the middle troposphere at dawn. On the other hand, the altitude of maximum convergence and that of the upward motion is lower during the convectively inactive period. The specific humidity peaks in the lower troposphere in the daytime and decreases in the middle troposphere. Spectral analyses of the time series of the infrared （IR） brightness temperature （TBB） and amounts of rainfall suggest multiscale temporal variation with a prominent diurnal cycle over land and oceanic regions such as the Intensive Flux Array （IFA） and the South Pacific Convergence Zone （SPCZ）. Over land, the daily maximum of deep convection associated with cloud top temperature less than 208 K appears at midnight due to the daytime radiative heating and the sea-land breeze. Over the ocean, convection usually tends to occur at dawn for the convectively active period while in the afternoon during the inactive period. Comparing the diurnal variation of convection with large-scale variables, the authors inferred that moisture in the middle troposphere contributes mostly to the development of nocturnal convection over the ocean during the convectively active period.
An atmosphere-ocean coupled mesoscale modeling system is developed and used to investigate the interactions between a squall line and the upper ocean observed over the western Paci?c warm pool during the Tropical Ocean/Global Atmosphere Coupled Ocean and Atmosphere Response Experiment (TOGA-COARE). The modeling system is developed by coupling the Advanced Regional Prediction Sys- tem (ARPS) to the Princeton Ocean Model (POM) through precipitation and two-way exchanges of mo- mentum, heat, and moisture across the air-sea interface. The results indicate that the interaction between the squall-line and the upper ocean produced noticeable di?erences in the sensible and latent heat ?uxes, as compared to the uncoupled cases. Precipitation, which is often ignored in air-sea heat ?ux estimates, played a major role in the coupling between the mesoscale convective system and the ocean. Precipitation a?ected the air-sea interaction through both freshwater ?ux and sensible heat ?ux. The former led to the formation of a thin stable ocean layer underneath and behind the precipitating atmospheric convection. The presence of this stable layer resulted in a more signi?cant convection-induced sea surface temperature (SST) change in and behind the precipitation zone. However, convection-induced SST changes do not seem to play an important role in the intsensi?cation of the existing convective system that resulted in the SST change, as the convection quickly moved away from the region of original SST response.
