Diurnal Variation Of Tropical Convection Research Articles

Quantitative Assessment of Diurnal Variation of Tropical Convection Simulated by a Global Nonhydrostatic Model without Cumulus Parameterization

Abstract This study investigated the resolution dependence of diurnal variation in tropical convective systems represented by a global nonhydrostatic model without cumulus parameterization. This paper describes the detailed characteristics of diurnal variation in surface precipitation based on three-dimensional data, with the aim of explicitly clarifying the mechanism that underlies the variation. The study particularly focused on the evolution in the size of the precipitation area for deep convective systems with an analysis of the vertical structure of thermodynamic fields. This analysis compares the results of simulations with horizontal grid sizes of 14 and 7 km (R14 and R7, respectively). Over land, the phase delay of diurnal variations in R7 is about 3 h less than that in R14. R7 produces a pronounced diurnal variation in the size distributions of precipitating area(s), especially for areas with a radius of 0–100 km; this characteristic is not found for R14. Such areas actively evolve between noon and evening, leading to the smooth development of larger-scale precipitating areas having a radius of 100–150 km. The maximum surface precipitation in R7 over land occurs at around 2000 local time throughout the tropics, approximately 2 h prior to the development of nighttime deep convection. Deep convective regimes are important as agents of vertical heat transport in the tropics. The present results suggest that precipitating areas with a radius <100 km make a strong contribution to the total amount of precipitation and to mass transport.

Resonant Wave Interactions in the Presence of a Diurnally Varying Heat Source

Abstract Resonant interactions among equatorial waves in the presence of a diurnally varying heat source are studied in the context of the diabatic version of the equatorial β-plane primitive equations for a motionless, hydrostatic, horizontally homogeneous and stably stratified background atmosphere. The heat source is assumed to be periodic in time and of small amplitude [i.e., O(ɛ)] and is prescribed to roughly represent the typical heating associated with deep convection in the tropical atmosphere. In this context, using the asymptotic method of multiple time scales, the free linear Rossby, Kelvin, mixed Rossby–gravity, and inertio-gravity waves, as well as their vertical structures, are obtained as leading-order solutions. These waves are shown to interact resonantly in a triad configuration at the O(ɛ) approximation, and the dynamics of these interactions have been studied in the presence of the forcing. It is shown that for the planetary-scale wave resonant triads composed of two first baroclinic equatorially trapped waves and one barotropic Rossby mode, the spectrum of the thermal forcing is such that only one of the triad components is resonant with the heat source. As a result, to illustrate the role of the diurnal forcing in these interactions in a simplified fashion, two kinds of triads have been analyzed. The first one refers to triads composed of a k = 0 first baroclinic geostrophic mode, which is resonant with the stationary component of the diurnal heat source, and two dispersive modes, namely, a mixed Rossby–gravity wave and a barotropic Rossby mode. The other class corresponds to triads composed of two first baroclinic inertio-gravity waves in which the highest-frequency wave resonates with a transient harmonic of the forcing. The integration of the asymptotic reduced equations for these selected resonant triads shows that the stationary component of the diurnal heat source acts as an “accelerator” for the energy exchanges between the two dispersive waves through the excitation of the catalyst geostrophic mode. On the other hand, since in the second class of triads the mode that resonates with the forcing is the most energetically active member because of the energy constraints imposed by the triad dynamics, the results show that the convective forcing in this case is responsible for a longer time scale modulation in the resonant interactions, generating a period doubling in the energy exchanges. The results suggest that the diurnal variation of tropical convection might play an important role in generating low-frequency fluctuations in the atmospheric circulation through resonant nonlinear interactions.

Diurnal variation of tropical convection during TOGA COARE IOP

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.

Cloud microphysical processes associated with the diurnal variations of tropical convection: A 2D cloud resolving modeling study

Cloud microphysical processes associated with the diurnal variations of tropical convection are investigated based on hourly data from a 2D coupled ocean-cloud resolving atmosphere simulation. The model is forced by the large-scale vertical velocity and zonal wind derived from TOGA COARE for a 50-day period. The diurnal composites are carried out in weak diurnal SST variations (case W) and strong diurnal SST signals (case S). The ice water path is larger than the liquid water path in case W than it is in case S. The difference is enhanced in the morning in case W and in the early afternoon in case S when the surface rain rates reach their peaks. Further comparison of cloud microphysics budgets, associated with rainfall peaks, between cases S and W shows that solar heating in case S warms air to reduce the contribution of vapor deposition to cloud growth, which decreases ice water path compared to those in case W. While the collection of cloud water by rain is a major contributor to the surface precipitation in both cases, the melting of precipitation ice (sum of snow and graupel) contributes less to the rainfall in case S than in case W.

Numerical Experiments Examining the Mechanism of Diurnal Variation of Tropical Convection

A regional atmospheric model was used to clarify the mechanisms that drive the diurnal cycle of tropical convection over the western Pacific Ocean. The diurnal cycle of convection was simulated using data from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Mechanisms suggested by previous investigators were confirmed through comparison experiments. The diurnal variation in precipitation was simulated without the effect of horizontal differences in radiative cooling between cloudy and clear regions. Horizontal differences of radiative cooling are not the main factor driving diurnal variation. Direct radiative cooling, and cloud radiative cooling effects destabilize the atmosphere during the night. Numerical experiments that reduced cloud radiative cooling demonstrated the importance of the cloud radiative cooling effect rather than the direct radiative cooling effect. Moreover, in the standard experiment, strong cloud radiative cooling appeared at both upper levels and at the top of the boundary layer. The experiment demonstrated that both the upper level cloud radiative cooling, and the boundary layer cloud radiative cooling, are important in driving the diurnal variation of convection. Boundary-layer cloud radiative cooling destabilizes the low layer atmosphere and promotes the development of shallow convection to deep convection. Diurnal variation of water vapor also plays a role in driving diurnal variation of convection. Moisture is transported upward from the boundary layer at night when radiative cooling of the boundary-layer cloud is intensified.

Diurnal Variations of Tropical Convection Observed during the TOGA-COARE.

Diurnal variations of tropical convection in the western Pacific are examined by using the data observed during the Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) Intensive Observing Period (IOP) (November 1992 to February 1993). High resolution data of satellite infrared histograms, MIT radar from the TOGA project at NASA/GSFC, upper-air soundings, and improved meteorological surface mooring (IMET) buoy data by the WHOI are utilized for analyses. Over tropical western Pacific, maximum convective activity occurs during the evening hours to midnight on large islands. On the other hand, in the vicinity of large islands, the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), the peak activity occurs during the morning hours to noon. The local time of peak activity varies depending on the focused cloud top height. Comparing the results in different regions, local time of peak convective events of different cloud top heights change little over large islands, but it varies among the surrounding ocean, ITCZ and SPCZ. Diurnal variations of precipitation tend to be more prominent in the case of heavy rainfall with a nocturnal maximum from the TOGA-COARE special data. Therefore, we focused on the days with nocturnal precipitation maxima and examined the diurnal variations of atmospheric vertical structures over the TOGA-COARE region. At the onset of convection in the evening, water vapor is increased in lower troposphere. During this time low-level clouds appear, and upward motion is observed in the lower layers. Convective activity reaches its peak around 00-03 LT, which coincides with the maximum precipitation. Large-scale upward motion, apparent heat source, and moisture sink are also observed at the peak of convective activity. While precipitation decreases gradually in the morning to noon, the activity of high-level clouds decay. It is suggested that the water vapor increase at low levels in the evening plays a role in the development of nocturnal convection. The maximum activity of convection over the TOGA-COARE region however, is observed about 6 hours prior to the peak of other convergence regions over the tropical western Pacific.