Large-scale Organization Of Convection Research Articles

Aerosol Indirect Effects on Tropical Convection Characteristics under Conditions of Radiative–Convective Equilibrium

AbstractThe impacts of enhanced aerosol concentrations such as those associated with dust intrusions on the trimodal distribution of tropical convection have been investigated through the use of large-domain (10 000 grid points), fine-resolution (1 km), long-duration (100 days), two-dimensional idealized cloud-resolving model simulations conducted under conditions of radiative–convective equilibrium (RCE). The focus of this research is on those aerosols that serve primarily as cloud condensation nuclei (CCN). The results demonstrate that the large-scale organization of convection, the domain-averaged precipitation, and the total cloud fraction show only show a weak response to enhanced aerosol concentrations. However, while the domainwide responses to enhanced aerosol concentrations are weak, aerosol indirect effects on the three tropical cloud modes are found to be quite significant and often opposite in sign, a fact that appears to contribute to the weaker domain response. The results suggest that aerosol indirect effects associated with shallow clouds may offset or compensate for the aerosol indirect effects associated with congestus and deep convection systems and vice versa, thus producing a more moderate domainwide response to aerosol indirect forcing. Finally, when assessing the impacts of aerosol indirect forcing associated with CCN on the characteristics of tropical convection, several aspects need to be considered, including which cloud mode or type is being investigated, the field of interest, and whether localized or systemwide responses are being examined.

Boreal Summer Intraseasonal Variability in Coupled Seasonal Hindcasts

Abstract The intraseasonal variability (ISV) of the Asian summer monsoon represented in seven coupled general circulation models (CGCMs) as part of the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) project is analyzed and evaluated against observations. The focus is on the spatial and seasonal variations of ISV of outgoing longwave radiation (OLR). The large-scale organization of convection, the propagation characteristics, and the air–sea coupling related to the monsoon ISV are also evaluated. A multivariate local mode analysis (LMA) reveals that most models produce less organized convection and ISV events of shorter duration than observed. Compared to the real atmosphere, these simulated patterns of perturbations are poorly reproducible from one event to the other. Most models simulate too weak sea surface temperature (SST) perturbations and systematic phase quadrature between OLR, surface winds, and SST—indicative of a slab-ocean-like response of the SST to surface flux perturbations. The relatively coarse vertical resolution of the different ocean GCMs (OGCMs) limits their ability to represent intraseasonal processes, such as diurnal warm layer formation, which are important for realistic simulation of the SST perturbations at intraseasonal time scales. Models with the same atmospheric GCM (AGCM) and different OGCMs tend to have similar biases of the simulated ISV, indicating the dominant role of atmospheric models in fixing the nature of the intraseasonal variability. It is, therefore, implied that improvements in the representation of ISV in coupled models have to fundamentally arise from fixing problems in the large-scale organization of convection in AGCMs.

Impact of Explicit Atmosphere–Ocean Coupling on MJO-Like Coherent Structures in Idealized Aquaplanet Simulations

Abstract This paper discusses the impact of the atmosphere–ocean coupling on the large-scale organization of tropical convection simulated by an idealized global model applying the Cloud-Resolving Convection Parameterization (CRCP; superparameterization). Because the organization resembles the Madden–Julian Oscillation (MJO), the results contribute to the debate concerning the role of atmosphere–ocean coupling in tropical intraseasonal oscillations. The modeling setup is an aquaplanet with globally uniform mean sea surface temperature (SST) of 30°C (tropics everywhere) in radiative–convective quasi equilibrium. The simulations apply an interactive radiation transfer model and a slab ocean model with a fixed oceanic mixed layer depth. Results from several 80- and 100-day-long simulations are discussed, where the only difference between the simulations is the prescribed oceanic mixed layer depth, which varied from 5 to 45 m. A simulation with a very deep oceanic mixed layer is also performed to represent constant-SST conditions. The simulations demonstrate that the interactive SST impedes the development of large-scale organization and has insignificant impact on the dynamics of mature MJO-like systems. The impediment is the result of a negative feedback between the large-scale organization of convection and SST, the convection–SST feedback. In this feedback, SST increases in regions of already suppressed convection and decreases in regions with enhanced convection, thus hindering the large-scale organization. Once developed, however, the MJO-like systems are equally strong in interactive and constant-SST simulations, and compare favorably with the observed MJO. The above impacts of the atmosphere–ocean coupling contradict the majority of previous studies using traditional general circulation models, where, typically, an enhancement of the intraseasonal signal occurs compared to prescribed-SST simulations. An explanation of this discrepancy is suggested.

Impact of ice microphysics on multiscale organization of tropical convection in two-dimensional cloud-resolving simulations

This paper documents the impact of ice microphysics on the multiscale organization of tropical convection as simulated in two-dimensional cloud-resolving model simulations. Results from simulations which apply warm-rain microphysics are compared with results from a simulation which includes ice processes presented previously in this journal. In these simulations, moist convection and large-scale dynamics interact on an unprecedented range of spatial and temporal scales, albeit within a limited two-dimensional dynamical framework. In general, the large-scale organization of convection is similar in all simulations, although the scale and propagation speed of convectively-coup led gravity waves are different between the warm-rain and ice simulations. The most striking differences are on the mesoscale. The warm-rain simulations feature mesoscale convective systems with a reduced stratiform component and shorter life cycle than mesoscale systems in the simulation with ice microphysics. These impacts are consistent with previous modelling studies of the impact of ice microphysics on organized convection. It is hypothesized that the shorter life cycle of mesoscale convective systems in warm-rain simulations influences scale selection of the large-scale convectively-coup led gravity waves. Moreover, the vertical transport of horizontal momentum is affected as well. To the author's knowledge this is the first time the impact of cloud microphysics on mesoscale convective systems is shown to affect the coupling between deep convection and large-scale perturbations in the tropics. Copyright © 2003 Royal Meteorological Society

MJO-like Coherent Structures: Sensitivity Simulations Using the Cloud-Resolving Convection Parameterization (CRCP)

Abstract Interaction between equatorially trapped disturbances and tropical convection is investigated using a nonhydrostatic global model that applies the cloud-resolving convection parameterization (CRCP). The CRCP represents subgrid scales of the global model by embedding a 2D cloud-resolving model in each column of the global model. The modeling setup is a constant-SST aquaplanet, with the size and rotation of earth, in radiative–convective quasi equilibrium. The global atmosphere is assumed to be initially at rest. No large-scale organization of convection is present outside the equatorial waveguide. Inside the waveguide, on the other hand, the model simulates spontaneous formation of coherent structures with deep convection on the leading edge and strong surface westerly winds to the west, the westerly wind bursts. These coherent structures resemble the Madden–Julian oscillation (MJO) observed in the terrestrial Tropics and are present in simulations applying prescribed and interactive radiation, an...