Equilibrium Vs. Activation Control of Large-Scale Variations of Tropical Deep Convection

Abstract The effects of turbulent surface fluxes and radiative heating on tropical deep convection are compared in a series of idealized cloud-system-resolving simulations with parameterized large-scale dynamics. Two methods of parameterizing the large-scale dynamics are used: the weak temperature gradient (WTG) approximation and the damped gravity wave (DGW) method. Both surface fluxes and radiative heating are specified, with radiative heating taken as constant in the vertical in the troposphere. All simulations are run to statistical equilibrium. In the precipitating equilibria, which result from sufficiently moist initial conditions, an increment in surface fluxes produces more precipitation than an equal increment of column-integrated radiative heating. This is straightforwardly understood in terms of the column-integrated moist static energy budget with constant normalized gross moist stability. Under both large-scale parameterizations, the gross moist stability does in fact remain close to constant over a wide range of forcings, and the small variations that occur are similar for equal increments of surface flux and radiative heating. With completely dry initial conditions, the WTG simulations exhibit hysteresis, maintaining a dry state with no precipitation for a wide range of net energy inputs to the atmospheric column. The same boundary conditions and forcings admit a rainy state also (for moist initial conditions), and thus multiple equilibria exist under WTG. When the net forcing (surface fluxes minus radiative heating) is increased enough that simulations that begin dry eventually develop precipitation, the dry state persists longer after initialization when the surface fluxes are increased than when radiative heating is increased. The DGW method, however, shows no multiple equilibria in any of the simulations.

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Response of Tropical Deep Convection to Localized Heating Perturbations: Implications for Aerosol-Induced Convective Invigoration

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-027.1 ◽

2013 ◽

Vol 70 (11) ◽

pp. 3533-3555 ◽

Cited By ~ 18

Author(s):

Hugh Morrison ◽

Wojciech W. Grabowski

Keyword(s):

Large Scale ◽

Deep Convection ◽

Warm Pool ◽

Tropospheric Temperature ◽

Surface Precipitation ◽

Tropical Warm Pool ◽

Adjustment Time ◽

Localized Heating ◽

Tropical Deep Convection ◽

Convective Adjustment

Abstract A cloud-system-resolving model is used to investigate the effects of localized heating/cooling perturbations on tropical deep convection, in the context of the aerosol “invigoration effect.” This effect supposes that a reduction of droplet collision–coalescence in polluted conditions leads to lofting of cloud water in convective updrafts and enhanced freezing, latent heating, and buoyancy. To specifically isolate and test this mechanism, heating perturbations were applied to updrafts with corresponding cooling applied in downdrafts. Ensemble simulations were run with either perturbed or unperturbed conditions and large-scale forcing from a 7.5-day period of active monsoon conditions during the 2006 Tropical Warm Pool–International Cloud Experiment. In the perturbed simulations there was an initial invigoration of convective updrafts and surface precipitation, but convection returned to its unperturbed state after about 24 h because of feedback with the larger-scale environment. This feedback led to an increase in the horizontally averaged mid-/upper-tropospheric temperature of about 1 K relative to unperturbed simulations. When perturbed conditions were applied to only part of the domain, gravity waves rapidly dispersed buoyancy anomalies in the perturbed region to the rest of the domain, allowing convective invigoration from the heating perturbations to be sustained over the entire simulation period. This was associated with a mean mesoscale circulation consisting of ascent (descent) at mid-/upper levels in the perturbed (unperturbed) region. In contrast to recent studies, it is concluded that the invigoration effect is intimately coupled with larger-scale dynamics through a two-way feedback, and in the absence of alterations in the larger-scale circulation there is limited invigoration beyond the convective adjustment time scale.

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Convective self–aggregation in a mean flow

10.5194/acp-2020-875 ◽

2020 ◽

Author(s):

Hyunju Jung ◽

Ann Kristin Naumann ◽

Bjorn Stevens

Keyword(s):

Large Scale ◽

Deep Convection ◽

Surface Flux ◽

Horizontal Wind ◽

Convective System ◽

Mean Flow ◽

Near Surface ◽

Tropical Deep Convection ◽

Large Scale Flow ◽

Self Aggregation

Abstract. Convective self-aggregation is an atmospheric phenomenon found in numerical simulations in a radiative convective equilibrium framework of which configuration captures the main characteristics of the real-world convection in the deep tropics. As tropical deep convection is typically embedded in a large-scale flow, we impose a background mean wind flow on convection-permitting simulations through the surface flux calculation. The simulations show that with imposing mean flow, the organized convective system propagates in the direction of the flow but slows down compared to what pure advection would suggest, and eventually becomes stationary relative to the surface after 15 simulation days. The termination of the propagation arises from momentum flux, which acts as a drag on the near-surface horizontal wind. In contrast, the thermodynamic response through the wind-induced surface heat exchange feedback is a relatively small effect, which slightly retards (by about 15 %) the convection relative to the mean wind.

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Probing the Response of Tropical Deep Convection to Aerosol Perturbations Using Idealized Cloud-Resolving Simulations with Parameterized Large-Scale Dynamics

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0351.1 ◽

2019 ◽

Vol 76 (9) ◽

pp. 2885-2897

Author(s):

Usama M. Anber ◽

Shuguang Wang ◽

Pierre Gentine ◽

Michael P. Jensen

Keyword(s):

Boundary Layer ◽

Large Scale ◽

Diabatic Heating ◽

Deep Convection ◽

Cloud Droplets ◽

Aerosol Loading ◽

Microphysical Properties ◽

Tropical Deep Convection ◽

Gross Moist Stability ◽

The Impact

Abstract A framework is introduced to investigate the indirect effect of aerosol loading on tropical deep convection using three-dimensional limited-domain idealized cloud-system-resolving model simulations coupled with large-scale dynamics over fixed sea surface temperature. The large-scale circulation is parameterized using the spectral weak temperature gradient (WTG) approximation that utilizes the dominant balance between adiabatic cooling and diabatic heating in the tropics. The aerosol loading effect is examined by varying the number of cloud condensation nuclei (CCN) available to form cloud droplets in the two-moment bulk microphysics scheme over a wide range of environments from 30 to 5000 cm−3. The radiative heating is held at a constant prescribed rate in order to isolate the microphysical effects. Analyses are performed over the period after equilibrium is achieved between convection and the large-scale environment. Mean precipitation is found to decrease modestly and monotonically when the aerosol number concentration increases as convection gets weaker, despite the increase in cloud liquid water in the warm-rain region and ice crystals aloft. This reduction is traced down to the reduction in surface enthalpy fluxes as an energy source to the atmospheric column induced by the coupling of the large-scale motion, though the gross moist stability remains constant. Increasing CCN concentration leads to 1) a cooler free troposphere because of a reduction in the diabatic heating and 2) a warmer boundary layer because of suppressed evaporative cooling. This dipole temperature structure is associated with anomalously descending large-scale vertical motion above the boundary layer and ascending motion at lower levels. Sensitivity tests suggest that changes in convection and mean precipitation are unlikely to be caused by the impact of aerosols on cloud droplets and microphysical properties but rather by accounting for the feedback from convective adjustment with the large-scale dynamics. Furthermore, a simple scaling argument is derived based on the vertically integrated moist static energy budget, which enables estimation of changes in precipitation given known changes in surfaces enthalpy fluxes and the constant gross moist stability. The impact on cloud hydrometeors and microphysical properties is also examined, and it is consistent with the macrophysical picture.

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Interaction of Tropical Deep Convection with the Large-Scale Circulation in the MJO

Journal of Climate ◽

10.1175/2009jcli3240.1 ◽

2010 ◽

Vol 23 (7) ◽

pp. 1837-1853 ◽

Cited By ~ 53

Author(s):

Eric Tromeur ◽

William B. Rossow

Keyword(s):

Large Scale ◽

Deep Convection ◽

Lower Troposphere ◽

Surface Fluxes ◽

Convective Heating ◽

Large Scale Circulation ◽

Tropical Deep Convection ◽

Upper Level ◽

Almost All ◽

Cloud Regimes

Abstract To better understand the interaction between tropical deep convection and the Madden–Julian oscillation (MJO), tropical cloud regimes are defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud-top pressure—optical thickness joint distributions from the D1 dataset covering 21.5 yr. An MJO index based solely on upper-level wind anomalies is used to study variations of the tropical cloud regimes. The MJO index shows that MJO events are present almost all the time; instead of the MJO event being associated with “on or off” deep convection, it is associated with weaker or stronger mesoscale organization of deep convection. Atmospheric winds and humidity from NCEP–NCAR reanalysis 1 are used to characterize the large-scale dynamics of the MJO; the results show that the large-scale motions initiate an MJO event by moistening the lower troposphere by horizontal advection. Increasingly strong convection transports moisture into the upper troposphere, suggesting a reinforcement of the convection itself. The change of convection organization shown by the cloud regimes indicates a strong interaction between the large-scale circulation and deep convection. The analysis is extended to the complete atmospheric diabatic heating by precipitation, radiation, and surface fluxes. The wave organizes stronger convective heating of the tropical atmosphere, which results in stronger winds, while there is only a passive response of the surface, directly linked to cloud radiative effects. Overall, the results suggest that an MJO event is an amplification of large-scale wave motions by stronger convective heating, which results from a dynamic reorganization of scattered deep convection into more intense mesoscale systems.

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Anvil Productivities of Tropical Deep Convective Clusters and Their Regional Differences

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0239.1 ◽

2016 ◽

Vol 73 (9) ◽

pp. 3467-3487 ◽

Cited By ~ 7

Author(s):

Min Deng ◽

Gerald. G. Mace ◽

Zhien Wang

Keyword(s):

Regional Differences ◽

Large Scale ◽

Regional Difference ◽

Deep Convection ◽

Life Stage ◽

Tropical Convection ◽

Mass Ratio ◽

Tropical Deep Convection ◽

Original Cluster

The anvil productivities of tropical deep convection are investigated and compared among eight climatological regions using 4 yr of collocated and combined CloudSat and CALIPSO data. For all regions, the convective clusters become deeper while they become wider and tend to be composed of multiple rainy cores. Two strong detrainment layers from deep convection are observed at 6–8 km and above 10 km, which is consistent with the trimodal characteristics of tropical convection that are associated with different divergence, cloud detrainment, and fractional cloudiness. The anvil productivity of tropical deep convection depends on the convection scale, convective life stage or intensity, and large-scale environment. Anvil ice mass ratio related to the whole cluster starts to level off or decrease when the cluster effective scales Weff (the dimension of an equivalent rectangular with the same volume and height as the original cluster) increase to about 200 km wide, while the ratios of anvil scale and volume keep increasing from 0.4 to 0.6 and 0.15 to 0.4, respectively. The anvil clouds above 12 km can count for more than 20% of cluster volume, or more than 50% of total anvil volume, but they only count less than about 2% of total ice mass in the cluster. Anvil production of younger convection of the same Weff is higher than that of the decaying convection. The regional difference in the composite anvil productivities of tropical convective clusters sorted by Weff is subtle, while the occurrence frequencies of different scales of convection vary substantially.

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Cloud-system resolving model simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-15573-2011 ◽

2011 ◽

Vol 11 (5) ◽

pp. 15573-15629 ◽

Cited By ~ 5

Author(s):

H. Morrison ◽

W. W. Grabowski

Keyword(s):

Indirect Effects ◽

Large Scale ◽

Deep Convection ◽

Radiative Heating ◽

Cloud System ◽

Direct Result ◽

Radiative Fluxes ◽

Aerosol Indirect Effects ◽

Ensemble Approach ◽

Tropical Deep Convection

Abstract. This paper presents results from 240-member ensemble simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment. Simulations using a two-dimensional cloud system-resolving model are run with pristine, polluted, or highly polluted aerosol conditions and large-scale forcing from a 6-day period of active monsoon conditions during the 2006 Tropical Warm Pool – International Cloud Experiment (TWP-ICE). Domain-mean surface precipitation is insensitive to aerosols primarily because the large-scale forcing is prescribed and dominates the water and static energy budgets. The spread of the top-of-atmosphere (TOA) shortwave and longwave radiative fluxes among different ensemble members for the same aerosol loading is surprisingly large, exceeding 25 W m−2 even when averaged over the 6-day period. This variability is caused by random fluctuations in the strength and timing of individual deep convective events. The ensemble approach demonstrates a small weakening of convection averaged over the 6-day period in the polluted simulations compared to pristine. Despite this weakening, the cloud top heights and anvil ice mixing ratios are higher in polluted conditions. This occurs because of the larger concentrations of cloud droplets that freeze, leading directly to higher ice particle concentrations, smaller ice particle sizes, and smaller fall velocities compared to simulations with pristine aerosols. Weaker convection in polluted conditions is a direct result of the changes in anvil ice characteristics and subsequent upper-tropospheric radiative heating and weaker tropospheric destabilization. Such a conclusion offers a different interpretation of recent satellite observations of tropical deep convection in pristine and polluted environments compared to the hypothesis of aerosol-induced convective invigoration. Sensitivity tests using the ensemble approach with modified microphysical parameters or domain configuration (horizontal gridlength, domain size) produce results that are similar to baseline, although there are quantitative differences in estimates of aerosol impacts on TOA radiative fluxes.

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Responses of Tropical Deep Convection to the QBO: Cloud-Resolving Simulations

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0035.1 ◽

2015 ◽

Vol 72 (9) ◽

pp. 3625-3638 ◽

Cited By ~ 38

Author(s):

Ji Nie ◽

Adam H. Sobel

Keyword(s):

Large Scale ◽

Deep Convection ◽

Vertical Motion ◽

Convective Precipitation ◽

Temperature Perturbation ◽

Quasi Biennial Oscillation ◽

Reference Profile ◽

Budget Analysis ◽

Tropical Deep Convection ◽

Sst Anomalies

Abstract Observational studies suggest that the stratospheric quasi-biennial oscillation (QBO) can modulate tropical deep convection. The authors use a cloud-resolving model with a limited domain, representing a convective column in the tropics, to study the mechanisms of this modulation. The large-scale circulation is parameterized using the weak temperature gradient (WTG) approximation, under which the parameterized large-scale vertical motion acts to relax the horizontal-mean temperature toward a specified reference profile. Temperature variations typically seen in easterly and westerly phases are imposed in the upper troposphere and lower stratosphere of this reference profile. The responses of convection are studied over different sea surface temperatures, holding the reference temperature profile fixed. This can be thought of as studying the response of convection to the QBO over different “relative SSTs” and also corresponds to different equilibrium precipitation rates in the control simulation. The equilibrium precipitation rate shows slight increases in response to a QBO easterly phase temperature perturbation over small SST anomalies and strong decreases over large SST anomalies, and vice versa for the QBO westerly phase perturbation. A column moist static energy budget analysis reveals that the QBO modulates the convective precipitation through two pathways: it changes the high-cloud properties and thus the column radiative cooling, and it alters the shape of the large-scale vertical motion and thus the efficiency of energy transport by the large-scale flow. The nonmonotonicity of the precipitation response with respect to relative SST results from the competition of these two effects.

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