The role of radiation in organizing convection in weak temperature gradient simulations

Sessions, Sharon; Sentić, Stipo; Herman, Michael J.

doi:10.1002/2015ms000587

Cited by 24 publications

(49 citation statements)

References 71 publications

(308 reference statements)

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“…Global-scale simulations, however, do represent these circulations (e.g., Coppin and Bony 2015), and smaller RCE simulations can impose them (e.g., Su et al 2000) or parameterize them using reference profiles and assumptions of weak temperature or pressure gradients (e.g., Sessions et al 2016). As these kinds of modeling studies progress, there will be more opportunities to evaluate their simulated relationships between aggregation and large-scale circulations using observations.…”

Section: Mean Wind and Wind Shearmentioning

confidence: 99%

Observing Convective Aggregation

et al. 2017

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Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of 123Surv Geophys (2017) 38:1199-1236 https://doi.org/10.1007/s10712-017-9419-1 model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a groundbased observational network.

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Section: Mean Wind and Wind Shearmentioning

confidence: 99%

Observing Convective Aggregation

et al. 2017

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“…One such tool is the weak temperature gradient (WTG) approximation [ Sobel and Bretherton , ; Raymond and Zeng , ; Herman and Raymond , ]. Based on the observed weak horizontal temperature gradients in the tropics, the WTG approximation parameterizes the effects of the large scale in limited domain simulations, and it is used to study the effects of changes in the large‐scale temperature and moisture on local convection [ Sobel and Bretherton , ; Raymond and Zeng , ; Raymond and Sessions , ; Sessions et al ., ]. Additionally, the WTG approximation allows for decoupling of dynamic and thermodynamic forcing to study their isolated effects on convection.…”

Section: Introductionmentioning

confidence: 99%

“…Additionally, the WTG approximation allows for decoupling of dynamic and thermodynamic forcing to study their isolated effects on convection. For instance, researchers have studied the convective response to radiation [ Anber et al ., ; Wang et al ., ; Sessions et al ., ], moisture treatment [ Wang and Sobel , ; Sessions et al ., ], surface fluxes [ Raymond and Zeng , ; Sessions et al ., ; Anber et al ., ], vertical wind shear [ Anber et al ., ], SSTs [ Sobel and Bretherton , ; Wang and Sobel , ; Daleu et al ., ], and changes in atmospheric stability and moisture [ Raymond and Sessions , ; Sessions et al ., ]. The WTG approximation has also been applied to modeling the evolution of the thermodynamic environment in tropical cyclogenesis [ Raymond and Sessions , ] and the Madden‐Julian Oscillation [ Wang et al ., ; Sentic et al ., ], both known instances of convective organization.…”

Section: Introductionmentioning

confidence: 99%

“…The WTG approximation can be used for studying convective organization by utilizing the hypothesized analogy [ Emanuel et al ., ; Anber et al ., ; Sessions et al ., ] between the moist and dry regions in domains with organized convection and multiple equilibria in precipitation exhibited in WTG simulations [ Raymond and Zeng , ; Sessions et al ., ; Anber et al ., ; Herman and Raymond , ; Sessions et al ., ]. Multiple equilibria in precipitation are defined as a dry or moist steady state for given boundary conditions when the domain is initialized dry or moist [e.g., Sobel et al ., ; Sessions et al ., ].…”

Section: Introductionmentioning

confidence: 99%

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Idealized modeling of convective organization with changing sea surface temperatures using multiple equilibria in weak temperature gradient simulations

Sentić

Sessions

2017

J Adv Model Earth Syst

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The weak temperature gradient (WTG) approximation is a method of parameterizing the influences of the large scale on local convection in limited domain simulations. WTG simulations exhibit multiple equilibria in precipitation; depending on the initial moisture content, simulations can precipitate or remain dry for otherwise identical boundary conditions. We use a hypothesized analogy between multiple equilibria in precipitation in WTG simulations, and dry and moist regions of organized convection to study tropical convective organization. We find that the range of wind speeds that support multiple equilibria depends on sea surface temperature (SST). Compared to the present SST, low SSTs support a narrower range of multiple equilibria at higher wind speeds. In contrast, high SSTs exhibit a narrower range of multiple equilibria at low wind speeds. This suggests that at high SSTs, organized convection might occur with lower surface forcing. To characterize convection at different SSTs, we analyze the change in relationships between precipitation rate, atmospheric stability, moisture content, and the large‐scale transport of moist entropy and moisture with increasing SSTs. We find an increase in large‐scale export of moisture and moist entropy from dry simulations with increasing SST, which is consistent with a strengthening of the up‐gradient transport of moisture from dry regions to moist regions in organized convection. Furthermore, the changes in diagnostic relationships with SST are consistent with more intense convection in precipitating regions of organized convection for higher SSTs.

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“…Since then, a number of papers have investigated this in further detail, with the feedback variously attributed to the radiative impact of high clouds [ Stephens et al ., ] and low clouds [ Muller and Held , ]. Both the long‐wave and short‐wave radiative impacts of the heterogeneous water vapor field can also drive convergence into moist regions [ Wing and Emanuel , ; Emanuel et al ., ; Sessions et al ., ]. Mapes and Zuidema [] document the changes in the vertical stability that can result from dry layers.…”

Section: Introductionmentioning

confidence: 99%

Organization of tropical convection in low vertical wind shears: Role of updraft entrainment

Tompkins

Semie

2017

J Adv Model Earth Syst

153

225

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Radiative‐convective equilibrium simulations with a 2 km horizontal resolution are conducted to investigate the impact on convective organization of different parameterizations for horizontal and vertical subgrid turbulence mixing. Three standard approaches for representing horizontal diffusion produce starkly differing mixing rates, particularly for the entrainment mixing into updrafts, which differ by more than an order of magnitude between the schemes. The simulations demonstrate that the horizontal subgrid mixing of water vapor is key, with high mixing rates a necessary condition for organization of convection to occur, since entrainment of dry air into updrafts suppresses convection. It is argued that diabatic budgets, while demonstrating the role of spatially heterogeneous radiative heating rates in driving organization, can overlook the role of physical processes such as updraft entrainment. These results may partially explain previous studies that showed that organization is more likely to occur at coarser resolutions, when entrainment is solely represented by subgrid‐scale turbulence schemes, highlighting the need for benchmark simulations of higher horizontal resolution. The recommendation is for the use of larger ensembles to ensure robustness of conclusions to subgrid‐scale parameterization assumptions when numerically investigating convective organization, possibly through a coordinated community model intercomparison effort.

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The role of radiation in organizing convection in weak temperature gradient simulations

Cited by 24 publications

References 71 publications

Observing Convective Aggregation

Observing Convective Aggregation

Idealized modeling of convective organization with changing sea surface temperatures using multiple equilibria in weak temperature gradient simulations

Organization of tropical convection in low vertical wind shears: Role of updraft entrainment

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