Response of Atmospheric Convection to Vertical Wind Shear: Cloud-System-Resolving Simulations with Parameterized Large-Scale Circulation. Part II: Effect of Interactive Radiation

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Two Madden-Julian Oscillation (MJO) events, observed during October and November 2011 in the equatorial Indian Ocean during the DYNAMO field campaign, are simulated in a limited-area cloudresolving model using parameterized large-scale dynamics. Three parameterizations of large-scale dynamics-the conventional weak temperature gradient (WTG) approximation, vertical mode-based spectral WTG (SWTG), and damped gravity wave coupling (DGW)-are employed. A number of changes to the implementation of the large-scale parameterizations, as well as the model itself, are made and lead to improvements in the results. Simulations using all three methods, with imposed time-dependent radiation and horizontal moisture advection, capture the time variations in precipitation associated with the two MJO events well. The three methods produce significant differences in the large-scale vertical motion profile, however. WTG produces the most top-heavy profile, while DGW's is less so, and SWTG produces a profile between the two, and in better agreement with observations. Numerical experiments without horizontal advection of moisture suggest that that process significantly reduces the precipitation and suppresses the top-heaviness of large-scale vertical motion during the MJO active phases. Experiments in which a temporally constant radiative heating profile is used indicate that radiative feedbacks significantly amplify the MJO. Experiments in which interactive radiation is used produce agreement with observations that is much better than that achieved in previous work, though not as good as that with imposed time-varying radiative heating. Our results highlight the importance of both horizontal advection of moisture and radiative feedbacks to the dynamics of the MJO.

Section: Resultssupporting

confidence: 79%

Section: Role Of Radiation In Simulated Mjo Dynamicssupporting

confidence: 83%

Modeling the MJO in a cloud‐resolving model with parameterized large‐scale dynamics: Vertical structure, radiation, and horizontal advection of dry air

Wang

Sobel

Nie

2016

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“…Though our model and the WTG implementation differs significantly from the Weather and Research Forecasting model used in Anber et al . [], we speculate that the most significant contribution to the different behavior is a consequence of the radiative cooling profiles generated by the two models (compare their Figure with Figure in this paper). Their cooling profiles exhibit strong (∼−10 K d −1 ) cooling at 12 km with 5 K d −1 heating at 8 km.…”

Section: Resultsmentioning

confidence: 54%

“…These cooling profiles—which include contributions from both long and shortwave radiation—are very different from those obtained in Anber et al . [], who considered the effect of wind shear and static versus interactive radiation on convection in WTG simulations. In their Figure , the radiative cooling profiles (which also include contributions from both long and shortwave radiation) show a cooling of about 1 K d −1 from the surface to about 12 km only for weak surface fluxes; stronger surface fluxes generate cooling patterns reminiscent of stratiform clouds with strong cooling (−8 to −12 K d −1 ) at about 12 km with strong warming (5–7 K d −1 ) at an approximate cloud base of 8 km.…”

Section: Numerical Experimentsmentioning

confidence: 99%

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

Sessions

Sentić

Herman

2016

Using a cloud system resolving model with the large scale parameterized by the weak temperature gradient approximation, we investigated the influence of interactive versus noninteractive radiation on the characteristics of convection and convective organization. The characteristics of convecting environments are insensitive to whether radiation is interactive compared to when it is not. This is not the case for nonconvecting environments; interactive radiative cooling profiles show strong cooling at the top of the boundary layer which induces a boundary layer circulation that ultimately exports moist entropy (or analogously moist static energy) from dry domains. This upgradient transport is associated with a negative gross moist stability, and it is analogous to boundary layer circulations in radiative convective equilibrium simulations of convective self-aggregation. This only occurs when radiation cools interactively. Whether radiation is static or interactive also affects the existence of multiple equilibria-steady states which either support precipitating convection or which remain completely dry depending on the initial moisture profile. Interactive radiation drastically increases the range of parameters which permit multiple equilibria compared to static radiation; this is consistent with the observation that self-aggregation in radiative-convective equilibrium simulations is more readily attained with interactive radiation. However, the existence of multiple equilibria in absence of interactive radiation suggests that other mechanisms may result in organization.

Coupling with ocean mixed layer leads to intraseasonal variability in tropical deep convection: Evidence from cloud‐resolving simulations

Anber

Wang

Sobel

2017

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The effect of coupling a slab ocean mixed layer to atmospheric convection is examined in cloud‐resolving model (CRM) simulations in vertically sheared and unsheared environments without Coriolis force, with the large‐scale circulation parameterized using the Weak Temperature Gradient (WTG) approximation. Surface fluxes of heat and moisture as well as radiative fluxes are fully interactive, and the vertical profile of domain‐averaged horizontal wind is strongly relaxed toward specified profiles with vertical shear that varies from one simulation to the next. Vertical wind shear is found to play a critical role in the simulated behavior. There exists a threshold value of the shear strength above which the coupled system develops regular oscillations between deep convection and dry nonprecipitating states, similar to those found earlier in a much more idealized model which did not consider wind shear. The threshold value of the vertical shear found here varies with the depth of the ocean mixed layer. The time scale of the spontaneously generated oscillations also varies with mixed layer depth, from 10 days with a 1 m deep mixed layer to 50 days with a 10 m deep mixed layer. The results suggest the importance of the interplay between convection organized by vertical wind shear, radiative feedbacks, large‐scale dynamics, and ocean mixed layer heat storage in real intraseasonal oscillations.