A mass-flux convection scheme for regional and global models

AbstractTropical convection that occurs on large-enough space and time scales may evolve in response to large-scale balanced circulations. In this scenario, large-scale midtropospheric vorticity anomalies modify the atmospheric stability by virtue of thermal wind gradient balance. The convective vertical mass flux and the moisture profile adjust to changes in atmospheric stability that affect moisture and entropy transport. We hypothesize that the convection observed during the 2011 DYNAMO field campaign evolves in response to balanced dynamics. Strong relationships between midtropospheric vorticity and atmospheric stability confirm the relationship between the dynamic and the thermodynamic environments, while robust relationships between the atmospheric stability, the vertical mass flux, and the saturation fraction provide evidence of moisture adjustment. These results are important because the part of convection that occurs as a response to balanced dynamics is potentially predictable. Furthermore, the diagnostics used in this work provide a simple framework for model evaluation, and suggest that one way to improve simulations of large-scale organized deep tropical convection in global models is to adequately capture the relationship between the dynamic and thermodynamic environments in convective parameterizations.

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A Parameterization of Convective Dust Storms for Models with Mass-Flux Convection Schemes

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0341.1 ◽

2015 ◽

Vol 72 (6) ◽

pp. 2545-2561 ◽

Cited By ~ 25

Author(s):

Florian Pantillon ◽

Peter Knippertz ◽

John H. Marsham ◽

Cathryn E. Birch

Keyword(s):

Conceptual Model ◽

Mass Flux ◽

Large Scale ◽

Dust Storms ◽

Cold Pool ◽

Moist Convection ◽

Convection Scheme ◽

Scale Models ◽

Strong Winds ◽

Convection Schemes

Abstract Cold pool outflows, generated by downdrafts from moist convection, can generate strong winds and therefore uplift of mineral dust. These so-called haboob convective dust storms occur over all major dust source areas worldwide and contribute substantially to emissions in northern Africa, the world’s largest source. Most large-scale models lack convective dust storms because they do not resolve moist convection, relying instead on convection schemes. The authors suggest a parameterization of convective dust storms to account for their contribution in such large-scale models. The parameterization is based on a simple conceptual model, in which the downdraft mass flux from the convection scheme spreads out radially in a cylindrical cold pool. The parameterization is tested with a set of Met Office Unified Model runs for June and July 2006 over West Africa. It is calibrated with a convection-permitting run and applied to a convection-parameterized run. The parameterization successfully produces the extensive area of dust-generating winds from cold pool outflows over the southern Sahara. However, this area extends farther to the east and dust-generating winds occur earlier in the day than in the convection-permitting run. These biases are caused by biases in the convection scheme. It is found that the location and timing of dust-generating winds are weakly sensitive to the parameters of the conceptual model. The results demonstrate that a simple parameterization has the potential to correct a major and long-standing limitation in global dust models.

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Updates in the NCEP GFS Cumulus Convection Schemes with Scale and Aerosol Awareness

Weather and Forecasting ◽

10.1175/waf-d-17-0046.1 ◽

2017 ◽

Vol 32 (5) ◽

pp. 2005-2017 ◽

Cited By ~ 27

Author(s):

Jongil Han ◽

Weiguo Wang ◽

Young C. Kwon ◽

Song-You Hong ◽

Vijay Tallapragada ◽

...

Keyword(s):

Mass Flux ◽

Deep Convection ◽

Threshold Value ◽

Turnover Time ◽

Cloud Base ◽

Cumulus Convection ◽

Quasi Equilibrium ◽

Convection Scheme ◽

Freezing Level ◽

Convection Schemes

Abstract The current operational NCEP Global Forecast System (GFS) cumulus convection schemes are updated with a scale-aware parameterization where the cloud mass flux decreases with increasing grid resolution. The ratio of advective time to convective turnover time is also taken into account for the scale-aware parameterization. In addition, the present deep cumulus convection closure using the quasi-equilibrium assumption is no longer used for grid sizes smaller than a threshold value. For the shallow cumulus convection scheme, the cloud-base mass flux is modified to be given by a function of mean updraft velocity. A simple aerosol-aware parameterization where rain conversion in the convective updraft is modified by aerosol number concentration is also included in the update. Along with the scale- and aerosol-aware parameterizations, more changes are made to the schemes. The cloud-base mass-flux computation in the deep convection scheme is modified to use convective turnover time as the convective adjustment time scale. The rain conversion rate is modified to decrease with decreasing air temperature above the freezing level. Convective inhibition in the subcloud layer is used as an additional trigger condition. Convective cloudiness is enhanced by considering suspended cloud condensate in the updraft. The lateral entrainment in the deep convection scheme is also enhanced to more strongly suppress convection in a drier environment. The updated NCEP GFS cumulus convection schemes display significant improvements especially in the summertime continental U.S. precipitation forecasts.

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Bulk Mass-Flux Perturbation Formulation for a Unified Approach of Deep Convection at High Resolution

Monthly Weather Review ◽

10.1175/mwr-d-15-0030.1 ◽

2015 ◽

Vol 143 (10) ◽

pp. 4038-4063 ◽

Cited By ~ 12

Author(s):

Luc Gerard

Keyword(s):

Mass Flux ◽

Deep Convection ◽

Limited Area ◽

Perturbation Approach ◽

Unified Approach ◽

Convection Scheme ◽

Bulk Mass ◽

Simplifying Assumptions ◽

Model Cloud ◽

The Mean

Abstract Parameterizing deep convection at model resolutions of a few kilometers or less requires diverging from several simplifying assumptions valid at coarser resolutions. The separation or complementarity between the deep-convection scheme and the model cloud scheme must be addressed properly to prevent a double counting of some phenomena, account for evolution in time, and keep consistent results while approaching resolutions where deep convection can be treated explicitly (without parameterization). In this paper, the author formulates and tests a perturbation approach of the bulk mass-flux representation of deep convective updrafts and downdrafts. The subgrid deep-convection scheme represents only the effect of the unresolved part of the real updrafts, complementing an explicit part associated with the mean gridbox vertical velocity. Special attention is paid to the ordering and interactions of the moist parameterizations, the formulation of the closure and of the triggering, and the accounting of time evolution aspects. The multiresolution behavior of the scheme is assessed in the operational numerical prediction model ALARO [a version of the Action de Recherche Petite Echelle Grande Echelle-Aire Limitée Adaptation Dynamique Développement International (ARPEGE-ALADIN) operational limited area model with a revised and modular structure of the physical parameterizations]. Unlike most mass-flux schemes, the parameterized part gradually fades out when the model resolution is increased, allowing the results to approach those of an explicit treatment of deep convection.

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