The Dynamics of Idealized Convection Schemes and Their Effect on the Zonally Averaged Tropical Circulation

2007 ◽  
Vol 64 (6) ◽  
pp. 1959-1976 ◽  
Author(s):  
Dargan M. W. Frierson
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Hadley Circulation ◽  
Horizontal Resolution ◽  
Shallow Convection ◽  
Tropical Circulation ◽  
Convection Scheme ◽  
Convection Schemes ◽  
Gross Moist Stability ◽  
The Tropics

In this paper, the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist GCM to obtain broad classifications of the influence of convection on the Tropics. This is accomplished with a simplified convection scheme in the style of Betts and Miller. The scheme is utilized in a moist GCM with simplified physical parameterizations (gray radiation, with zonally symmetric, slab mixed layer ocean boundary conditions). Comparisons are made with simulations without a convection scheme [i.e., with large-scale condensation (LSC) only], with the moist convective adjustment (MCA) parameterization, and with various formulations and parameter sets with a simplified Betts–Miller (SBM) scheme. With the control run using the SBM scheme, the Tropics become quieter and less dependent on horizontal resolution as compared with the LSC or MCA simulations. The Hadley circulation mass transport is significantly reduced with the SBM scheme, as is the ITCZ precipitation. An important factor determining this behavior is the parameterization of shallow convection: without shallow convection, the convection scheme is largely ineffective at preventing convection from occurring at the grid scale. The sensitivities to convection scheme parameters are also examined. The simulations are remarkably insensitive to the convective relaxation time, and only mildly sensitive to the relative humidity of the reference profile, provided significant large-scale condensation is not allowed to occur. The changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere.

scholarly journals Convectively Coupled Kelvin Waves in an Idealized Moist General Circulation Model

2007 ◽  
Vol 64 (6) ◽  
pp. 2076-2090 ◽  
Author(s):  
Dargan M. W. Frierson
Keyword(s):  
Relaxation Time ◽  
General Circulation Model ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Kelvin Waves ◽  
Convection Scheme ◽  
Gross Moist Stability ◽  
The Tropics ◽  
The Waves

The dynamics of convectively coupled Kelvin waves and their dependence on convection scheme parameters are studied within a simplified moist general circulation model. The model consists of the primitive equations on the sphere over zonally symmetric aquaplanet, slab mixed layer ocean boundary conditions, and idealized physical parameterizations including gray radiative transfer and a simplified Betts–Miller convection scheme. This framework allows the authors to study the dependence of Kelvin waves on quantities such as the gross moist stability in a clean manner. A control simulation with the model produces convectively coupled Kelvin waves that are remarkably persistent and dominate the variability within the Tropics. These waves propagate with an equivalent depth of ≈40 m. Linear regression analysis with respect to a Kelvin-filtered time series shows that the waves are driven by evaporation–wind feedback and have structures broadly consistent with theoretical predictions for Kelvin waves. Next, the determination of the speed and structure of the Kelvin waves is studied by examining the response of the waves to changes in convection scheme parameters. When the convective relaxation time is lengthened, the waves are damped and eventually are completely eliminated. The propagation speed additionally increases with longer relaxation time. Then changes to a convection scheme parameter that essentially controls the fraction of convective versus large-scale precipitation are examined. When some large-scale precipitation occurs, the waves increase in strength, propagate more slowly, and move to larger scales. However, when mostly large-scale precipitation occurs, the Kelvin wave disappears, and the Tropics are dominated by tropical storm–like variability. The decrease in speed is related here to the gross moist stability of the atmosphere, which is reduced with increased large-scale precipitation.

Dynamic Radiative–Convective Equilibria Using GCM Column Physics

2007 ◽  
Vol 64 (1) ◽  
pp. 228-238 ◽  
Author(s):  
Isaac M. Held ◽  
Ming Zhao ◽  
Bruce Wyman
Keyword(s):  
Fluid Dynamics ◽  
Large Scale ◽  
Homogeneous Model ◽  
Horizontal Resolution ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Geophysical Fluid Dynamics ◽  
Convection Scheme ◽  
Cloud Feedbacks ◽  
The Tropics ◽  
Convective Organization

Abstract The behavior of a GCM column physics package in a nonrotating, doubly periodic, homogeneous setting with prescribed SSTs is examined. This radiative–convective framework is proposed as a useful tool for studying some of the interactions between convection and larger-scale dynamics and the effects of differing modeling assumptions on convective organization and cloud feedbacks. For the column physics utilized here, from the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 model, many of the properties of the homogeneous, nonrotating model are closely tied to the fraction of precipitation that is large-scale, rather than convective. Significant large-scale precipitation appears above a critical temperature and then increases with further increases in temperature. The amount of large-scale precipitation is a function of horizontal resolution and can also be controlled by modifying the convection scheme, as is illustrated here by modifying assumptions concerning entrainment into convective plumes. Significant similarities are found between the behavior of the homogeneous model and that of the Tropics of the parent GCM when ocean temperatures are increased and when the convection scheme is modified.

Double and Single ITCZs with and without Clouds

2017 ◽  
Vol 30 (22) ◽  
pp. 9147-9166 ◽  
Author(s):  
Max Popp ◽  
Levi G. Silvers
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Convective Available Potential Energy ◽  
Circulation Model ◽  
Hadley Circulation ◽  
Large Scale Circulation ◽  
Low Level ◽  
Convergence Zones ◽  
Gross Moist Stability ◽  
Static Energy

A major bias in tropical precipitation over the Pacific in climate simulations stems from the models’ tendency to produce two strong distinct intertropical convergence zones (ITCZs) too often. Several mechanisms have been proposed that may contribute to the emergence of two ITCZs, but current theories cannot fully explain the bias. This problem is tackled by investigating how the interaction between atmospheric cloud-radiative effects (ACREs) and the large-scale circulation influences the ITCZ position in an atmospheric general circulation model. Simulations are performed in an idealized aquaplanet setup and the longwave and shortwave ACREs are turned off individually or jointly. The low-level moist static energy (MSE) is shown to be a good predictor of the ITCZ position. Therefore, a mechanism is proposed that explains the changes in MSE and thus ITCZ position due to ACREs consistently across simulations. The mechanism implies that the ITCZ moves equatorward if the Hadley circulation strengthens because of the increased upgradient advection of low-level MSE off the equator. The longwave ACRE increases the meridional heating gradient in the tropics and as a response the Hadley circulation strengthens and the ITCZ moves equatorward. The shortwave ACRE has the opposite effect. The total ACRE pulls the ITCZ equatorward. This mechanism is discussed in other frameworks involving convective available potential energy, gross moist stability, and the energy flux equator. It is thus shown that the response of the large-scale circulation to the shortwave and longwave ACREs is a fundamental driver of changes in the ITCZ position.

The tropical atmospheric conveyor belt: a Lagrangian perspective of the large-scale tropical circulation

2020 ◽  
Author(s):  
Dana Raiter ◽  
Eli Galanti ◽  
Yohai Kaspi
Keyword(s):  
Large Scale ◽  
Meridional Circulation ◽  
Hadley Circulation ◽  
Warm Pool ◽  
Conveyor Belt ◽  
Jet Stream ◽  
Tropical Circulation ◽  
Pacific Warm Pool ◽  
Actual Movement ◽  
The Tropics

<div> <div>The Hadley circulation (HC) is a key element of the climate system. It is traditionally defined as the zonally averaged meridional circulation in the tropics, therefore treated as a zonally symmetric phenomenon. However, differences in temperature between land and sea cause zonal asymmetries on Earth, dramatically affecting the circulation. The longitudinal dependence of the HC evokes questions about where and when the actual large scale tropical circulation occurs. In this study, we look into the connection between the longitudinally dependent HC and the actual large scale movement of air in the tropics using a coupled Eulerian and Lagrangian approach. Decomposing the velocity field, we identify the components affecting the actual circulation. In addition, we calculate trajectories of air parcels to analyze the actual movement. We propose an alternative definition for the circulation, that describes the actual path of air parcels in the tropics, as a tropical conveyor belt. The Indo-Pacific warm pool is the driver of the circulation, where air converges and ascends, then moves westward and poleward before entering the jet stream, moving eastward with it, eventually beginning its descent near the Americas. Furthermore, using an idealized moist GCM, we explore how tropical asymmetries affect the circulation and discuss the possible mechanisms controlling the tropical conveyor belt.</div> </div>

Convectively Coupled Equatorial Wave Simulations Using the ECMWF IFS and the NOAA GFS Cumulus Convection Schemes in the NOAA GFS Model

2019 ◽  
Vol 147 (11) ◽  
pp. 4005-4025 ◽  
Author(s):  
Lisa Bengtsson ◽  
Juliana Dias ◽  
Maria Gehne ◽  
Peter Bechtold ◽  
Jeffrey Whitaker ◽  
...  
Keyword(s):  
Large Scale ◽  
Cumulus Convection ◽  
Convection Scheme ◽  
Equatorial Wave ◽  
Gfs Model ◽  
Convection Schemes ◽  
Tropical Wave ◽  
Consistent Treatment ◽  
The Tropics ◽  
Moisture Profiles

Abstract There is a longstanding challenge in numerical weather and climate prediction to accurately model tropical wave variability, including convectively coupled equatorial waves (CCEWs) and the Madden–Julian oscillation. For subseasonal prediction, the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) has been shown to be superior to the NOAA Global Forecast System (GFS) in simulating tropical variability, suggesting that the ECMWF model is better at simulating the interaction between cumulus convection and the large-scale tropical circulation. In this study, we experiment with the cumulus convection scheme of the ECMWF IFS in a research version of the GFS to understand which aspects of the IFS cumulus convection scheme outperform those of the GFS convection scheme in the tropics. We show that the IFS cumulus convection scheme produces significantly different tropical moisture and temperature tendency profiles from those simulated by the GFS convection scheme when it is coupled with other physics schemes in the GFS physics package. We show that a consistent treatment of the interaction between parameterized convective plumes in the GFS planetary boundary layer (PBL) and the IFS convection scheme is required for the GFS to replicate the tropical temperature and moisture profiles simulated by the IFS model. The GFS model with the IFS convection scheme, and the consistent treatment between the convection and PBL schemes, produces much more organized convection in the tropics, and generates tropical waves that propagate more coherently than the GFS in its default configuration due to better simulated interaction between low-level convergence and precipitation.

scholarly journals Analysis of the circulation and shelf-slope exchanges in the continental margin of the northwestern Mediterranean

Ocean Science ◽  
2006 ◽  
Vol 2 (2) ◽  
pp. 173-181 ◽  
Author(s):  
A. Jordi ◽  
G. Basterretxea ◽  
A. Orfila ◽  
J. Tintoré
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Horizontal Resolution ◽  
Ocean Model ◽  
Scale Model ◽  
Open Boundaries ◽  
Shelf Slope ◽  
Large Scale Model ◽  
High Horizontal Resolution ◽  
Northwestern Mediterranean

Abstract. In this paper, we present the results from a high horizontal resolution numerical simulation of the northwestern Mediterranean using a z-level, non-hydrostatic, primitive equation ocean model (DieCAST). The high resolution allows an accurate representation of the submarine canyons that presides in the region. The model is one-way coupled to a large scale model of the Mediterranean Sea through open boundaries and uses the atmospheric forcing fields provided in terms of HIRLAM outputs by the Spanish National Institute of Meteorology. Results show that the model can successfully reproduce the complex general circulation characteristics of the area, including the modifications induced by canyons in their vicinity and other phenomena observed such as instabilities and coastal trapped waves. The sea surface temperature is similar to satellite observations except that simulated temperatures are slightly warmer near the coast than observations and colder near the open boundaries. An important topic of this work is the computation of the shelf-slope exchanges, which are able to renew shelf waters in a few months.

scholarly journals Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model?

2019 ◽  
Vol 49 (5) ◽  
pp. 1141-1157 ◽  
Author(s):  
Patrick Wagner ◽  
Siren Rühs ◽  
Franziska U. Schwarzkopf ◽  
Inga Monika Koszalka ◽  
Arne Biastoch
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Eddy Diffusivity ◽  
Ocean General Circulation Model ◽  
Horizontal Resolution ◽  
Ocean Model ◽  
Time Step ◽  
Lagrangian Particles ◽  
Cape Basin

AbstractTo model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin.

scholarly journals An Explanation for the Sensitivity of the Mean State of the Community Atmosphere Model to Horizontal Resolution on Aquaplanets

2017 ◽  
Vol 30 (13) ◽  
pp. 4781-4797 ◽  
Author(s):  
Adam R. Herrington ◽  
Kevin A. Reed
Keyword(s):  
Moisture Transport ◽  
General Circulation ◽  
Large Scale ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Hadley Cell ◽  
Community Atmosphere Model ◽  
The Mean ◽  
Atmosphere Model ◽  
Mean State

The sensitivity of the mean state of the Community Atmosphere Model to horizontal resolutions typical of present-day general circulation models is investigated in an aquaplanet configuration. Nonconvergence of the mean state is characterized by a progressive drying of the atmosphere and large reductions in cloud coverage with increasing resolution. Analyses of energy and moisture budgets indicate that these trends are balanced by variations in moisture transport by the resolved circulation, and a reduction in activity of the convection scheme. In contrast, the large-scale precipitation rate increases with resolution, which is approximately balanced by greater advection of dry static energy associated with more active resolved vertical motion in the ascent region of the Hadley cell. An explanation for the sensitivity of the mean state to horizontal resolution is proposed, based on linear Boussinesq theory. The authors hypothesize that an increase in horizontal resolution in the model leads to a reduction in horizontal scale of the diabatic forcing arising from the column physics, facilitating finescale flow and faster resolved convective updrafts within the dynamical core, and steering the coupled system toward a new mean state. This hypothesis attempts to explain the underlying mechanism driving the variations in moisture transport observed in the simulations.

Simulation of the Diurnal Cycle in Tropical Rainfall and Circulation during Boreal Summer with a High-Resolution GCM

2010 ◽  
Vol 138 (9) ◽  
pp. 3434-3453 ◽  
Author(s):  
Jeffrey J. Ploshay ◽  
Ngar-Cheung Lau
Keyword(s):  
Diurnal Cycle ◽  
General Circulation ◽  
Large Scale ◽  
Surface Wind ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
Boreal Summer ◽  
Tropical Rainfall ◽  
Early Afternoon ◽  
Seaward Migration

Abstract The simulation of the diurnal cycle (DC) of precipitation and surface wind pattern by a general circulation model (GCM) with a uniform horizontal resolution of 50 km over the global domain is evaluated. The model output is compared with observational counterparts based on datasets provided by the Tropical Rainfall Measuring Mission and reanalysis products of the European Centre for Medium-Range Weather Forecasts. The summertime diurnal characteristics over tropical regions in Asia, the Americas, and Africa are portrayed using the amplitude and phase of the first harmonic of the 24-h cycle, departures of data fields during selected hours from the daily mean, and differences between extreme phases of the DC. There is general agreement between the model and observations with respect to the large-scale land–sea contrasts in the DC. Maximum land precipitation, onshore flows, and landward migration of rainfall signals from the coasts occur in the afternoon, whereas peak maritime rainfall and offshore flows prevail in the morning. Seaward migration of precipitation is discernible over the western Bay of Bengal and South China Sea during nocturnal and morning hours. The evolution from low-intensity rainfall in the morning/early afternoon to heavier precipitation several hours later is also evident over selected continental sites. However, the observed incidence of rainfall with very high intensity in midafternoon is not reproduced in the model atmosphere. Although the model provides an adequate simulation of the daytime upslope and nighttime downslope winds in the vicinity of mountain ranges, valleys, and basins, there are notable discrepancies between model and observations in the DC of precipitation near some of these orographic features. The model does not reproduce the observed seaward migration of precipitation from the western coasts of Myanmar (Burma) and India, and from individual islands of the Indonesian Archipelago at nighttime.

scholarly journals Simulation of winter and summer climates with PRL Atmospheric General Circulation Model

MAUSAM ◽  
2021 ◽  
Vol 50 (4) ◽  
pp. 391-400
Author(s):  
BIJU THOMAS ◽  
S.V. KASTURE ◽  
S. V. SATYAN
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Large Scale ◽  
Atmospheric General Circulation Model ◽  
Initial Conditions ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
Hemispheric Differences ◽  
Physical Research Laboratory ◽  
Atmospheric General Circulation

A global, spectral Atmospheric General Circulation Model (AGCM) has been developed indigenously at Physical Research Laboratory (PRL) for climate studies. The model has six a levels in the vertical and has horizontal resolution of 21 waves with rhomboidal truncation. The model includes smooth topography, planetary boundary layer, deep convection, large scale condensation, interactive hydrology, radiation with interactive clouds and diurnal cycle. Sea surface temperature and sea ice values were fixed based on climatological data for different calender months.   The model was integrated for six years starting with an isothermal atmosphere (2400K), zero winds initial conditions and forcing from incoming solar radiation. After one year the model stabilizes. The seasonal averages of various fields of the last five years are discussed in this paper. It is found that the model reproduces reasonably well the seasonal features of atmospheric circulation, seasonal variability and hemispheric differences.

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