Upscale Impact of Mesoscale Convective Systems and Its Parameterization in an Idealized GCM for an MJO Analog above the Equator

2019 ◽  
Vol 76 (3) ◽  
pp. 865-892 ◽  
Author(s):  
Qiu Yang ◽  
Andrew J. Majda ◽  
Mitchell W. Moncrieff
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Interaction Mechanism ◽  
Global Climate Models ◽  
Mesoscale Convective Systems ◽  
Vertical Shear ◽  
Collective Effects ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Westerly Wind Burst

Abstract The Madden–Julian oscillation (MJO) typically contains several superclusters and numerous embedded mesoscale convective systems (MCSs). It is hypothesized here that the poorly simulated MJOs in current coarse-resolution global climate models (GCMs) is related to the inadequate treatment of unresolved MCSs. So its parameterization should provide the missing collective effects of MCSs. However, a satisfactory understanding of the upscale impact of MCSs on the MJO is still lacking. A simple two-dimensional multicloud model is used as an idealized GCM with clear deficiencies. Eddy transfer of momentum and temperature by the MCSs, predicted by the mesoscale equatorial synoptic dynamics (MESD) model, is added to this idealized GCM. The upscale impact of westward-moving MCSs promotes eastward propagation of the MJO analog, consistent with the theoretical prediction of the MESD model. Furthermore, the upscale impact of upshear-moving MCSs significantly intensifies the westerly wind burst because of two-way feedback between easterly vertical shear and eddy momentum transfer with low-level eastward momentum forcing. Finally, a basic parameterization of the upscale impact of upshear-moving MCSs modulated by deep heating excess and vertical shear strength significantly improves key features of the MJO analog in the idealized GCM with clear deficiencies. A three-way interaction mechanism between the MJO analog, parameterized upscale impact of MCSs, and background vertical shear is identified.

scholarly journals A 17 year climatology of the macrophysical properties of convection in Darwin

2018 ◽  
Vol 18 (23) ◽  
pp. 17687-17704 ◽  
Author(s):  
Robert C. Jackson ◽  
Scott M. Collis ◽  
Valentin Louf ◽  
Alain Protat ◽  
Leon Majewski
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Sea Breeze ◽  
Active Phase ◽  
Global Climate Models ◽  
Convective System ◽  
Wet Season ◽  
Australian Monsoon ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract. The validation of convective processes in global climate models (GCMs) could benefit from the use of large datasets that provide long-term climatologies of the spatial statistics of convection. To that regard, echo top heights (ETHs), convective areas, and frequencies of mesoscale convective systems (MCSs) from 17 years of data from a C-band polarization (CPOL) radar are analyzed in varying phases of the Madden–Julian Oscillation (MJO) and northern Australian monsoon in order to provide ample validation statistics for GCM validation. The ETHs calculated using velocity texture and reflectivity provide similar results, showing that the ETHs are insensitive to various techniques that can be used. Retrieved ETHs are correlated with those from cloud top heights retrieved by Multifunctional Transport Satellites (MTSATs), showing that the ETHs capture the relative variability in cloud top heights over seasonal scales. Bimodal distributions of ETH, likely attributable to the cumulus congestus clouds and mature stages of convection, are more commonly observed when the active phase of the MJO is over Australia due to greater mid-level moisture during the active phase of the MJO. The presence of a convectively stable layer at around 5 km altitude over Darwin inhibiting convection past this level can explain the position of the modes at around 2–4 km and 7–9 km. Larger cells were observed during break conditions compared to monsoon conditions, but only during the inactive phase of the MJO. The spatial distributions show that Hector, a deep convective system that occurs almost daily during the wet season over the Tiwi Islands, and sea-breeze convergence lines are likely more common in break conditions. Oceanic MCSs are more common during the night over Darwin. Convective areas were generally smaller and MCSs more frequent during active monsoon conditions. In general, the MJO is a greater control on the ETHs in the deep convective mode observed over Darwin, with higher distributions of ETH when the MJO is active over Darwin.

scholarly journals Understanding Intermodel Variability in Future Projections of a Sahelian Storm Proxy and Southern Saharan Warming

2021 ◽  
Vol 34 (2) ◽  
pp. 509-525
Author(s):  
David P. Rowell ◽  
Rory G. J. Fitzpatrick ◽  
Lawrence S. Jackson ◽  
Grace Redmond
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Global Climate ◽  
Vertical Wind Shear ◽  
Global Climate Models ◽  
Tropospheric Temperature ◽  
Storm Activity ◽  
Convective Systems ◽  
The North ◽  
Projected Changes

AbstractProjected changes in the intensity of severe rain events over the North African Sahel—falling from large mesoscale convective systems—cannot be directly assessed from global climate models due to their inadequate resolution and parameterization of convection. Instead, the large-scale atmospheric drivers of these storms must be analyzed. Here we study changes in meridional lower-tropospheric temperature gradient across the Sahel (ΔTGrad), which affect storm development via zonal vertical wind shear and Saharan air layer characteristics. Projected changes in ΔTGrad vary substantially among models, adversely affecting planning decisions that need to be resilient to adverse risks, such as increased flooding. This study seeks to understand the causes of these projection uncertainties and finds three key drivers. The first is intermodel variability in remote warming, which has strongest impact on the eastern Sahel, decaying toward the west. Second, and most important, a warming–advection–circulation feedback in a narrow band along the southern Sahara varies in strength between models. Third, variations in southern Saharan evaporative anomalies weakly affect ΔTGrad, although for an outlier model these are sufficiently substantive to reduce warming here to below that of the global mean. Together these uncertain mechanisms lead to uncertain southern Saharan/northern Sahelian warming, causing the bulk of large intermodel variations in ΔTGrad. In the southern Sahel, a local negative feedback limits the contribution to uncertainties in ΔTGrad. This new knowledge of ΔTGrad projection uncertainties provides understanding that can be used, in combination with further research, to constrain projections of severe Sahelian storm activity.

Evaluation of Organized Convection Parameterization in Global Climate Models

2020 ◽  
Author(s):  
Chih-Chieh Chen ◽  
Changhai Liu ◽  
Mitch Moncrieff ◽  
Yaga Richter
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Earth System Model ◽  
Global Climate Models ◽  
System Model ◽  
Moist Convection ◽  
Earth System ◽  
Mesoscale Convective ◽  
Convective Organization ◽  
Organized Convection

<p>The importance of convective organization on the global circulation has been recognized for a long time, but parameterizations of the associated processes are missing in global climate models. Contemporary convective parameterizations commonly use a convective plume model (or a spectrum of plumes). This is perhaps appropriate for unorganized convection but the assumption of a gap between the small cumulus scale and the large-scale motion fails to recognize mesoscale dynamics manifested in mesoscale convective systems (MCSs) and multi-scale cloud systems associated with the MJO. Organized convection is abundant in environments featuring vertical wind shear, and significantly modulates the life cycle of moist convection, the transport of heat and momentum, and accounts for a large percentage of precipitation in the tropics. Mesoscale convective organization is typically associated with counter-gradient momentum transport, and distinct heating profiles between the convective and stratiform regions.</p><p>Moncrieff, Liu and Bogenschutz (2017) recently developed a dynamical based parameterization of organized moisture convection, referred to as multiscale coherent structure parameterization (MCSP), for global climate models. A prototype version of MCSP has been implemented in the NCAR Community Earth System Model (CESM) and the Energy Exascale Earth System Model (E3SM), positively affecting the distribution of tropical precipitation, convectively coupled tropical waves, and the Madden-Julian oscillation. We will show the further development of the MCSP and its impact on the simulation of mean precipitation and variability in the two global climate models.</p>

Simulation, Modeling, and Dynamically Based Parameterization of Organized Tropical Convection for Global Climate Models

2017 ◽  
Vol 74 (5) ◽  
pp. 1363-1380 ◽  
Author(s):  
Mitchell W. Moncrieff ◽  
Changhai Liu ◽  
Peter Bogenschutz
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Global Climate ◽  
Tropical Convection ◽  
Global Climate Models ◽  
Momentum Transport ◽  
Vertical Shear ◽  
Convergence Zone ◽  
Convective Momentum Transport ◽  
Organized Convection

Abstract A new approach for treating organized convection in global climate models (GCMs) referred to as multiscale coherent structure parameterization (MCSP) introduces physical and dynamical effects of organized convection that are missing from contemporary parameterizations. The effects of vertical shear are approximated by a nonlinear slantwise overturning model based on Lagrangian conservation principles. Simulation of the April 2009 Madden–Julian oscillation event during the Year of Tropical Convection (YOTC) over the Indian Ocean using the Weather Research and Forecasting (WRF) Model at 1.3-km grid spacing identifies self-similar properties for squall lines, MCSs, and superclusters embedded in equatorial waves. The slantwise overturning model approximates this observed self-similarity. The large-scale effects of MCSP are examined in two categories of GCM. First, large-scale convective systems simulated in an aquaplanet model are approximated by slantwise overturning with attention to convective momentum transport. Second, MCSP is utilized in the Community Atmosphere Model, version 5.5 (CAM5.5), as tendency equations for second-baroclinic heating and convective momentum transport. The difference between MCSP and CAM5.5 is a direct measure of the global effects of organized convection. Consistent with TRMM measurements, the MCSP generates large-scale precipitation patterns in the tropical warm pool and the adjoining locale; improves precipitation in the intertropical convergence zone (ITCZ), South Pacific convergence zone (SPCZ), and Maritime Continent regions; and affects tropical wave modes. In conclusion, the treatment of organized convection by MCSP is salient for the next generation of GCMs.

scholarly journals 100 Years of Research on Mesoscale Convective Systems

2018 ◽  
Vol 59 ◽  
pp. 17.1-17.54 ◽  
Author(s):  
Robert A. Houze
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Lower Troposphere ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Time Space ◽  
Weather Events ◽  
Mesoscale Convective ◽  
Cumulonimbus Clouds ◽  
Insight Into

Abstract When cumulonimbus clouds aggregate, developing into a single entity with precipitation covering a horizontal scale of hundreds of kilometers, they are called mesoscale convective systems (MCSs). They account for much of Earth’s precipitation, generate severe weather events and flooding, produce prodigious cirriform anvil clouds, and affect the evolution of the larger-scale circulation. Understanding the inner workings of MCSs has resulted from developments in observational technology and modeling. Time–space conversion of ordinary surface and upper-air observations provided early insight into MCSs, but deeper understanding has followed field campaigns using increasingly sophisticated radars, better aircraft instrumentation, and an ever-widening range of satellite instruments, especially satellite-borne radars. High-resolution modeling and theoretical insights have shown that aggregated cumulonimbus clouds induce a mesoscale circulation consisting of air overturning on a scale larger than the scale of individual convective up- and downdrafts. These layers can be kilometers deep and decoupled from the boundary layer in elevated MCSs. Cooling in the lower troposphere and heating aloft characterize the stratiform regions of MCSs. As a result, long-lived MCSs with large stratiform regions have a top-heavy heating profile that generates potential vorticity in midlevels, thus influencing the larger-scale circulation within which the MCSs occur. Global satellite data show MCSs varying in structure, depending on the prevailing large-scale circulation and topography. These patterns are likely to change with global warming. In addition, environmental pollution affects MCS structure and dynamics subtly. Feedbacks of MCSs therefore need to be included or parameterized in climate models.

scholarly journals Robust Observational Quantification of the Contribution of Mesoscale Convective Systems to Rainfall in the Tropics

2014 ◽  
Vol 27 (13) ◽  
pp. 4952-4958 ◽  
Author(s):  
R. Roca ◽  
J. Aublanc ◽  
P. Chambon ◽  
T. Fiolleau ◽  
N. Viltard
Keyword(s):  
Water Budget ◽  
Climate Models ◽  
Mesoscale Convective Systems ◽  
Future Evolution ◽  
Convective Systems ◽  
Tropical Water ◽  
Regional Features ◽  
Mesoscale Convective ◽  
The Tropics ◽  
Selection Of

Satellite estimation of precipitation and satellite-derived statistics of mesoscale convective systems (MCS) are analyzed conjunctively to quantify the contribution of the various types of MCS to the water budget of the tropics. This study focuses on two main mesoscale characteristics of the systems: duration and propagation. Overall, the systems lasting more than 12 h are shown to account for around 75% of the tropical rainfall, and 60% of the rainfall is due to systems traveling more than 250 km, a typical GCM grid. A number of regional features are also revealed by factoring in the convective systems’ morphological parameters in the water budget computation. These findings support the challenging effort to account for such mesoscale features when considering the theory on the future evolution of the water budget as well as the physical parameterizations of climate models. Finally, this analysis provides a simple metric for evaluating high-resolution numerical simulations of the tropical water budget. Furthermore, results are shown to be robust to the selection of the satellite rainfall products.

Long-lived mesoscale convective systems over eastern South Africa

2021 ◽  
pp. 1-66
Author(s):  
D.M. Morake ◽  
R. C. Blamey ◽  
C.J.C. Reason
Keyword(s):  
South Africa ◽  
Seasonal Cycle ◽  
Southern Annular Mode ◽  
Early Summer ◽  
Mesoscale Convective Systems ◽  
Lower Atmosphere ◽  
Vertical Shear ◽  
Convective Systems ◽  
Early Afternoon ◽  
Mesoscale Convective

AbstractA climatology of large, long-lived mesoscale convective systems (MCSs) over eastern South Africa for the extended austral summer (September-April) from 1985-2008 is presented. On average, 63 MCSs occur here in summer, but with considerable interannual variability in frequency. The systems mainly occur between November and March, with a December peak. This seasonal cycle in MCS activity is shown to coincide with favorable CAPE and vertical shear profiles across the domain. Most systems tend to occur along the eastern escarpment and adjacent warm waters of the northern Agulhas Current with a nocturnal life cycle. Typically, initiation begins in the early afternoon, MCS status is reached mid-afternoon, maximum extent early in the night and termination around midnight or shortly thereafter. It is found that most MCSs initiate over land, but systems that initiate over the ocean tend to last longer than those that develop over land. The results also show that there are differences in the seasonal cycle between continental and oceanic MCSs, with oceanic systems containing two intraseasonal peaks (December and April). There is a relatively strong positive relationship between the Southern Annular Mode (SAM) and early summer MCSs frequency. For the late summer, the frequency of MCSs appears related to the strength of the Mascarene High and Mozambique Channel Trough which modulate the inflow of moisture into eastern South Africa and the stability of the lower atmosphere over the region.

Modeling of Mesoscale-Convective Systems Downstream of Mountain Regions

2021 ◽  
Author(s):  
Andreas F. Prein
Keyword(s):  
Climate Models ◽  
Flash Flood ◽  
Climate Change Impacts ◽  
Mesoscale Convective Systems ◽  
The Tibetan Plateau ◽  
Mountain Regions ◽  
Convective Initiation ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Model Setup

<p>Mesoscale-Convective Systems (MCSs) are prolific rain-producers and are responsible for most flash flood events in mid-latitudes. Global hotspots of MCS occurrence are downstream of major mountain regions such as the Rocky Mountains, the Andes, and the Himalayas. This is because of the effects of mountain barriers on circulation patterns, moisture transport, and convective initiation. Realistically simulating MCSs in climate models is essential for representing the water and energy cycle and flood and severe convective weather assessments. However, state-of-the-art climate models have substantial biases in simulating MCSs and orographic impacts on downstream environments resulting in large uncertainties and errors in assessing climate change impacts on water availability and extreme events. Here we present that kilometer-scale models, which have an improved representation of orography and can represent deep convective processes explicitly, show a step improvement in simulating organized convective storms compared to coarser-resolution models. We will show examples of these improvements from kilometer-scale simulations over the Tibetan Plateau, North- and South America. We will also show sensitivities to the model setup and feedback processes and end with discussing remaining challenges and future prospects.</p>

Evaluating Diurnal and Semi-Diurnal Cycle of Precipitation in CMIP6 Models Using Satellite- and Ground-Based Observations

2021 ◽  
pp. 1-56
Author(s):  
Shuaiqi Tang ◽  
Peter Gleckler ◽  
Shaocheng Xie ◽  
Jiwoo Lee ◽  
Min-Seop Ahn ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Climate Models ◽  
Early Morning ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Ensemble Mean ◽  
Mesoscale Convective ◽  
Diurnal Cycle Of Precipitation

AbstractThe diurnal and semi-diurnal cycle of precipitation simulated from CMIP6 models during 1996-2005 are evaluated globally between 60°S and 60°N, as well as at ten selected locations representing three categories of diurnal cycle of precipitation: (1) afternoon precipitation over land, (2) early morning precipitation over ocean, and (3) nocturnal precipitation over land. Three satellite-based and two ground-based rainfall products are used to evaluate the climate models. Globally, the ensemble mean of CMIP6 models shows a diurnal phase of 3 to 4 hours earlier over land and 1 to 2 hours earlier over ocean, when compared with the latest satellite products. These biases are in line with what were found in previous versions of climate models but reduced compared to the CMIP5 ensemble mean. Analysis at the selected locations complimented with in-situ measurements further reinforces these results. Several CMIP6 models have shown a significant improvement in the diurnal cycle of precipitation compared to their CMIP5 counterparts, notably on delaying afternoon precipitation over land. This can be attributed to the use of more sophisticated convective parameterizations. Most models are still unable to capture the nocturnal peak associated with elevated convection and propagating mesoscale convective systems, with a few exceptions that allow convection to be initiated above the boundary layer to capture nocturnal elevated convection. We also quantify an encouraging consistency between the satellite- and ground-based precipitation measurements despite differing spatiotemporal resolutions and sampling periods, which provides confidence in using them to evaluate the diurnal and semi-diurnal cycle of precipitation in climate models.

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