Shallow Meridional Circulations in the Tropical Atmosphere

2008 ◽  
Vol 21 (14) ◽  
pp. 3453-3470 ◽  
Author(s):  
Chidong Zhang ◽  
David S. Nolan ◽  
Christopher D. Thorncroft ◽  
Hanh Nguyen
Keyword(s):  
Large Scale ◽  
Meridional Circulation ◽  
Indian Subcontinent ◽  
Meridional Flow ◽  
Trade Wind ◽  
Seasonal Cycles ◽  
Tropical Atmosphere ◽  
Moist Convection ◽  
Meridional Overturning ◽  
Deep Moist Convection

Abstract A shallow meridional circulation (SMC) in the tropical atmosphere features a low-level (e.g., 700 hPa) flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above. Representations of the SMC in three global reanalyses show both similarities and astonishing discrepancies. While the SMC over West Africa appears to be the strongest, it also exists over the eastern Atlantic and eastern Pacific Oceans, and over the Indian subcontinent, with different strength and structure. All SMCs undergo marked seasonal cycles. The SMCs are summarized into two types: one associated with the marine ITCZ and the other with the summer monsoon. The large-scale conditions for these two types of SMCs are similar: a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection. The processes responsible for these conditions are different for the two types of SMCs, as are their structures relative to moist convection, associated precipitation, and deep meridional overturning circulations. It is suggested that discrepancies among the representations of the SMC in the three global reanalyses stem from different treatment of physical parameterizations, especially for cumulus convection, in the models used for the data assimilation.

scholarly journals Improvements in the Subgrid-Scale Representation of Moist Convection in a Cumulus Parameterization Scheme: The Single-Column Test and Its Impact on Seasonal Prediction

2007 ◽  
Vol 135 (6) ◽  
pp. 2135-2154 ◽  
Author(s):  
Young-Hwa Byun ◽  
Song-You Hong
Keyword(s):  
Large Scale ◽  
Meridional Circulation ◽  
Random Selection ◽  
The Other ◽  
Moist Convection ◽  
Subgrid Scale ◽  
Convection Scheme ◽  
Single Column ◽  
Convective Momentum Transport ◽  
Selection Of

Abstract This study describes a revised approach for the subgrid-scale convective properties of a moist convection scheme in a global model and evaluates its effects on a simulated model climate. The subgrid-scale convective processes tested in this study comprise three components: 1) the random selection of cloud top, 2) the inclusion of convective momentum transport, and 3) a revised large-scale destabilization effect considering synoptic-scale forcing in the cumulus convection scheme of the National Centers for Environmental Prediction medium-range forecast model. Each component in the scheme has been evaluated within a single-column model (SCM) framework forced by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment data. The impact of the changes in the scheme on seasonal predictions has been examined for the boreal summers of 1996, 1997, and 1999. In the SCM simulations, an experiment that includes all the modifications reproduces the typical convective heating and drying feature. The simulated surface rainfall is in good agreement with the observed precipitation. Random selection of the cloud top effectively moistens and cools the upper troposphere, and it induces drying and warming below the cloud-top level due to the cloud–radiation feedback. However, the two other components in the revised scheme do not play a significant role in the SCM simulations. On the other hand, the role of each modification component in the scheme is significant in the ensemble seasonal simulations. The random selection process of the cloud top preferentially plays an important role in the adjustment of the thermodynamic profile in a manner similar to that in the SCM framework. The inclusion of convective momentum transport in the scheme weakens the meridional circulation. The revised large-scale destabilization process plays an important role in the modulation of the meridional circulation when this process is combined with other processes; on the other hand, this process does not induce significant changes in large-scale fields by itself. Consequently, the experiment that involves all the modifications shows a significant improvement in the seasonal precipitation, thereby highlighting the importance of nonlinear interaction between the physical processes in the model and the simulated climate.

Diurnal Radiation Cycle Impact on the Pregenesis Environment of Hurricane Karl (2010)

2014 ◽  
Vol 71 (4) ◽  
pp. 1241-1259 ◽  
Author(s):  
Christopher Melhauser ◽  
Fuqing Zhang
Keyword(s):  
Tropical Cyclones ◽  
Large Scale ◽  
Environmental Stability ◽  
Shortwave Radiation ◽  
Radiative Cooling ◽  
Moist Convection ◽  
Heating And Cooling ◽  
Highly Sensitive ◽  
Deep Moist Convection ◽  
The Impact

Abstract Through convection-permitting ensemble and sensitivity experiments, this study examines the impact of the diurnal radiation cycle on the pregenesis environment of Hurricane Karl (2010). It is found that the pregenesis environmental stability and the intensity of deep moist convection can be considerably modulated by the diurnal extremes in radiation. Nighttime destabilization of the local and large-scale environment through radiative cooling may promote deep moist convection and increase the genesis potential, likely enhancing the intensity of the resultant tropical cyclones. Modified longwave and shortwave radiation experiments found tropical cyclone development to be highly sensitive to the periodic cycle of heating and cooling, with suppressed formation in the daytime-only and no-radiation experiments and quicker intensification compared with the control for nighttime-only experiments.

Influence of Equatorial Waves on the Genesis of Super Typhoon Haiyan (2013)

2015 ◽  
Vol 72 (12) ◽  
pp. 4591-4613 ◽  
Author(s):  
Shoujuan Shu ◽  
Fuqing Zhang
Keyword(s):  
Environmental Conditions ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Sensitivity Analyses ◽  
Tropical Region ◽  
Moist Convection ◽  
Super Typhoon ◽  
Equatorial Wave ◽  
Deep Moist Convection ◽  
Cyclonic Disturbance

Abstract The influence of equatorial wave disturbances on the genesis of Super Typhoon Haiyan (2013) is investigated through spectral, composite, and ensemble sensitivity analysis of various observational datasets in combination with predictions from an operational ensemble. Under the favorable large-scale environmental conditions of the Asian monsoon combined with the Madden–Julian oscillation (MJO), the incipient Haiyan develops from a cyclonic disturbance that originates from a train of westward-propagating mixed Rossby–gravity (MRG) waves. Haiyan eventually develops in the monsoon trough region at the leading edge of the moist MJO phase that has strong low-level convergence, high moisture content, and weak shear, along with high sea surface temperature. These favorable environmental conditions promote the intensification of deep moist convection that facilitates the development of the cyclonic disturbance from an MRG wave into a tropical depression, which later intensifies rapidly into Super Typhoon Haiyan, one of the world’s strongest and most destructive tropical storms ever recorded. Results from ensemble sensitivity analyses are consistent with this finding and further show that the uncertainties in tropical waves and their interactions can impact the large-scale environment surrounding Haiyan’s precursor and therefore limit the predictability of tropical cyclone formation and intensity. The better-performing members tend to have a stronger initial MRG wave disturbance, which provides a stronger initial seed for the later development of the storm, as well as a stronger moist MJO wave in the tropical region, which not only promotes deep convection near the precursor location, but also reduces the environmental vertical wind shear by strengthening the tropical westerlies.

scholarly journals The behavior of high-CAPE (convective available potential energy) summer convection in large-domain large-eddy simulations with ICON

2021 ◽  
Vol 21 (6) ◽  
pp. 4285-4318
Author(s):  
Harald Rybka ◽  
Ulrike Burkhardt ◽  
Martin Köhler ◽  
Ioanna Arka ◽  
Luca Bugliaro ◽  
...  
Keyword(s):  
Large Scale ◽  
Data Sets ◽  
Motion Field ◽  
Moist Convection ◽  
Water Path ◽  
Ice Water Path ◽  
Large Eddy ◽  
Deep Moist Convection ◽  
Cloud Ice ◽  
Ice Water

Abstract. Current state-of-the-art regional numerical weather prediction (NWP) models employ kilometer-scale horizontal grid resolutions, thereby simulating convection within the grey zone. Increasing resolution leads to resolving the 3D motion field and has been shown to improve the representation of clouds and precipitation. Using a hectometer-scale model in forecasting mode on a large domain therefore offers a chance to study processes that require the simulation of the 3D motion field at small horizontal scales, such as deep summertime moist convection, a notorious problem in NWP. We use the ICOsahedral Nonhydrostatic weather and climate model in large-eddy simulation mode (ICON-LEM) to simulate deep moist convection and distinguish between scattered, large-scale dynamically forced, and frontal convection. We use different ground- and satellite-based observational data sets, which supply information on ice water content and path, ice cloud cover, and cloud-top height on a similar scale as the simulations, in order to evaluate and constrain our model simulations. We find that the timing and geometric extent of the convectively generated cloud shield agree well with observations, while the lifetime of the convective anvil was, at least in one case, significantly overestimated. Given the large uncertainties of individual ice water path observations, we use a suite of observations in order to better constrain the simulations. ICON-LEM simulates a cloud ice water path that lies between the different observational data sets, but simulations appear to be biased towards a large frozen water path (all frozen hydrometeors). Modifications of parameters within the microphysical scheme have little effect on the bias in the frozen water path and the longevity of the anvil. In particular, one of our convective days appeared to be very sensitive to the initial and boundary conditions, which had a large impact on the convective triggering but little impact on the high frozen water path and long anvil lifetime bias. Based on this limited set of sensitivity experiments, the evolution of locally forced convection appears to depend more on the uncertainty of the large-scale dynamical state based on data assimilation than of microphysical parameters. Overall, we judge ICON-LEM simulations of deep moist convection to be very close to observations regarding the timing, geometrical structure, and cloud ice water path of the convective anvil, but other frozen hydrometeors, in particular graupel, are likely overestimated. Therefore, ICON-LEM supplies important information for weather forecasting and forms a good basis for parameterization development based on physical processes or machine learning.

scholarly journals The behavior of high-CAPE summer convection in large-domain large-eddy simulations with ICON

2020 ◽  
Author(s):  
Harald Rybka ◽  
Ulrike Burkhardt ◽  
Martin Köhler ◽  
Ioanna Arka ◽  
Luca Bugliaro ◽  
...  
Keyword(s):  
Large Scale ◽  
Life Time ◽  
Data Sets ◽  
Motion Field ◽  
Moist Convection ◽  
Water Path ◽  
Ice Water Path ◽  
Deep Moist Convection ◽  
Cloud Ice ◽  
Ice Water

Abstract. Current state of the art regional numerical weather prediction (NWP) models employ kilometre scale horizontal grid resolutions thereby simulating convection within its grey-zone. Increasing resolution leads to resolving the 3D motion field and has been shown to improve the representation of clouds and precipitation. Using a hectometer-scale model in forecasting mode on a large domain therefore offers a chance to study processes that require the simulation of the 3D motion field at small horizontal scales, such as deep summertime moist convection, a notorious problem in NWP. We use the Icosahedral Nonhydrostatic weather and climate model in large-eddy simulation mode (ICON-LEM) to simulate deep moist convection distinguishing between scattered, large scale dynamically forced and frontal convection. We use different ground and satellite based observational data sets, that supply information on ice water content and path, ice cloud cover and cloud top height on a similar scale as the simulations, in order to evaluate and constrain our model simulations. We find that the timing and geometric extent of the convectively generated cloud shield agrees well with observations while the life time of the convective anvil was, at least in one case, significantly overestimated. Given the large uncertainties of individual ice water path observations, we use a suite of observations in order to better constrain the simulations. ICON-LEM simulates cloud ice water path that lies in-between the different observational data sets but simulations appear to be biased towards a large frozen water path (all frozen hydrometeors). The bias in frozen water path and the longevity of the anvil are little affected by modifications of parameters within the microphysical scheme. In particular one of our convective days appeared to be very sensitive to the initial and boundary conditions which had a large impact on the convective triggering, but little impact on the high frozen water path and long anvil life time bias. Based on this limited set of sensitivity experiments, the evolution of locally forced convection appears to depend more on the uncertainty of the large-scale dynamical state based on data assimilation than of microphysical parameters. Overall, we judge ICON-LEM simulations of deep moist convection to be very close to observations regarding timing, geometrical structure and cloud ice water path of the convective anvil, but other frozen hydrometeors, in particular graupel, are likely overestimated. Therefore, ICON-LEM supplies important information for weather forecasting and forms a good basis for parameterization development based on physical processes or machine learning.

scholarly journals Aquaplanets, Climate Sensitivity, and Low Clouds

2008 ◽  
Vol 21 (19) ◽  
pp. 4974-4991 ◽  
Author(s):  
Brian Medeiros ◽  
Bjorn Stevens ◽  
Isaac M. Held ◽  
Ming Zhao ◽  
David L. Williamson ◽  
...  
Keyword(s):  
Climate Sensitivity ◽  
Large Scale ◽  
Climate Models ◽  
Trade Wind ◽  
Tropical Atmosphere ◽  
Leading Role ◽  
Low Clouds ◽  
Scale Models ◽  
Simulated Climate ◽  
The Tropics

Abstract Cloud effects have repeatedly been pointed out as the leading source of uncertainty in projections of future climate, yet clouds remain poorly understood and simulated in climate models. Aquaplanets provide a simplified framework for comparing and understanding cloud effects, and how they are partitioned as a function of regime, in large-scale models. This work uses two climate models to demonstrate that aquaplanets can successfully predict a climate model’s sensitivity to an idealized climate change. For both models, aquaplanet climate sensitivity is similar to that of the realistic configuration. Tropical low clouds appear to play a leading role in determining the sensitivity. Regions of large-scale subsidence, which cover much of the tropics, are most directly responsible for the differences between the models. Although cloud effects and climate sensitivity are similar for aquaplanets and realistic configurations, the aquaplanets lack persistent stratocumulus in the tropical atmosphere. This, and an additional analysis of the cloud response in the realistically configured simulations, suggests the representation of shallow (trade wind) cumulus convection, which is ubiquitous in the tropics, is largely responsible for differences in the simulated climate sensitivity of these two models.

Quasi-idealized numerical simulations of processes involved in orogenic convection initiation over the Sierras de Córdoba mountains

2021 ◽  
Author(s):  
Christopher A. Davis
Keyword(s):  
Numerical Simulations ◽  
Mountain Range ◽  
Moist Convection ◽  
Low Level ◽  
Physical Mechanisms ◽  
Convergence Line ◽  
Synoptic Conditions ◽  
Deep Moist Convection ◽  
Outflow Boundary ◽  
Convection Initiation

Abstract The Sierras de Córdoba (SDC) mountain range in Argentina is a hotspot of deep moist convection initiation (CI). Radar climatology indicates that 44% of daytime CI events that occur near the SDC in spring and summer seasons and that are not associated with the passage of a cold front or an outflow boundary involve a northerly LLJ, and these events tend to preferentially occur over the southeast quadrant of the main ridge of the SDC. To investigate the physical mechanisms acting to cause CI, idealized convection-permitting numerical simulations with a horizontal grid spacing of 1 km were conducted using CM1. The sounding used for initializing the model featured a strong northerly LLJ, with synoptic conditions resembling those in a previously postulated conceptual model of CI over the region, making it a canonical case study. Differential heating of the mountain caused by solar insolation in conjunction with the low-level northerly flow sets up a convergence line on the eastern slopes of the SDC. The southern portion of this line experiences significant reduction in convective inhibition, and CI occurs over the SDC southeast quadrant. Thesimulated storm soon acquires supercellular characteristics, as observed. Additional simulations with varying LLJ strength also show CI over the southeast quadrant. A simulation without background flow generated convergence over the ridgeline, with widespread CI across the entire ridgeline. A simulation with mid- and upper-tropospheric westerlies removed indicates that CI is minimally influenced by gravity waves. We conclude that the low-level jet is sufficient to focus convection initiation over the southeast quadrant of the ridge.

scholarly journals Observing the Tropical Atmosphere in Moisture Space

2018 ◽  
Vol 75 (10) ◽  
pp. 3313-3330 ◽  
Author(s):  
Hauke Schulz ◽  
Bjorn Stevens
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Circulation Pattern ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Low Rank ◽  
Substantial Part ◽  
Tropical Atmosphere ◽  
Shallow Clouds ◽  
Self Aggregation

Measurements from the Barbados Cloud Observatory are analyzed to identify the processes influencing the distribution of moist static energy and the large-scale organization of tropical convection. Five years of water vapor and cloud profiles from a Raman lidar and cloud radar are composed to construct the structure of the observed atmosphere in moisture space. The large-scale structure of the atmosphere is similar to that now familiar from idealized studies of convective self-aggregation, with shallow clouds prevailing over a moist marine layer in regions of low-rank humidity, and deep convection in a nearly saturated atmosphere in regions of high-rank humidity. With supplementary reanalysis datasets the overall circulation pattern is reconstructed in moisture space, and shows evidence of a substantial lower-tropospheric component to the circulation. This shallow component of the circulation helps support the differentiation between the moist and dry columns, similar to what is found in simulations of convective self-aggregation. Radiative calculations show that clear-sky radiative differences can explain a substantial part of this circulation, with further contributions expected from cloud radiative effects. The shallow component appears to be important for maintaining the low gross moist stability of the convecting column. A positive feedback between a shallow circulation driven by differential radiative cooling and the low-level moisture gradients that help support it is hypothesized to play an important role in conditioning the atmosphere for deep convection. The analysis suggests that the radiatively driven shallow circulations identified by modeling studies as contributing to the self-aggregation of convection in radiative–convective equilibrium similarly play a role in shaping the intertropical convergence zone and, hence, the large-scale structure of the tropical atmosphere.

scholarly journals Oceanic Overturning and Heat Transport: The Role of Background Diffusivity

2019 ◽  
Vol 32 (3) ◽  
pp. 701-716 ◽  
Author(s):  
Magnus Hieronymus ◽  
Jonas Nycander ◽  
Johan Nilsson ◽  
Kristofer Döös ◽  
Robert Hallberg
Keyword(s):  
Heat Transport ◽  
Large Scale ◽  
Energy Transport ◽  
Climate Model ◽  
Potential Temperature ◽  
Meridional Heat Transport ◽  
Sea Ice Extent ◽  
Antarctic Sea Ice ◽  
Meridional Overturning

The role of oceanic background diapycnal diffusion for the equilibrium climate state is investigated in the global coupled climate model CM2G. Special emphasis is put on the oceanic meridional overturning and heat transport. Six runs with the model, differing only by their value of the background diffusivity, are run to steady state and the statistically steady integrations are compared. The diffusivity changes have large-scale impacts on many aspects of the climate system. Two examples are the volume-mean potential temperature, which increases by 3.6°C between the least and most diffusive runs, and the Antarctic sea ice extent, which decreases rapidly as the diffusivity increases. The overturning scaling with diffusivity is found to agree rather well with classical theoretical results for the upper but not for the lower cell. An alternative empirical scaling with the mixing energy is found to give good results for both cells. The oceanic meridional heat transport increases strongly with the diffusivity, an increase that can only partly be explained by increases in the meridional overturning. The increasing poleward oceanic heat transport is accompanied by a decrease in its atmospheric counterpart, which keeps the increase in the planetary energy transport small compared to that in the ocean.

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