scholarly journals Local Impact of Stochastic Shallow Convection on Clouds and Precipitation in the Tropical Atlantic

2021 ◽  
Author(s):  
Mirjana Sakradzija ◽  
Fabian Senf ◽  
Leonhard Scheck ◽  
Maike Ahlgrimm ◽  
Daniel Klocke
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Sensible Heat Flux ◽  
Convective Heating ◽  
Climate Modelling ◽  
Tropical Atlantic ◽  
Shallow Convection ◽  
Convection Scheme ◽  
Local Boundary ◽  
Local Impact

<p>Local impact of a stochastic shallow convection scheme on clouds and precipitation is tested in a case study over the tropical Atlantic on 20th December 2013 using the Icosahedral Nonhydrostatic Model (ICON) of the German Weather Service. ICON is used at a grid resolution of 2.5 km and is tested in several configurations that differ in their treatment of shallow convection. Two versions of a scale-aware stochastic shallow convection scheme are compared to the operational deterministic scheme and a case with no representation of shallow convection. The model is evaluated by comparing synthetically generated irradiance data for both visible and infrared wavelengths against actual satellite observations. The experimental approach is designed to distinguish the local effects of parameterized shallow convection (or lack thereof) within the trades versus the ITCZ. <br>The stochastic cases prove to be superior in reproducing low-level cloud cover, deep convection and its organization, as well as the distribution of precipitation in the tropical Atlantic ITCZ. In these cases, convective heating in the subcloud layer is substantial, boundary layer depth is increased as a result of the heating, while evaporation is enhanced at the expense of sensible heat flux at the ocean’s surface. The stochastic case where subgrid shallow convection is deactivated below the resolved deep updrafts shows that local boundary-layer convection is crucial for a better representation of deep convection. Based on these results, our study points to a necessity to further develop parameterizations of shallow convection for the use at the convection-permitting resolutions and to assuredly include them in weather and climate modelling efforts. </p>

scholarly journals Local Impact of Stochastic Shallow Convection on Clouds and Precipitation in the Tropical Atlantic

2020 ◽  
Vol 148 (12) ◽  
pp. 5041-5062
Author(s):  
Mirjana Sakradzija ◽  
Fabian Senf ◽  
Leonhard Scheck ◽  
Maike Ahlgrimm ◽  
Daniel Klocke
Keyword(s):  
Boundary Layer ◽  
Climate Models ◽  
Deep Convection ◽  
Sensible Heat Flux ◽  
Convective Heating ◽  
Tropical Atlantic ◽  
Shallow Convection ◽  
Local Boundary ◽  
Stochastic Case ◽  
Local Impact

AbstractThe local impact of stochastic shallow convection on clouds and precipitation is tested in a case study over the tropical Atlantic on 20 December 2013 using the Icosahedral Nonhydrostatic Model (ICON). ICON is used at a grid resolution of 2.5 km and is tested in several configurations that differ in their treatment of shallow convection. A stochastic shallow convection scheme is compared to the operational deterministic scheme and a case with no representation of shallow convection. The model is evaluated by comparing synthetically generated irradiance data for both visible and infrared wavelengths against actual satellite observations. The experimental approach is designed to distinguish the local effects of parameterized shallow convection (or lack thereof) within the trades versus the ITCZ. The stochastic cases prove to be superior in reproducing low-level cloud cover, deep convection, and its organization, as well as the distribution of precipitation in the tropical Atlantic ITCZ. In these cases, convective heating in the subcloud layer is substantial, and boundary layer depth is increased as a result of the heating, while evaporation is enhanced at the expense of sensible heat flux at the ocean’s surface. The stochastic case where subgrid shallow convection is deactivated below the resolved deep updrafts indicates that local boundary layer convection is crucial for a better representation of deep convection. Based on these results, our study points to a necessity to further develop parameterizations of shallow convection for use at the convection-permitting resolutions and to assuredly include them in weather and climate models even as their imperfect versions.

scholarly journals Impacts of Shallow Convection on MJO Simulation: A Moist Static Energy and Moisture Budget Analysis

2013 ◽  
Vol 26 (8) ◽  
pp. 2417-2431 ◽  
Author(s):  
Qiongqiong Cai ◽  
Guang J. Zhang ◽  
Tianjun Zhou
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Lower Troposphere ◽  
Longwave Radiation ◽  
Reanalysis Data ◽  
Specific Humidity ◽  
Moist Static Energy ◽  
Shallow Convection ◽  
Budget Analysis ◽  
Static Energy

Abstract The role of shallow convection in Madden–Julian oscillation (MJO) simulation is examined in terms of the moist static energy (MSE) and moisture budgets. Two experiments are carried out using the NCAR Community Atmosphere Model, version 3.0 (CAM3.0): a “CTL” run and an “NSC” run that is the same as the CTL except with shallow convection disabled below 700 hPa between 20°S and 20°N. Although the major features in the mean state of outgoing longwave radiation, 850-hPa winds, and vertical structure of specific humidity are reasonably reproduced in both simulations, moisture and clouds are more confined to the planetary boundary layer in the NSC run. While the CTL run gives a better simulation of the MJO life cycle when compared with the reanalysis data, the NSC shows a substantially weaker MJO signal. Both the reanalysis data and simulations show a recharge–discharge mechanism in the MSE evolution that is dominated by the moisture anomalies. However, in the NSC the development of MSE and moisture anomalies is weaker and confined to a shallow layer at the developing phases, which may prevent further development of deep convection. By conducting the budget analysis on both the MSE and moisture, it is found that the major biases in the NSC run are largely attributed to the vertical and horizontal advection. Without shallow convection, the lack of gradual deepening of upward motion during the developing stage of MJO prevents the lower troposphere above the boundary layer from being preconditioned for deep convection.

scholarly journals Improving subtropical boundary layer cloudiness in the 2011 NCEP GFS

2014 ◽  
Vol 7 (2) ◽  
pp. 2249-2291 ◽  
Author(s):  
J. K. Fletcher ◽  
C. S. Bretherton ◽  
H. Xiao ◽  
R. Sun ◽  
J. Han
Keyword(s):  
Boundary Layer ◽  
Radiative Forcing ◽  
Current Knowledge ◽  
Ocean Model ◽  
Climate Modelling ◽  
Shallow Convection ◽  
Shallow Cumulus ◽  
Single Column ◽  
Low Cloud ◽  
Environmental Prediction

Abstract. The current operational version of National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) shows significant low cloud bias. These biases also appear in the Coupled Forecast System (CFS), which is developed from the GFS. These low cloud biases degrade seasonal and longer climate forecasts, particularly of shortwave cloud radiative forcing, and affect predicted sea-surface temperature. Reducing this bias in the GFS will aid the development of future CFS versions and contributes to NCEP's goal of unified weather and climate modelling. Changes are made to the shallow convection and planetary boundary layer parametrisations to make them more consistent with current knowledge of these processes and to reduce the low cloud bias. These changes are tested in a single-column version of GFS and in global simulations with GFS coupled to a dynamical ocean model. In the single column model, we focus on changing parameters that set the following: the strength of shallow cumulus lateral entrainment, the conversion of updraught liquid water to precipitation and grid-scale condensate, shallow cumulus cloud top, and the effect of shallow convection in stratocumulus environments. Results show that these changes improve the single-column simulations when compared to large eddy simulations, in particular through decreasing the precipitation efficiency of boundary layer clouds. These changes, combined with a few other model improvements, also reduce boundary layer cloud and albedo biases in global coupled simulations.

scholarly journals Entrainment and its dependency on environmental conditions and convective organization in convection-permitting simulations

2021 ◽  
Author(s):  
Tobias Becker ◽  
Cathy Hohenegger
Keyword(s):  
South America ◽  
West Africa ◽  
Deep Convection ◽  
Static Stability ◽  
Tropical Atlantic ◽  
Shallow Convection ◽  
Squall Lines ◽  
Environmental Humidity ◽  
Turbulence Generation ◽  
Convective Organization

AbstractIn this study, we estimate bulk entrainment rates for deep convection in convection-permitting simulations, conducted over the tropical Atlantic Ocean, encompassing parts of Africa and South America. We find that, even though entrainment rates decrease with height in all regions, they are, when averaging between 600 and 800 hPa, generally higher over land than over ocean. This is so because, over Amazonia, shallow convection causes an increase of bulk entrainment rates at lower levels and because, over West Africa, where entrainment rates are highest, convection is organized in squall lines. These squall lines are associated with strong mesoscale convergence, causing more intense updrafts and stronger turbulence generation in the vicinity of updrafts, increasing the entrainment rates. With the exception of West Africa, entrainment rates differ less across regions than across different environments within the regions. In contrast to what is usually assumed in convective parameterizations, entrainment rates increase with environmental humidity. Moreover, over ocean, they increase with static stability, while over land, they decrease. In addition, confirming the results of a recent idealized study, entrainment rates increase with convective aggregation, except in regions dominated by squall lines, like over West Africa.

scholarly journals Simulating deep convection with a shallow convection scheme

2011 ◽  
Vol 11 (20) ◽  
pp. 10389-10406 ◽  
Author(s):  
C. Hohenegger ◽  
C. S. Bretherton
Keyword(s):  
Climate Modeling ◽  
Global Climate ◽  
Deep Convection ◽  
Convective Precipitation ◽  
Cloud Base ◽  
Gradual Transition ◽  
Shallow Convection ◽  
Convection Scheme ◽  
Community Atmosphere Model ◽  
Simulated Precipitation

Abstract. Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.

Comparison Studies of Cloud- and Convection-Related Processes Simulated by the Canadian Regional Climate Model over the Pacific Ocean

2008 ◽  
Vol 136 (11) ◽  
pp. 4168-4187 ◽  
Author(s):  
Yanjun Jiao ◽  
Colin Jones
Keyword(s):  
Boundary Layer ◽  
Cross Section ◽  
Cloud Cover ◽  
Large Scale ◽  
Climate Model ◽  
Deep Convection ◽  
Regional Climate ◽  
Eddy Diffusivity ◽  
Shallow Convection ◽  
The Tropical Pacific

Abstract This paper presents results from the Canadian Regional Climate Model (CRCM) contribution to the Global Energy and Water Cycle Experiment (GEWEX) Pacific Cross-section Intercomparison Project. This experiment constitutes a simulation of stratocumulus, trade cumulus, and deep convective transitions along a cross section in the tropical Pacific. The simulated seasonal mean cloud and convection are compared between an original version of CRCM (CRCM4) and a modified version (CRCMM) with refined parameterizations. Results are further compared against available observations and reanalysis data. The specific parameterization refinements touch upon the triggering and closure of shallow convection, the cloud and updraft characteristics of deep convection, the parameterization of large-scale cloud fraction, the calculation of the eddy diffusivity in the boundary layer, and the evaporation of falling large-scale precipitation. CRCMM shows substantial improvement in many aspects of the simulated seasonal mean cloud, convection, and precipitation over the tropical Pacific, CRCMM-simulated total column water vapor, total cloud cover, and precipitation are in better agreement with observations than in the original CRCM4 model. The maximum frequency of the shallow convection shifts from the ITCZ region in CRCM4 to the subtropics in CRCMM; accordingly, excessive cloud in the shallow cumulus region in CRCM4 is greatly diminished. Finally, CRCMM better simulates the vertical structure of relative humidity, cloud cover, and vertical velocity, at least when compared to the 40-yr ECMWF Re-Analysis. Analyses of sensitivity experiments assessing specific effects of individual parameterization changes indicate that the modification to the eddy diffusivity in the boundary layer and changes to deep convection contribute most significantly to the overall model improvements.

Dynamics of the Shallow Meridional Circulation around Intertropical Convergence Zones

2007 ◽  
Vol 64 (7) ◽  
pp. 2262-2285 ◽  
Author(s):  
David S. Nolan ◽  
Chidong Zhang ◽  
Shu-hua Chen
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Meridional Circulation ◽  
Deep Convection ◽  
Hadley Circulation ◽  
Sea Breeze ◽  
Return Flow ◽  
Convective Heating ◽  
Moist Convection ◽  
East Pacific

Abstract The generally accepted view of the meridional circulation in the tropical east Pacific is that of a single deep overturning cell driven by deep convective heating in the intertropical convergence zone (ITCZ), similar to the zonal mean Hadley circulation. However, recent observations of the atmosphere from the tropical eastern Pacific have called this view into question. In several independent datasets, significant meridional return flows out of the ITCZ region were observed, not only at high altitudes, but also at low altitudes, just above the atmospheric boundary layer. This paper presents a theory and idealized simulations to understand the causes and dynamics of this shallow meridional circulation (SMC). Fundamentally, the SMC can be seen as a large-scale sea-breeze circulation driven by sea surface temperature gradients when deep convection is absent in the ITCZ region. A simple model of this circulation is presented. Using observed values, the sea-breeze model shows that the pressure gradient above the boundary can indeed reverse, leading to the pressure force that drives the shallow return flow out of the ITCZ. The Weather Research and Forecast Model (WRF) is used to simulate an idealized Hadley circulation driven by moist convection in a tropical channel. The SMC is reproduced, with reasonable similarity to the circulation observed in the east Pacific. The simulations confirm that the SMC is driven by a reversal of the pressure gradient above the boundary layer, and that the return flow is strongest when deep convection is absent in the ITCZ, and weakest when deep convection is active. The model also shows that moisture transport out of the ITCZ region is far greater in the low-level shallow return flow than in the high-altitude return flow associated with the deep overturning, and that a budget for water transport in and out of the ITCZ region is grossly incomplete without it. Much of the moisture carried in the shallow return flow is recycled into the boundary layer, but does not appear to contribute to enhanced cloudiness in the subtropical stratocumulus poleward of the ITCZ.

scholarly journals Simulating deep convection with a shallow convection scheme

2011 ◽  
Vol 11 (3) ◽  
pp. 8385-8430 ◽  
Author(s):  
C. Hohenegger ◽  
C. S. Bretherton
Keyword(s):  
Climate Modeling ◽  
Global Climate ◽  
Deep Convection ◽  
Convective Precipitation ◽  
Cloud Base ◽  
Gradual Transition ◽  
Shallow Convection ◽  
Convection Scheme ◽  
Community Atmosphere Model ◽  
Simulated Precipitation

Abstract. Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-Column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.

scholarly journals A Density Current Parameterization Coupled with Emanuel’s Convection Scheme. Part I: The Models

2010 ◽  
Vol 67 (4) ◽  
pp. 881-897 ◽  
Author(s):  
Jean-Yves Grandpeix ◽  
Jean-Philippe Lafore
Keyword(s):  
Evolution Equations ◽  
Deep Convection ◽  
Leading Edge ◽  
Power Input ◽  
Water Yield ◽  
Density Current ◽  
Convective Heating ◽  
Convection Scheme ◽  
Global Circulation Models ◽  
New Variables

Abstract The aim of the present series of papers is to develop a density current parameterization for global circulation models. This first paper is devoted to the presentation of this new wake parameterization coupled with Emanuel’s convective scheme. The model represents a population of identical circular cold pools (the wakes) with vertical frontiers. The wakes are cooled by the precipitating downdrafts while the outside area is warmed by the subsidence induced by the saturated drafts. The budget equations for mass, energy, and water yield evolution equations for the prognostic variables (the vertical profiles of the temperature and humidity differences between the wakes and their exterior). They also provide additional terms for the equations of the mean variables. The driving terms of the wake equations are the differential heating and drying due to convective drafts. The action of the convection on the wakes is implemented by splitting the convective tendency and attributing the effect of the precipitating downdrafts to the wake region and the effect of the saturated drafts to their exterior. Conversely, the action of the wakes on convection is implemented by introducing two new variables representing the convergence at the leading edge of the wakes. The available lifting energy (ALE) determines the triggers of deep convection: convection occurs when ALE exceeds the convective inhibition. The available lifting power (ALP) determines the intensity of convection; it is equal to the power input into the system by the collapse of the wakes. The ALE/ALP closure, together with the splitting of the convective heating and drying, implements the full coupling between wake and convection. The coupled wake–convection scheme thus created makes it possible to represent the moist convective processes more realistically, to prepare the coupling of convection with boundary layer and orographic processes, and to consider simulating the propagation of convective systems.

scholarly journals Improving subtropical boundary layer cloudiness in the 2011 NCEP GFS

2014 ◽  
Vol 7 (5) ◽  
pp. 2107-2120 ◽  
Author(s):  
J. K. Fletcher ◽  
C. S. Bretherton ◽  
H. Xiao ◽  
R. Sun ◽  
J. Han
Keyword(s):  
Boundary Layer ◽  
Radiative Forcing ◽  
Current Knowledge ◽  
Short Wave ◽  
Ocean Model ◽  
Climate Modelling ◽  
Shallow Convection ◽  
Shallow Cumulus ◽  
Single Column ◽  
Low Cloud

Abstract. The current operational version of National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) shows significant low cloud bias. These biases also appear in the Coupled Forecast System (CFS), which is developed from the GFS. These low cloud biases degrade seasonal and longer climate forecasts, particularly of short-wave cloud radiative forcing, and affect predicted sea surface temperature. Reducing this bias in the GFS will aid the development of future CFS versions and contributes to NCEP's goal of unified weather and climate modelling. Changes are made to the shallow convection and planetary boundary layer parameterisations to make them more consistent with current knowledge of these processes and to reduce the low cloud bias. These changes are tested in a single-column version of GFS and in global simulations with GFS coupled to a dynamical ocean model. In the single-column model, we focus on changing parameters that set the following: the strength of shallow cumulus lateral entrainment, the conversion of updraught liquid water to precipitation and grid-scale condensate, shallow cumulus cloud top, and the effect of shallow convection in stratocumulus environments. Results show that these changes improve the single-column simulations when compared to large eddy simulations, in particular through decreasing the precipitation efficiency of boundary layer clouds. These changes, combined with a few other model improvements, also reduce boundary layer cloud and albedo biases in global coupled simulations.

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