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

Monthly Weather Review ◽

10.1175/mwr-d-20-0229.1 ◽

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.

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Linking horizontal and vertical transports of biomass fire emissionsto the tropical Atlantic ozone paradox during the Northern Hemisphere winter season: climatology

Atmospheric Chemistry and Physics ◽

10.5194/acp-4-449-2004 ◽

2004 ◽

Vol 4 (2) ◽

pp. 449-469 ◽

Cited By ~ 17

Author(s):

G. S. Jenkins ◽

J.-H. Ryu

Keyword(s):

South America ◽

West Africa ◽

Northern Hemisphere ◽

Outgoing Longwave Radiation ◽

Winter Season ◽

Longwave Radiation ◽

Tropical Atlantic ◽

Hemisphere Winter ◽

Column Ozone ◽

Northern Hemisphere Winter

Abstract. During the Northern hemisphere winter season, biomass burning is widespread in West Africa, yet the total tropospheric column ozone values (<30DU) over much of the Tropical Atlantic Ocean (15°N-5°S) are relatively low. At the same time, the tropospheric column ozone values in the Southern Tropical Atlantic are higher than those in the Northern Hemisphere (ozone paradox). We examine the causes for low tropospheric column ozone values by considering the horizontal and vertical transport of biomass fire emissions in West Africa during November through March, using observed data which characterizes fires, aerosols, horizontal winds, precipitation, lightning and outgoing longwave radiation. We have found that easterly winds prevail in the lower troposphere but transition to westerly winds at pressure levels lower than 500hPa. A persistent anticyclone over West Africa at 700hPa is responsible for strong easterly winds, which causes a net outflow of ozone/ozone precursors from biomass burning in West Africa across the Atlantic Ocean towards South America. The lowest outgoing longwave radiation (OLR) and highest precipitation rates are generally found over the central Atlantic, some distance downstream of fires in West Africa making the vertical transport of ozone and ozone precursors less likely and ozone destruction more likely. However, lightning over land areas in Central Africa and South America can lead to enhanced ozone levels in the upper troposphere especially over the Southern tropical Atlantic during the Northern Hemisphere winter season.

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Space-borne observations link the tropical atlantic ozone maximum and paradox to lightning

Atmospheric Chemistry and Physics ◽

10.5194/acp-4-361-2004 ◽

2004 ◽

Vol 4 (2) ◽

pp. 361-375 ◽

Cited By ~ 25

Author(s):

G. S. Jenkins ◽

J.-H. Ryu

Keyword(s):

South America ◽

West Africa ◽

Biomass Burning ◽

Central Africa ◽

Tropical Atlantic ◽

Upper Troposphere ◽

The North ◽

Seasonal Basis ◽

June July ◽

Column Ozone

Abstract. The potential enhancement of tropospheric column ozone values over the Tropical Atlantic Ocean on a seasonal basis by lightning is investigated using satellite derived ozone data, TRMM lightning data, ozonesonde data and NCEP reanalysis during 1998-2001. Our results show that the number of lightning flashes in Africa and South America reach a maximum during September, October and November (SON). The spatial patterns of winds in combination with lightning from West Africa, Central Africa and South America is likely responsible for enriching middle/upper troposphere ozone over the Tropical South Atlantic during SON. Moreover, lightning flashes are high in the hemisphere opposite to biomass burning during December, January, and February (DJF) and June, July and August (JJA). This pattern leads to an enrichment of ozone in the middle/upper troposphere in the Southern Hemisphere Tropics during DJF and the Northern Hemisphere Tropics during JJA. During JJA the largest numbers of lightning flashes are observed in West Africa, enriching tropospheric column ozone to the north of 5S in the absence of biomass burning. During DJF, lightning is concentrated in South America and Central Africa enriching tropospheric column ozone south of the Equator in the absence of biomass burning.

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Space-borne observations link the tropical Atlantic ozone maximum and paradox to lightning

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-3-5725-2003 ◽

2003 ◽

Vol 3 (6) ◽

pp. 5725-5754 ◽

Cited By ~ 2

Author(s):

G. S. Jenkins ◽

J.-H. Ryu

Keyword(s):

South America ◽

West Africa ◽

Atlantic Ocean ◽

Biomass Burning ◽

Central Africa ◽

Tropical Atlantic ◽

Tropical Atlantic Ocean ◽

The North ◽

June July ◽

Column Ozone

Abstract. The causes of high tropospheric column ozone values over the Tropical Atlantic Ocean during September, October, and November (SON) are investigated by examining lightning during 1998–2001. The cause for high tropospheric column ozone in the hemisphere opposite of biomass burning (tropical ozone paradox) is also examined. Our results show that lightning is central to high tropospheric column ozone during SON and responsible for the tropical ozone paradox during December, January, and February (DJF) and June, July and August (JJA). During SON large numbers of flashes are observed in South America, Central and West Africa enriching the tropospheric column ozone over the Tropical Atlantic Ocean. During JJA the largest numbers of lightning flashes are observed in West Africa, enriching tropospheric column ozone to the north of 5° S in the absence biomass burning. During DJF, lightning is concentrated in South America and Central Africa enriching tropospheric column ozone south of the Equator in the absence of biomass burning.

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Atlantic Tropical Cyclogenesis: A Three-Way Interaction between an African Easterly Wave, Diurnally Varying Convection, and a Convectively Coupled Atmospheric Kelvin Wave

Monthly Weather Review ◽

10.1175/mwr-d-11-00122.1 ◽

2012 ◽

Vol 140 (4) ◽

pp. 1108-1124 ◽

Cited By ~ 41

Author(s):

Michael J. Ventrice ◽

Christopher D. Thorncroft ◽

Matthew A. Janiga

Keyword(s):

West Africa ◽

Diurnal Cycle ◽

Deep Convection ◽

Active Phase ◽

Boreal Summer ◽

Tropical Cyclogenesis ◽

Tropical Atlantic ◽

Kelvin Wave ◽

African Easterly Wave ◽

Easterly Wave

This paper explores a three-way interaction between an African easterly wave (AEW), the diurnal cycle of convection over the Guinea Highlands (GHs), and a convectively coupled atmospheric equatorial Kelvin wave (CCKW). These interactions resulted in the genesis of Tropical Storm Debby over the eastern tropical Atlantic during late August 2006. The diurnal cycle of convection downstream of the GHs during the month of August is explored. Convection associated with the coherent diurnal cycle is observed off the coast of West Africa during the morning. Later, convection initiates over and downstream of the GHs during the afternoon. These convective features were pronounced during the passage of the pre-Debby AEW. The superposition between the convectively active phase of a strong CCKW and the pre-Debby AEW occurred shortly after merging with the diurnally varying convection downstream of the GHs. The CCKW–AEW interaction preceded tropical cyclogenesis by 18 h. The CCKW provided a favorable environment for deep convection. An analysis of high-amplitude CCKWs over the tropical Atlantic and West Africa during the Northern Hemisphere boreal summer (1979–2009) highlights a robust relationship between CCKWs and the frequency of tropical cyclogenesis. Tropical cyclogenesis is found to be less frequent immediately prior to the passage of the convectively active phase of the CCKW, more frequent during the passage, and most frequent just after the passage.

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Numerical Simulation of Radial Cloud Bands within the Upper-Level Outflow of an Observed Mesoscale Convective System

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3531.1 ◽

2010 ◽

Vol 67 (9) ◽

pp. 2990-2999 ◽

Cited By ~ 36

Author(s):

S. B. Trier ◽

R. D. Sharman ◽

R. G. Fovell ◽

R. G. Frehlich

Keyword(s):

Deep Convection ◽

Vertical Wind Shear ◽

Mesoscale Convective System ◽

Static Stability ◽

Vertical Shear ◽

Convective System ◽

Shallow Convection ◽

Band Formation ◽

Mesoscale Convective ◽

Radial Outflow

Abstract Turbulence affecting aircraft is frequently reported within bands of cirrus anvil cloud extending radially outward from upstream deep convection in mesoscale convective systems (MCSs). A high-resolution convection permitting model is used to simulate bands of this type observed on 17 June 2005. The timing, location, and orientation of these simulated bands are similar to those in satellite imagery for this case. The 10–20-km horizontal spacing between the bands is also similar to typical spacing found in a recent satellite-based climatology of MCS-induced radial outflow bands. The simulated bands result from shallow convection in the near-neutral to weakly unstable MCS outer anvil. The weak stratification of the anvil, the ratio of band horizontal wavelength to the depth of the near-neutral anvil layer (5:1 to 10:1), and band orientation approximately parallel to the vertical shear within the same layer are similar to corresponding aspects of horizontal convective rolls in the atmospheric boundary layer, which result from thermal instability. The vertical shear in the MCS outflow is important not only in influencing the orientation of the radial bands but also for its role, through differential temperature advection, in helping to thermodynamically destabilize the environment in which they originate. High-frequency gravity waves emanating from the parent deep convection are trapped in a layer of strong static stability and vertical wind shear beneath the near-neutral anvil and, consistent with satellite studies, are oriented approximately normal to the developing radial bands. The wave-generated vertical displacements near the anvil base may aid band formation in the layer above.

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

10.5194/egusphere-egu21-13321 ◽

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.&#160;<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&#8217;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.&#160;</p>

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

Monthly Weather Review ◽

10.1175/mwr-d-20-0107.1 ◽

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.

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Understanding the variability of the rainfall dipole in West Africa using the EC-Earth last millennium simulation

Climate Dynamics ◽

10.1007/s00382-021-05696-x ◽

2021 ◽

Author(s):

Qiong Zhang ◽

Ellen Berntell ◽

Qiang Li ◽

Fredrik Charpentier Ljungqvist

Keyword(s):

West Africa ◽

Model Simulation ◽

Rainfall Variability ◽

Low Pressure ◽

Last Millennium ◽

Tropical Atlantic ◽

Gulf Of Guinea ◽

Dipole Pattern ◽

Sahel Region ◽

Heat Low

AbstractThere is a well-known mode of rainfall variability associating opposite hydrological conditions over the Sahel region and the Gulf of Guinea, forming a dipole pattern. Previous meteorological observations show that the dipole pattern varies at interannual timescales. Using an EC-Earth climate model simulation for last millennium (850–1850 CE), we investigate the rainfall variability in West Africa over longer timescales. The 1000-year-long simulation data show that this rainfall dipole presents at decadal to multidecadal and centennial variability and long-term trend. Using the singular value decomposition (SVD) analysis, we identified that the rainfall dipole present in the first SVD mode with 60% explained variance and associated with the variabilities in tropical Atlantic sea surface temperature (SST). The second SVD mode shows a monopole rainfall variability pattern centred over the Sahel, associated with the extra-tropical Atlantic SST variability. We conclude that the rainfall dipole-like pattern is a natural variability mode originated from the local ocean–atmosphere-land coupling in the tropical Atlantic basin. The warm SST anomalies in the equatorial Atlantic Ocean favour an anomalous low pressure at the tropics. This low pressure weakens the meridional pressure gradient between the Saharan Heat Low and the tropical Atlantic. It leads to anomalous northeasterly, reduces the southwesterly moisture flux into the Sahel and confines the Gulf of Guinea's moisture convergence. The influence from extra-tropical climate variability, such as Atlantic multidecadal oscillation, tends to modify the rainfall dipole pattern to a monopole pattern from the Gulf of Guinea to Sahara through influencing the Sahara heat low. External forcing—such as orbital forcing, solar radiation, volcanic and land-use—can amplify/dampen the dipole mode through thermal forcing and atmosphere dynamical feedback.

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