Daytime Cycle of Low-Level Clouds and the Tropical Convective Boundary Layer in Southwestern Amazonia

C. Strong; J. D. Fuentes; M. Garstang; A. K. Betts

doi:10.1175/jam2266.1

Daytime Cycle of Low-Level Clouds and the Tropical Convective Boundary Layer in Southwestern Amazonia

Journal of Applied Meteorology ◽

10.1175/jam2266.1 ◽

2005 ◽

Vol 44 (10) ◽

pp. 1607-1619 ◽

Cited By ~ 16

Author(s):

C. Strong ◽

J. D. Fuentes ◽

M. Garstang ◽

A. K. Betts

Keyword(s):

Boundary Layer ◽

Mixed Layer ◽

Solar Irradiance ◽

Large Scale ◽

Numerical Models ◽

Cloud Amount ◽

Layer Growth ◽

Cloud Layer ◽

Water Vapor Transport ◽

Low Level

Abstract During the wet season in the southwestern Amazon region, daytime water transport out of the atmospheric mixed layer into the deeper atmosphere is shown to depend upon cloud amounts and types and synoptic-scale velocity fields. Interactions among clouds, convective conditions, and subcloud-layer properties were estimated for two dominant flow regimes observed during the 1999 Tropical Rainfall Measuring Mission component of the Brazilian Large-Scale Biosphere–Atmosphere (TRMM-LBA) field campaign. During daytime the cloud and subcloud layers were coupled by radiative, convective, and precipitation processes. The properties of cloud and subcloud layers varied according to the different convective influences of easterly versus westerly lower-tropospheric flows. The most pronounced flow-regime effects on composite cloud cycles occurred under persistent lower-tropospheric flows, which produced strong convective cloud growth with a near absence of low-level stratiform clouds, minimal cumulative attenuation of incoming solar irradiance (∼25%), rapid daytime mixed-layer growth (>100 m h−1), and boundary layer drying (0.22 g kg−1 h−1), high convective velocities (>1.5 m s−1), high surface buoyancy flux (>200 W m−2), and high latent heat flux (600 W m−2) into cloud layer. In contrast, persistent westerly flows were less convective, showing a strong morning presence of low-level stratiform genera (>0.9 cloud amount), greater cumulative attenuation of incoming solar irradiance (∼47%), slower mixed-layer growth (<50 m h−1) with a slight tendency for mixed-layer moistening, and a delayed peak in the low-level cumuliform cloud cycle (2000 versus 1700 UTC). The results reported in this article indicate that numerical models need to account for cloud amounts and types when estimating water vapor transport to the cloud layer.

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Cloud Feedbacks, Rapid Adjustments, and the Forcing–Response Relationship in a Transient CO2 Reversibility Scenario

Journal of Climate ◽

10.1175/jcli-d-13-00421.1 ◽

2014 ◽

Vol 27 (4) ◽

pp. 1799-1818 ◽

Cited By ~ 17

Author(s):

Timothy Andrews ◽

Mark A. Ringer

Keyword(s):

Boundary Layer ◽

Radiative Forcing ◽

Large Scale ◽

Climate Model ◽

Surface Air Temperature ◽

Cloud Amount ◽

Cloud Optical Depth ◽

Low Level ◽

Radiative Feedbacks ◽

Hadley Centre

Abstract The Hadley Centre Global Environment Model, version 2–Earth System (HadGEM2-ES) climate model is forced by a 1% yr−1 compound increase in atmospheric CO2 for 140 years, followed by a 1% yr−1 CO2 decrease back to the starting level. Analogous atmosphere-only simulations are performed to diagnose the component of change associated with the effective radiative forcing and rapid adjustments. The residual change is associated with radiative feedbacks that are shown to be linearly related to changes in global-mean surface air temperature and are found to be reversible under this experimental design, even for regional cloud feedback changes. The cloud adjustment is related to changes in cloud amount, with little indication of any large-scale changes in cloud optical depth. Plant physiological forcing plays a significant role in determining the cloud adjustment in this model and is the dominant contribution to the low-level cloud changes over land. Low-level cloud adjustments are associated with changes in surface turbulent fluxes and lower tropospheric stability, with significant adjustments in boundary layer cloud types and in the depth of the boundary layer itself. The linearity of simple forcing–response frameworks are examined and found to be generally applicable. Small regional departures from linearity occur during the early part of the ramp-down phase, where the Southern Ocean and eastern tropical Pacific continue to warm for a few decades, despite the reversal in radiative forcing and global temperatures. The importance of considering time-varying patterns of warming and regional phenomena when diagnosing and understanding feedbacks in a coupled atmosphere–ocean framework is highlighted.

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Dynamic and Thermodynamic Relations of Distinctive Stratus Clouds on the Lee Side of the Tibetan Plateau in the Cold Season

Journal of Climate ◽

10.1175/jcli-d-13-00009.1 ◽

2013 ◽

Vol 26 (21) ◽

pp. 8378-8391 ◽

Cited By ~ 19

Author(s):

Yi Zhang ◽

Rucong Yu ◽

Jian Li ◽

Weihua Yuan ◽

Minghua Zhang

Keyword(s):

Tibetan Plateau ◽

Large Scale ◽

Eastern China ◽

Cold Season ◽

Water Vapor Transport ◽

The Tibetan Plateau ◽

Primary Role ◽

Moist Air ◽

Low Level ◽

Stratus Clouds

Abstract Given the large discrepancies that exist in climate models for shortwave cloud forcing over eastern China (EC), the dynamic (vertical motion and horizontal circulation) and thermodynamic (stability) relations of stratus clouds and the associated cloud radiative forcing in the cold season are examined. Unlike the stratus clouds over the southeastern Pacific Ocean (as a representative of marine boundary stratus), where thermodynamic forcing plays a primary role, the stratus clouds over EC are affected by both dynamic and thermodynamic factors. The Tibetan Plateau (TP)-forced low-level large-scale lifting and high stability over EC favor the accumulation of abundant saturated moist air, which contributes to the formation of stratus clouds. The TP slows down the westerly overflow through a frictional effect, resulting in midlevel divergence, and forces the low-level surrounding flows, resulting in convergence. Both midlevel divergence and low-level convergence sustain a rising motion and vertical water vapor transport over EC. The surface cold air is advected from the Siberian high by the surrounding northerly flow, causing low-level cooling. The cooling effect is enhanced by the blocking of the YunGui Plateau. The southwesterly wind carrying warm, moist air from the east Bay of Bengal is uplifted by the HengDuan Mountains via topographical forcing; the midtropospheric westerly flow further advects the warm air downstream of the TP, moistening and warming the middle troposphere on the lee side of the TP. The low-level cooling and midlevel warming together increase the stability. The favorable dynamic and thermodynamic large-scale environment allows for the formation of stratus clouds over EC during the cold season.

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Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-5811-2016 ◽

2016 ◽

Vol 16 (9) ◽

pp. 5811-5839 ◽

Cited By ~ 11

Author(s):

Jan Kazil ◽

Graham Feingold ◽

Takanobu Yamaguchi

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Large Scale ◽

Layer Growth ◽

Radiative Properties ◽

Cloud Base ◽

Latent Heat Release ◽

Marine Stratocumulus ◽

Stratocumulus Clouds ◽

Boundary Layer Growth

Abstract. Observed and projected trends in large-scale wind speed over the oceans prompt the question: how do marine stratocumulus clouds and their radiative properties respond to changes in large-scale wind speed? Wind speed drives the surface fluxes of sensible heat, moisture, and momentum and thereby acts on cloud liquid water path (LWP) and cloud radiative properties. We present an investigation of the dynamical response of non-precipitating, overcast marine stratocumulus clouds to different wind speeds over the course of a diurnal cycle, all else equal. In cloud-system resolving simulations, we find that higher wind speed leads to faster boundary layer growth and stronger entrainment. The dynamical driver is enhanced buoyant production of turbulence kinetic energy (TKE) from latent heat release in cloud updrafts. LWP is enhanced during the night and in the morning at higher wind speed, and more strongly suppressed later in the day. Wind speed hence accentuates the diurnal LWP cycle by expanding the morning–afternoon contrast. The higher LWP at higher wind speed does not, however, enhance cloud top cooling because in clouds with LWP ⪆ 50 g m−2, longwave emissions are insensitive to LWP. This leads to the general conclusion that in sufficiently thick stratocumulus clouds, additional boundary layer growth and entrainment due to a boundary layer moistening arises by stronger production of TKE from latent heat release in cloud updrafts, rather than from enhanced longwave cooling. We find that large-scale wind modulates boundary layer decoupling. At nighttime and at low wind speed during daytime, it enhances decoupling in part by faster boundary layer growth and stronger entrainment and in part because shear from large-scale wind in the sub-cloud layer hinders vertical moisture transport between the surface and cloud base. With increasing wind speed, however, in decoupled daytime conditions, shear-driven circulation due to large-scale wind takes over from buoyancy-driven circulation in transporting moisture from the surface to cloud base and thereby reduces decoupling and helps maintain LWP. The total (shortwave + longwave) cloud radiative effect (CRE) responds to changes in LWP and cloud fraction, and higher wind speed translates to a stronger diurnally averaged total CRE. However, the sensitivity of the diurnally averaged total CRE to wind speed decreases with increasing wind speed.

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How is the marine atmospheric boundary layer turbulence organized in the trades ?

10.5194/egusphere-egu21-12339 ◽

2021 ◽

Author(s):

Pierre-Etienne Brilouet ◽

Marie Lothon ◽

Sandrine Bony

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Vertical Velocity ◽

Large Scale ◽

Layer Structure ◽

Potential Temperature ◽

Surface Wind ◽

Cloud Amount ◽

Velocity Spectrum ◽

Marine Atmospheric Boundary Layer

Tradewind clouds can exhibit a wide diversity of mesoscale organizations, and the turbulence of marine atmospheric boundary layer (MABL) can exhibit coherent structures and mesoscale circulations. One of the objectives of the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field experiment was to better understand the tight interplay between the mesoscale organization of clouds, boundary-layer processes, and the large-scale environment.During the experiment, that took place East of Barbados over the Western Tropical Atlantic Ocean in Jan-Feb 2020, the French ATR-42 research aircraft was devoted to the characterization of the cloud amount and of the subcoud layer structure. During its 17 research flights, it sampled a large diversity of large scale conditions and cloud patterns. Multiple sensors onboard the aircraft measured high-frequency fluctuations of potential temperature, water vapour mixing ratio and wind , allowing for an extensive characterization of the turbulence within the subcloud layer. A quality-controled and calibrated turbulence dataset was produced on the basis of these measurements, which is now available on the EUREC4A AERIS data portal.The MABL turbulent structure is studied using this dataset, through a spectral analysis of the vertical velocity. Vertical profiles of characteristic length scales reveal a non-isotropic structure with a stretching of the eddies along the mean wind. The organization strength of the turbulent field is also explored by defining a diagnostic based on the shape of the vertical velocity spectrum. The structure and the degree of organization of the subcloud layer are characterized for different types of mesoscale convective pattern and as a function of the large-scale environment, including near-surface wind and lower-tropospheric stability conditions.&#160;

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Evaluation of a Statistical Model of Cloud Vertical Structure Using Combined CloudSat and CALIPSO Cloud Layer Profiles

Journal of Climate ◽

10.1175/2010jcli3734.1 ◽

2010 ◽

Vol 23 (24) ◽

pp. 6641-6653 ◽

Cited By ~ 25

Author(s):

William B. Rossow ◽

Yuanchong Zhang

Keyword(s):

Statistical Model ◽

Vertical Structure ◽

Spatial Scales ◽

Three Dimensional ◽

Cloud Amount ◽

Cloud Layer ◽

Middle Level ◽

Polar Regions ◽

Total Cloud Amount ◽

Low Level

Abstract A model of the three-dimensional distribution of clouds was developed from the statistics of cloud layer occurrence from the International Satellite Cloud Climatology Project (ISCCP) and the statistics of cloud vertical structure (CVS) from an analysis of radiosonde humidity profiles. The CVS model associates each cloud type, defined by cloud-top pressure of the topmost cloud layer and total column optical thickness, with a particular CVS. The advent of satellite cloud radar (CloudSat) and lidar [Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)] measurements (together C&C) of CVS allows for a quantitative evaluation of this statistical model. The zonal monthly-mean cloud layer distribution from the ISCCP CVS agrees with that from C&C to within 10% (when normalized to the same total cloud amount). The largest differences are an overestimate of middle-level cloudiness in winter polar regions, an overestimate of cloud-top pressures of the highest-level clouds, especially in the tropics, and an underestimate of low-level cloud amounts over southern midlatitude oceans. A more severe test of the hypothesized relationship is made by comparing CVS for individual satellite pixels. The agreement of CVS is good for isolated low-level clouds and reasonably good when the uppermost cloud layer is a high-level cloud; however, the agreement is not good when the uppermost cloud layer is a middle-level cloud, even when ISCCP correctly locates cloud top. An improved CVS model combining C&C and ISCCP may require classification at spatial scales larger than individual satellite pixels.

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Large-Scale Atmospheric Forcing by Southeast Pacific Boundary Layer Clouds: A Regional Model Study*

Journal of Climate ◽

10.1175/jcli3302.1 ◽

2005 ◽

Vol 18 (7) ◽

pp. 934-951 ◽

Cited By ~ 38

Author(s):

Yuqing Wang ◽

Shang-Ping Xie ◽

Bin Wang ◽

Haiming Xu

Keyword(s):

Boundary Layer ◽

Large Scale ◽

Inversion Layer ◽

Regional Model ◽

Cloud Layer ◽

Eastern Pacific ◽

Radiative Effect ◽

Boundary Layer Clouds ◽

Southeast Pacific ◽

Stratocumulus Clouds

Abstract A regional model is used to study the radiative effect of boundary layer clouds over the southeast Pacific on large-scale atmosphere circulation during August–October 1999. With the standard settings, the model simulates reasonably well the large-scale circulation over the eastern Pacific, precipitation in the intertropical convergence zone (ITCZ) north of the equator, and marine boundary layer stratocumulus clouds to the south. In a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern Pacific artificially removed, boundary layer clouds south of the equator almost disappear and precipitation in the ITCZ is reduced by 15%–20%, indicating that the stratocumulus clouds over the southeast Pacific have both local and cross-equatorial effects. Examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer. In response to this cloud-induced cooling, an in situ anomalous high pressure system develops in the boundary layer and an anomalous shallow meridional circulation develops in the lower troposphere over the equatorial eastern Pacific. At the lower branch of this shallow circulation, anomalous boundary layer southerlies blow from the boundary layer high toward the northern ITCZ where the air ascends. An anomalous returning flow (northerly) just above the cloud layer closes the shallow circulation. This low-level anomalous shallow circulation enhances the subsidence over the southeast Pacific above the cloud layer, helping to maintain boundary layer clouds and temperature inversion there. Meanwhile, the strengthened cross-equatorial flow near the surface enhances moisture convergence and convection in the ITCZ north of the equator. This in turn strengthens the local, deep Hadley circulation and hence the large-scale subsidence and boundary layer clouds over the southeast Pacific. This positive feedback therefore enhances the interhemispheric climate asymmetry over the tropical eastern Pacific.

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Combined effects of surface conditions, boundary layer dynamics and chemistry on diurnal SOA-evolution

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-9331-2012 ◽

2012 ◽

Vol 12 (4) ◽

pp. 9331-9375 ◽

Cited By ~ 2

Author(s):

R. H. H. Janssen ◽

J. Vilà-Guerau de Arellano ◽

L. N. Ganzeveld ◽

P. Kabat ◽

J. L. Jimenez ◽

...

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Land Surface ◽

Large Scale ◽

Organic Aerosol ◽

Layer Growth ◽

Combined Effects ◽

Free Troposphere ◽

Surface Conditions ◽

Boundary Layer Dynamics

Abstract. We study the combined effects of land surface conditions, atmospheric boundary layer dynamics and chemistry on the diurnal evolution of biogenic secondary organic aerosol in the atmospheric boundary layer, using a model that contains the essentials of all these components. First, we evaluate the model for a case study in Hyytiälä, Finland, and find that it is able to well reproduce the observed dynamics and gas-phase chemistry. We show that the exchange of organic aerosol between the free troposphere and the boundary layer (entrainment) must be taken into account in order to explain the observed diurnal cycle in organic aerosol (OA) concentration. An examination of the budgets of organic aerosol and terpene concentration shows that the former is dominated by entrainment, while the latter is mainly driven by emission and chemical transformation. We systematically examine the role of the land surface, which governs both the surface energy balance partitioning and terpene-emissions, and the large-scale atmospheric process of vertical subsidence. Entrainment is especially important for the dilution of organic aerosol concentrations under conditions of dry soils and low terpene-emissions. Subsidence suppresses boundary layer growth while enhancing entrainment. Therefore it influences the relationship between organic aerosol and terpene-concentrations. Our findings indicate that the diurnal evolution of SOA in the boundary layer is the result of coupled effects of the land surface, dynamics of the atmospheric boundary layer, chemistry, and free troposphere conditions. This has potentially some consequences for the design of both field campaigns and large-scale modeling studies.

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Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-31527-2013 ◽

2013 ◽

Vol 13 (12) ◽

pp. 31527-31562 ◽

Cited By ~ 3

Author(s):

E. Blay-Carreras ◽

D. Pino ◽

A. Van de Boer ◽

O. De Coster ◽

C. Darbieu ◽

...

Keyword(s):

Boundary Layer ◽

Mixed Layer ◽

Convective Boundary Layer ◽

Large Scale ◽

Temporal Evolution ◽

Residual Layer ◽

Mixing Ratio ◽

Convective Boundary ◽

Previous Night ◽

Morning Transition

Abstract. Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted airplane systems and two aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the numerical simulations are inspired by some of these observations. The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable to model the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. The simulation shows a large increase of the entrainment buoyancy heat flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed and, for the studied day, it is necessary to be considered to reproduce the afternoon observations. Additionally, we investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.

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A New Mixing-Length Formulation for the Parameterization of Dry Convection: Implementation and Evaluation in a Mesoscale Model

Monthly Weather Review ◽

10.1175/mwr2808.1 ◽

2004 ◽

Vol 132 (11) ◽

pp. 2698-2707 ◽

Cited By ~ 11

Author(s):

J. Teixeira ◽

J. P. Ferreira ◽

P. M. A. Miranda ◽

T. Haack ◽

J. Doyle ◽

...

Keyword(s):

Boundary Layer ◽

Convective Boundary Layer ◽

Large Scale ◽

Mesoscale Model ◽

Climate Impact ◽

Layer Growth ◽

Mixing Length ◽

Convective Boundary ◽

Realistic Representation ◽

New Formulation

Abstract A realistic representation of the evolution of the dry convective boundary layer in mesoscale and large-scale atmospheric models has been an elusive goal for many years. In this paper the performance of a new mixing-length formulation for the dry convective boundary layer is evaluated in the context of the Coupled Ocean– Atmosphere Mesoscale Prediction System (COAMPS). In this new formulation, the mixing length is proportional to a time scale and to the square root of the turbulent kinetic energy. The model results are tested against observations from the Climate Impact of Changes in Land Use (CICLUS) field experiment in the south of Portugal. It is shown that COAMPS with the new formulation produces a more realistic simulation of the boundary layer growth. A data assimilation experiment performed with COAMPS shows that the improvements provided by the new formulation are significant, particularly in terms of the humidity vertical distribution. Finally, one-dimensional simulations are used to confirm that the new formulation provides more accurate results because of a more realistic representation of the entrainment and of the vertical mixing in general.

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Low-Level Cloud Variability over the Equatorial Cold Tongue in Observations and Models

Journal of Climate ◽

10.1175/jcli4073.1 ◽

2007 ◽

Vol 20 (8) ◽

pp. 1555-1570 ◽

Cited By ~ 20

Author(s):

David K. Mansbach ◽

Joel R. Norris

Keyword(s):

Boundary Layer ◽

Surface Temperature ◽

Climate Models ◽

Cloud Amount ◽

Global Climate Models ◽

Geophysical Fluid Dynamics Laboratory ◽

Cold Tongue ◽

Multilinear Regression ◽

Near Surface ◽

Low Level

Abstract Examination of cloud and meteorological observations from satellite, surface, and reanalysis datasets indicates that monthly anomalies in low-level cloud amount and near-surface temperature advection are strongly negatively correlated on the southern side of the equatorial Pacific cold tongue. This inverse correlation occurs independently of relationships between cloud amount and sea surface temperature (SST) or lower tropospheric static stability (LTS), and the combination of advection plus SST or LTS explains significantly more interannual cloud variability in a multilinear regression than does SST or LTS alone. Warm anomalous advection occurs when the equatorial cold tongue is well defined and the southeastern Pacific trade winds bring relatively warm air over colder water. Ship meteorological reports and soundings show that the atmospheric surface layer becomes stratified under these conditions, thus inhibiting the upward mixing of moisture needed to sustain cloudiness against subsidence and entrainment drying. Cold anomalous advection primarily occurs when the equatorial cold tongue is weak or absent and the air–sea temperature difference is substantially negative. These conditions favor a more convective atmospheric boundary layer, greater cloud amount, and less frequent occurrence of clear sky. Examination of output from global climate models developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and the National Center for Atmospheric Research (NCAR) indicates that both models generally fail to simulate the cloud–advection relationships observed on the northern and southern sides of the equatorial cold tongue. Although the GFDL atmosphere model does reproduce the expected signs of cloud-advection correlations when forced with prescribed historical SST variations, it does not consistently do so when coupled to an ocean model. The NCAR model has difficulty reproducing the observed correlations in both atmosphere-only and coupled versions. This suggests that boundary layer cloud parameterizations could be improved through better representation of the effects of advection over varying SST.

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