On the Growth of Layers of Nonprecipitating Cumulus Convection

2007 ◽  
Vol 64 (8) ◽  
pp. 2916-2931 ◽  
Author(s):  
Bjorn Stevens
Keyword(s):  
Boundary Layer ◽  
Inversion Layer ◽  
Bowen Ratio ◽  
Cloud Layer ◽  
Buoyancy Flux ◽  
Cloud Base ◽  
Moist Convection ◽  
Wide Range ◽  
Surface Buoyancy ◽  
Convective Layer

A prototype problem of a nonprecipitating convective layer growing into a layer of uniform stratification and exponentially decreasing humidity is introduced to study the mechanism by which the cumulus-topped boundary layer grows. The problem naturally admits the surface buoyancy flux, outer layer stratification, and moisture scale as governing parameters. Large-eddy simulations show that many of the well-known properties of the cumulus-topped boundary layer (including a well-mixed subcloud layer, a cloud-base transition layer, a conditionally unstable cloud layer, and an inversion layer) emerge naturally in the simulations. The simulations also quantify the differences between nonprecipitating moist convection and its dry counterpart. Whereas dry penetrative convective layers grow proportionally to the square root of time (diffusively) the cumulus layers grow proportionally to time (ballistically). The associated downward transport of warm, dry air results in a significant decrease in the surface Bowen ratio. The linear-in-time growth of the cloud layer is shown to result from the transport and subsequent evaporation of liquid water into the inversion layer. This process acts as a sink of buoyancy, which acts to imbue the free troposphere with the properties of the cloud layer. A simple model, based on this mechanism, and formulated in terms of an effective dry buoyancy flux (which is constrained by the subcloud layer’s similarity to a dry convective layer), is shown to provide good predictions of the growth of the layer across a wide range of governing parameters.

scholarly journals A Closer Look at Boundary Layer Inversion in Large-Eddy Simulations and Bulk Models: Buoyancy-Driven Case

2015 ◽  
Vol 72 (2) ◽  
pp. 728-749 ◽  
Author(s):  
Pierre Gentine ◽  
Gilles Bellon ◽  
Chiel C. van Heerwaarden
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Weather Prediction ◽  
Inversion Layer ◽  
Large Eddy Simulations ◽  
Buoyancy Flux ◽  
Convective Boundary ◽  
Strong Inversion ◽  
Wide Range ◽  
Large Eddy

Abstract The inversion layer (IL) of a clear-sky, buoyancy-driven convective boundary layer is investigated using large-eddy simulations covering a wide range of convective Richardson numbers. A new model of the IL is suggested and tested. The model performs better than previous first-order models of the entrainment and provides physical insights into the main controls of the mixed-layer and IL growths. A consistent prognostic equation of the IL growth is derived, with explicit dependence on the position of the minimum buoyancy flux, convective Richardson number, and relative stratification across the inversion G. The IL model expresses the interrelationship between the position and magnitude of the minimum buoyancy flux and inversion-layer depth. These relationships emphasize why zero-order jump models of the convective boundary layer perform well under a strong inversion and show that these models miss the additional parameter G to fully characterize the entrainment process under a weak inversion. Additionally, the position of the minimum buoyancy flux within the new IL model is shown to be a key component of convective boundary layer entrainment. The new IL model is sufficiently simple to be used in numerical weather prediction or general circulation models as a way to resolve the IL in a low-vertical-resolution model.

scholarly journals Is There a Diurnal Cycle in the Summer Cloud-Capped Arctic Boundary Layer?

2007 ◽  
Vol 64 (11) ◽  
pp. 3970-3986 ◽  
Author(s):  
Michael Tjernström
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Inversion Layer ◽  
Cloud Layer ◽  
The Arctic ◽  
Cloud Base ◽  
Marine Stratocumulus ◽  
Diurnal Variability ◽  
Near Surface ◽  
Strong Control

Abstract Data from the Arctic Ocean Experiment 2001 (AOE-2001) are used to study the vertical structure and diurnal cycle of the summertime central Arctic cloud-capped boundary layer. Mean conditions show a shallow stratocumulus-capped boundary layer, with a nearly moist neutrally stratified cloud layer, although cloud tops often penetrated into the stable inversion. The subcloud layer was more often stably stratified. Conditions near the surface were relatively steady, with a strong control on temperature and moisture by the melting ice surface. A statistically significant diurnal cycle was found in many parameters, although weak in near-surface temperature and moisture. Near-surface wind speed and direction and friction velocity had a pronounced cycle, while turbulent kinetic energy showed no significant diurnal variability. The cloud layer had the most pronounced diurnal variability, with lowest cloud-base height midday followed by enhanced drizzle and temporarily higher cloud-top heights in the afternoon. This is opposite to the cycle found in midlatitude or subtropical marine stratocumulus. The cloud layer was warmest (coolest) and more (less) stably stratified midafternoon (midmorning), coinciding with the coolest (warmest) but least (most) stably stratified capping inversion layer. It is speculated that drizzle is important in regulating the diurnal variability in the cloud layer, facilitated by enhanced midday mixing due to a differential diurnal variability in cloud and subcloud layer stability. Changing the Arctic aerosol climate could change these clouds to a more typical “marine stratocumulus structure,” which could act as a negative feedback on Arctic warming.

scholarly journals The diurnal stratocumulus-to-cumulus transition over land in southern West Africa

2020 ◽  
Vol 20 (5) ◽  
pp. 2735-2754 ◽  
Author(s):  
Xabier Pedruzo-Bagazgoitia ◽  
Stephan R. de Roode ◽  
Bianca Adler ◽  
Karmen Babić ◽  
Cheikh Dione ◽  
...  
Keyword(s):  
Boundary Layer ◽  
West Africa ◽  
Wind Shear ◽  
Numerical Experiments ◽  
Inversion Layer ◽  
Surface Fluxes ◽  
Cloud Layer ◽  
Cloud Base ◽  
Scale Models ◽  
The Impact

Abstract. The misrepresentation of the diurnal cycle of boundary layer clouds by large-scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary to improve its representation. To this end, the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) measurement campaign gathered a unique dataset of observations of the frequent stratocumulus-to-cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using large eddy simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget. Finally, we study the impact of mean wind and wind shear at the cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from the surface to the cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for the maintenance leading to a net thickening of the cloud layer of about 18 g m−2 h−1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at the cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at the cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence, wind shear at the cloud top accelerates the breakup and transition by about 2 h. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger-scale models.

scholarly journals The diurnal stratocumulus-to-cumulus transition over land

2019 ◽  
Author(s):  
Xabier Pedruzo-Bagazgoitia ◽  
Stephan R. de Roode ◽  
Bianca Adler ◽  
Karmen Babić ◽  
Cheikh Dione ◽  
...  
Keyword(s):  
Boundary Layer ◽  
West Africa ◽  
Wind Shear ◽  
Numerical Experiments ◽  
Inversion Layer ◽  
Surface Fluxes ◽  
Cloud Layer ◽  
Cloud Base ◽  
Scale Models ◽  
The Impact

Abstract. The misrepresentation of the diurnal cycle of boundary-layer clouds by large scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary to improve its representation. To this end, the DACCIWA measurement campaign gathered a unique dataset of observations of the frequent stratocumulus to cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using Large Eddy Simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget. Finally, we study the impact of mean wind and wind shear at cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from surface to cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for the maintenance leading to a net thickening of the cloud layer of about 18 g m−2 h−1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence wind shear at cloud top accelerates the breakup and transition by about 2 hours. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger scale models.

scholarly journals Transport and chemical transformations influenced by shallow cumulus over land

2005 ◽  
Vol 5 (5) ◽  
pp. 8811-8849
Author(s):  
J. Vilà-Guerau de Arellano ◽  
S.-W. Kim ◽  
M. C. Barth ◽  
E. G. Patton
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Cloud Layer ◽  
Cloud Formation ◽  
Cloud Base ◽  
Chemical Transformations ◽  
Mixing Ratios ◽  
Shallow Cumulus ◽  
Photolysis Rates ◽  
Good Agreement

Abstract. The distribution and evolution of reactive species in a boundary layer characterized by the presence of shallow cumulus over land is studied by means of two large-eddy simulation models: the NCAR and WUR codes. The study focuses on two physical processes that can influence the chemistry: the enhancement of the vertical transport by the buoyant convection associated with cloud formation and the perturbation of the photolysis rates below, in and above the clouds. It is shown that the dilution of the reactant mixing ratio caused by the deepening of the atmospheric boundary layer is an important process and that it can decrease reactant mixing ratios by 10 to 50 percent compared to very similar conditions but with no cloud formation. Additionally, clouds transport chemical species to higher elevations in the boundary layer compared to the case with no clouds which influences the reactant mixing ratios of the nocturnal residual layers following the collapse of the daytime boundary layer. Estimates of the rate of reactant transport based on the calculation of the integrated flux divergence range from to −0.2 ppb hr−1 to −1 ppb hr−1, indicating a net loss of sub-cloud layer air transported into the cloud layer. A comparison of this flux to a parameterized mass flux shows good agreement in mid-cloud, but at cloud base the parameterization underestimates the mass flux. Scattering of radiation by cloud drops perturbs photolysis rates. It is found that these perturbed photolysis rates substantially (10–40%) affect mixing ratios locally (spatially and temporally), but have little effect on mixing ratios averaged over space and time. We find that the ultraviolet radiance perturbation becomes more important for chemical transformations that react with a similar order time scale as the turbulent transport in clouds. Finally, the detailed intercomparison of the LES results shows very good agreement between the two codes when considering the evolution of the reactant mean, flux and (co-)variance vertical profiles.

Penetrative turbulent convection into a rotating two-layer fluid

1996 ◽  
Vol 321 ◽  
pp. 299-313 ◽  
Author(s):  
Siavash Narimousa
Keyword(s):  
Convective Flow ◽  
Bottom Layer ◽  
Turbulent Convection ◽  
Rossby Number ◽  
Coriolis Parameter ◽  
Density Interface ◽  
Fluid Systems ◽  
Wide Range ◽  
Surface Buoyancy ◽  
Convective Layer

Turbulent convection into stratified two-layer fluid systems in the presence of rotation has been investigated in a cylindrical laboratory tank. For a wide range of conditions the vertical stability of the flow depends only on the Richardson number Ri = g′h0/(B0R)2/3 (here, g′ = gδ ρi/ρ0 is the jump in the reduced gravity across the density interface, δ ρi is the jump in density itself, h0 is the depth of the top layer, B0 is the surface buoyancy flux and R is the radius of the source). We have found that for values of Ri greater than a critical value of Ric ≈ 11, the convective flow did not penetrate through the density interface, regardless of the values of the convective Rossby number Ro* = (B0/f3h02)1/2 of the flow (here f is the Coriolis parameter). In this case after the convective layer interacted with the density interface the mixed fluid, of intermediate density, propagated radially along the interface in the form of an intrusion. Later, if Ro* was less than approximately 5, mesoscale vortices with mean diameter D ≈ 8(RoR)2/3 and maximum swirl velocity v ≈ (B0R)1/3 were generated at the edge of this propagating front (here RoR = (B0/f3R2)1/2 is the Rossby number based on R). When Ri was less than 11, the convective flow eventually penetrated through the density interface and into the bottom layer. This occurred through the formation of discrete ‘convective holes’ beneath the source. At large values of Ro*, and at early times, one turbulent plume penetrated into the bottom layer from each hole. Later the initial holes coalesced to create one large lesion beneath the source.

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

2005 ◽  
Vol 18 (7) ◽  
pp. 934-951 ◽  
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.

scholarly journals Atmosphere Boundary Layer Height (ABLH) Determination under Multiple-Layer Conditions Using Micro-Pulse Lidar

2019 ◽  
Vol 11 (3) ◽  
pp. 263 ◽  
Author(s):  
Ruijun Dang ◽  
Yi Yang ◽  
Hong Li ◽  
Xiao-Ming Hu ◽  
Zhiting Wang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Correlation Coefficient ◽  
Cloud Layer ◽  
Horizontal Wind ◽  
Cloud Base ◽  
Accurate Estimation ◽  
Lidar Measurement ◽  
Height Distribution ◽  
Layer Height ◽  
Boundary Layer Height

Accurate estimation of the atmospheric boundary layer height (ABLH) is critically important and it mainly relies on the detection of the vertical profiles of atmosphere variables (temperature, humidity,’ and horizontal wind speed) or aerosols. Aerosol Lidar is a powerful remote sensing instrument frequently used to retrieve ABLH through the detection of the vertical distribution of aerosol concentration. A challenge is that cloud, residual layer (RL), and local signal structure seriously interfere with the lidar measurement of ABLH. A new objective technique presenting as giving a top limiter altitude is introduced to reduce the interference of RL and cloud layer on ABLH determination. Cloud layers are identified by looking for the rapid increase and sharp attenuation of the signal combined with the relative increase in the signal. The cloud layers weather overlay are classified or are decoupled from the ABL by analyzing the continuity of the signal below the cloud base. For cloud layer capping of the ABL, the limiter is determined to be the altitude where a positive signal gradient first occurs above the cloud upper edge. For a cloud that is decoupled from the ABL, the cloud base is considered to be the altitude limiter. For RL in the morning, the altitude limiter is the greatest positive gradient altitude below the RL top. The ABLH will be determined below the top limiter altitude using Haar wavelet (HM) and the curve fitting method (CFM). Besides, the interference of local signal noise is eliminated through consideration of the temporal continuity. While comparing the lidar-determined ABLH by HM (or CFM) and nearby radiosonde measurements of the ABLH, a reasonable concordance is found with a correlation coefficient of 0.94 (or 0.96) and 0.79 (or 0.74), presenting a mean of the relative absolute differences with respect to radiosonde measurements of 10.5% (or 12.3%) and 22.3% (or 17.2%) for cloud-free and cloudy situations, respectively. The diurnal variations in the ABLH determined from HM and CFM on four selected cases show good agreement with a mean correlation coefficient higher than 0.99 and a mean absolute bias of 0.22 km. Also, the determined diurnal ABLH are consistent with surface turbulent kinetic energy (TKE) combined with the time-height distribution of the equivalent potential temperature.

Soil Moisture Feedbacks on Convection Triggers: The Role of Soil–Plant Hydrodynamics

2009 ◽  
Vol 10 (1) ◽  
pp. 96-112 ◽  
Author(s):  
Mario Siqueira ◽  
Gabriel Katul ◽  
Amilcare Porporato
Keyword(s):  
Boundary Layer ◽  
Soil Moisture ◽  
Water Vapor ◽  
Negative Feedback ◽  
Stomatal Closure ◽  
Inversion Layer ◽  
Self Regulation ◽  
Surface Fluxes ◽  
Plant Hydraulics ◽  
Wide Range

Abstract The linkages between soil moisture dynamics and convection triggers, defined here as the first crossing between the boundary layer height (hBL) and lifting condensation level (hLCL), are complicated by a large number of interacting processes occurring over a wide range of space and time scales. To progress on this problem, a soil–plant hydrodynamics model was coupled to a simplified ABL budget to explore the feedback of soil moisture on convection triggers. The soil–plant hydraulics formulation accounted mechanistically for features such as root water uptake, root water redistribution, and midday stomatal closure, all known to affect diurnal cycles of surface fluxes and, consequently, ABL growth. The ABL model considered the convective boundary layer as a slab with a discontinuity at the inversion layer. The model was parameterized using the wealth of data already collected for a maturing Loblolly pine plantation situated in the southeastern United States. A 30-day dry-down simulation was used to investigate the possible feedback mechanisms between soil moisture and convective rainfall triggers. Previous studies, which made use of surface flux measurements to drive an ABL model, have postulated that a negative feedback was possible, which could award the ecosystem with some degree of self-regulation of its water status. According to model simulation results here, this negative feedback is unlikely. However, drastic changes in external water sources to the ABL are needed for triggering convection when soil moisture is depleted. The apparent negative feedback originated from a decoupling between the water vapor sources needed to produce convection triggers and surface water vapor fluxes.

scholarly journals Scaling Laws for the Heterogeneously Heated Free Convective Boundary Layer

2014 ◽  
Vol 71 (11) ◽  
pp. 3975-4000 ◽  
Author(s):  
Chiel C. van Heerwaarden ◽  
Juan Pedro Mellado ◽  
Alberto De Lozar
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Convective Boundary Layer ◽  
Scaling Laws ◽  
Fixed Ratio ◽  
Buoyancy Flux ◽  
Convective Boundary ◽  
Optimal State ◽  
Surface Buoyancy ◽  
Surface Buoyancy Flux

Abstract The heterogeneously heated free convective boundary layer (CBL) is investigated by means of dimensional analysis and results from large-eddy simulations (LES) and direct numerical simulations (DNS). The investigated physical model is a CBL that forms in a linearly stratified atmosphere heated from the surface by square patches with a high surface buoyancy flux. Each simulation has been run long enough to show the formation of a peak in kinetic energy, corresponding to the “optimal” heterogeneity size with strong secondary circulations, and the subsequent transition into a horizontally homogeneous CBL. Scaling laws for the time of the optimal state and transition and for the vertically integrated kinetic energy (KE) have been developed. The laws show that the optimal state and transition do not occur at a fixed ratio of the heterogeneity size to the CBL height. Instead, these occur at a higher ratio for simulations with increasing heterogeneity sizes because of the development of structures in the downward-moving air that grow faster than the CBL thickness. The moment of occurrence of the optimal state and transition are strongly related to the heterogeneity amplitude: stronger amplitudes result in an earlier optimal state and a later transition. Furthermore, a decrease in patch size combined with a compensating increase in patch surface buoyancy flux to maintain the energy input results in decreasing KE and a later transition. The simulations suggest that a CBL with a heterogeneity size smaller than the initial CBL height has less entrainment than a horizontally homogeneous CBL, whereas one with a larger heterogeneity size has more.

Sign in / Sign up

Export Citation Format

Share Document