Do Undiluted Convective Plumes Exist in the Upper Tropical Troposphere?

2010 ◽  
Vol 67 (2) ◽  
pp. 468-484 ◽  
Author(s):  
David M. Romps ◽  
Zhiming Kuang
Keyword(s):  
Kinetic Energy ◽  
Deep Convection ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Heat Of Fusion ◽  
Cloud Base ◽  
Base Temperature ◽  
Neutral Buoyancy ◽  
Environmental Air ◽  
Ice Phase

Abstract Using a passive tracer, entrainment is studied in cloud-resolving simulations of deep convection in radiative–convective equilibrium. It is found that the convective flux of undiluted parcels decays with height exponentially, indicating a constant probability per vertical distance of mixing with environmental air. This probability per distance is sufficiently large that undiluted updrafts are negligible above a height of 4–5 km and virtually absent above 10 km. These results are shown to be independent of the horizontal grid size within the range of 3.2 km to 100 m. Plumes that do reach the tropopause are found to be highly diluted. An equivalent potential temperature is defined that is exactly conserved for all reversible adiabatic transformations, including those with ice. Using this conserved variable, it is shown that the latent heat of fusion (from both freezing and deposition) causes only a small increase in the level of neutral buoyancy near the tropopause. In fact, when taken to sufficiently low pressures, a parcel with an ice phase ends up colder than it would without an ice phase. Nevertheless, the contribution from fusion to a parcel’s kinetic energy is quite large. Using an ensemble of tracers, information is encoded in parcels at the cloud base and decoded where the parcel is observed in the free troposphere. Using this technique, clouds at the tropopause are diagnosed for their cloud-base temperature, specific humidity, and vertical velocity. Using these as the initial values for a Lagrangian parcel model, it is shown that fusion provides the kinetic energy required for diluted parcels to reach the tropopause.

scholarly journals Triggering Deep Convection with a Probabilistic Plume Model

2014 ◽  
Vol 71 (11) ◽  
pp. 3881-3901 ◽  
Author(s):  
Fabio D’Andrea ◽  
Pierre Gentine ◽  
Alan K. Betts ◽  
Benjamin R. Lintner
Keyword(s):  
Climate Models ◽  
Proper Time ◽  
Deep Convection ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Cloud Base ◽  
Moist Convection ◽  
Density Currents ◽  
Shallow Convection ◽  
Plume Model

Abstract A model unifying the representation of the planetary boundary layer and dry, shallow, and deep convection, the probabilistic plume model (PPM), is presented. Its capacity to reproduce the triggering of deep convection over land is analyzed in detail. The model accurately reproduces the timing of shallow convection and of deep convection onset over land, which is a major issue in many current general climate models. PPM is based on a distribution of plumes with varying thermodynamic states (potential temperature and specific humidity) induced by surface-layer turbulence. Precipitation is computed by a simple ice microphysics, and with the onset of precipitation, downdrafts are initiated and lateral entrainment of environmental air into updrafts is reduced. The most buoyant updrafts are responsible for the triggering of moist convection, causing the rapid growth of clouds and precipitation. Organization of turbulence in the subcloud layer is induced by unsaturated downdrafts, and the effect of density currents is modeled through a reduction of the lateral entrainment. The reduction of entrainment induces further development from the precipitating congestus phase to full deep cumulonimbus. Model validation is performed by comparing cloud base, cloud-top heights, timing of precipitation, and environmental profiles against cloud-resolving models and large-eddy simulations for two test cases. These comparisons demonstrate that PPM triggers deep convection at the proper time in the diurnal cycle and produces reasonable precipitation. On the other hand, PPM underestimates cloud-top height.

Composited structure of shallow cumulus clouds

2021 ◽  
Author(s):  
Chris Holloway ◽  
Jian-Feng Gu ◽  
Bob Plant ◽  
Todd Jones
Keyword(s):  
Kinetic Energy ◽  
Vertical Velocity ◽  
Inversion Layer ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Theoretical Models ◽  
Turbulence Kinetic Energy ◽  
Cloud Base ◽  
Asymmetric Structures ◽  
Negatively Buoyant

<div> <div> <div> <div> <p>The normalized distributions of thermodynamic and dynamical variables both within and outside shallow clouds are investigated through a composite algorithm using large eddy simulation of the BOMEX case. The normalized magnitude is maximum near cloud center and decreases outwards. While relative humidity (RH) and cloud liquid water (<em>q<sub>l </sub></em>) decrease smoothly to match the environment, the vertical velocity, virtual potential temperature (<em>θ<sub>v </sub></em>) and potential temperature (<em>θ</em>) perturbations have more complicated behaviour towards the cloud boundary. Below the inversion layer, <em>θ<sub>v</sub></em> becomes <span>negative before the vertical velocity has turned from updraft to subsiding shell outside the cloud, indicating the presence of a transition zone where the updraft is negatively buoyant. Due to the downdraft outside the cloud and the enhanced horizontal turbulent mixing across the edge, the normalized turbulence kinetic energy (TKE) and horizontal turbulence kinetic energy (HTKE) decrease more slowly from the cloud center outwards than the thermodynamic variables. The distributions all present asymmetric structures in response to the vertical wind shear, with more negatively buoyant air, stronger downdrafts and larger TKE on the downshear side. We discuss several implications of the distributions for theoretical models and parameterizations. Positive buoyancy near cloud base is mostly due to </span><span>the virtual effect of water vapor, emphasising the role of moisture in triggering. The mean vertical velocity is found </span><span>to be approximately half the maximum vertical velocity within each cloud, providing a constraint on some models. Finally, products of normalized distributions for different variables are shown to be able to well represent the vertical heat and moisture fluxes, but they underestimate fluxes in the inversion layer because they do not capture cloud top downdrafts.</span></p> </div> </div> </div> </div>

scholarly journals GoAmazon2014/5 campaign points to deep-inflow approach to deep convection across scales

2018 ◽  
Vol 115 (18) ◽  
pp. 4577-4582 ◽  
Author(s):  
Kathleen A. Schiro ◽  
Fiaz Ahmed ◽  
Scott E. Giangrande ◽  
J. David Neelin
Keyword(s):  
Climate Models ◽  
Climate Model ◽  
Deep Convection ◽  
Potential Temperature ◽  
Strong Relationship ◽  
Field Campaign ◽  
Convective Systems ◽  
Tropical Oceans ◽  
Mesoscale Convective ◽  
Environmental Air

A substantial fraction of precipitation is associated with mesoscale convective systems (MCSs), which are currently poorly represented in climate models. Convective parameterizations are highly sensitive to the assumptions of an entraining plume model, in which high equivalent potential temperature air from the boundary layer is modified via turbulent entrainment. Here we show, using multiinstrument evidence from the Green Ocean Amazon field campaign (2014–2015; GoAmazon2014/5), that an empirically constrained weighting for inflow of environmental air based on radar wind profiler estimates of vertical velocity and mass flux yields a strong relationship between resulting buoyancy measures and precipitation statistics. This deep-inflow weighting has no free parameter for entrainment in the conventional sense, but to a leading approximation is simply a statement of the geometry of the inflow. The structure further suggests the weighting could consistently apply even for coherent inflow structures noted in field campaign studies for MCSs over tropical oceans. For radar precipitation retrievals averaged over climate model grid scales at the GoAmazon2014/5 site, the use of deep-inflow mixing yields a sharp increase in the probability and magnitude of precipitation with increasing buoyancy. Furthermore, this applies for both mesoscale and smaller-scale convection. Results from reanalysis and satellite data show that this holds more generally: Deep-inflow mixing yields a strong precipitation–buoyancy relation across the tropics. Deep-inflow mixing may thus circumvent inadequacies of current parameterizations while helping to bridge the gap toward representing mesoscale convection in climate models.

scholarly journals Why Does Deep Convection Have Different Sensitivities to Temperature Perturbations in the Lower versus Upper Troposphere?

2018 ◽  
Vol 76 (1) ◽  
pp. 27-41 ◽  
Author(s):  
Yang Tian ◽  
Zhiming Kuang
Keyword(s):  
Water Content ◽  
Vertical Velocity ◽  
Liquid Water ◽  
Deep Convection ◽  
Liquid Water Content ◽  
Cloud Base ◽  
Base Temperature ◽  
Upper Troposphere ◽  
Convective Parameterization ◽  
Parameterization Schemes

Abstract Previous studies have documented that deep convection responds more strongly to above-the-cloud-base temperature perturbations in the lower troposphere than to those in the upper troposphere, a behavior that is important to the dynamics of large-scale moist flows, such as convectively coupled waves. A number of factors may contribute to this differing sensitivity, including differences in buoyancy, vertical velocity, and/or liquid water content in cloud updrafts in the lower versus upper troposphere. Quantifying the contributions from these factors can help to guide the development of convective parameterization schemes. We tackle this issue by tracking Lagrangian particles embedded in cloud-resolving simulations within a linear response framework. The results show that both the differences in updraft buoyancy and vertical velocity play a significant role, with the vertical velocity being the more important, and the effect of liquid water content is only secondary compared to the other two factors. These results indicate that cloud updraft vertical velocities need to be correctly modeled in convective parameterization schemes in order to properly account for the differing convective sensitivities to temperature perturbations at different heights of the free troposphere.

Diurnal differences in tropical maritime anvil cloud evolution

2021 ◽  
pp. 1-66
Author(s):  
Adam B. Sokol ◽  
Casey J. Wall ◽  
Dennis L. Hartmann ◽  
Peter N. Blossey
Keyword(s):  
Diurnal Cycle ◽  
Deep Convection ◽  
Radiative Heating ◽  
Cloud Base ◽  
Diurnal Difference ◽  
Size Number ◽  
Cloud Resolving Model ◽  
Ice Crystal Size ◽  
Environmental Air ◽  
Cloud Evolution

Abstract Satellite observations of tropical maritime convection indicate an afternoon maximum in anvil cloud fraction that cannot be explained by the diurnal cycle of deep convection peaking at night. We use idealized cloud-resolving model simulations of single anvil cloud evolution pathways, initialized at different times of the day, to show that tropical anvil clouds formed during the day are more widespread and longer lasting than those formed at night. This diurnal difference is caused by shortwave radiative heating, which lofts and spreads anvil clouds via a mesoscale circulation that is largely absent at night, when a different, longwave-driven circulation dominates. The nighttime circulation entrains dry environmental air that erodes cloud top and shortens anvil lifetime. Increased ice nucleation in more turbulent nighttime conditions supported by the longwave cloud top cooling and cloud base heating dipole cannot overcompensate for the effect of diurnal shortwave radiative heating. Radiative-convective equilibrium simulations with a realistic diurnal cycle of insolation confirm the crucial role of shortwave heating in lofting and sustaining anvil clouds. The shortwave-driven mesoscale ascent leads to daytime anvils with larger ice crystal size, number concentration, and water content at cloud top than their nighttime counterparts.

scholarly journals CAPE in Tropical Cyclones

2012 ◽  
Vol 69 (8) ◽  
pp. 2452-2463 ◽  
Author(s):  
John Molinari ◽  
David M. Romps ◽  
David Vollaro ◽  
Leon Nguyen
Keyword(s):  
Tropical Cyclones ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Specific Humidity ◽  
Heat Of Fusion ◽  
Entrainment Rate ◽  
The Core ◽  
Conditional Instability ◽  
Abundance And Distribution ◽  
The Vertical Distribution

Abstract Convective available potential energy (CAPE) and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the NOAA Gulfstream-IV aircraft. Calculations were done with and without the effects of condensate loading, entrainment, and the latent heat of fusion. CAPE showed larger values downshear than upshear within 400 km of the center, consistent with the observed variation of convective intensity. The larger downshear CAPE arose from (i) higher surface specific humidity, (ii) lower midtropospheric temperature, and, for entraining CAPE, (iii) larger free-tropospheric relative humidity. Reversible CAPE had only one-half the magnitude of pseudoadiabatic CAPE. As shown previously, reversible CAPE with fusion closely resembled pseudoadiabatic CAPE without fusion. Entrainment had the most dramatic impact. Entraining CAPE was consistent with the observed radial distribution of convective intensity, displaying the largest values downshear at inner radii. Without entrainment, downshear CAPE was smallest in the core and increased outward to the 600-km radius. The large number of sondes allowed the examination of soundings at the 90th percentile of conditional instability, which reflect the conditions leading to the most vigorous updrafts. Observations of convection in tropical cyclones prescribe the correct method for calculating this conditional instability. In particular, the abundance and distribution of vigorous deep convection is most accurately reflected by calculating CAPE with condensate retention and a fractional entrainment rate in the range of 5%–10% km−1.

scholarly journals Supersaturation, buoyancy, and moist convective dynamics

2021 ◽  
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Cloud Model ◽  
Deep Convection ◽  
Potential Temperature ◽  
Lower Troposphere ◽  
Entrainment Rate ◽  
Latent Heating ◽  
Microphysics Scheme ◽  
Neutral Buoyancy ◽  
Factors Affecting ◽  
Freezing Level

Abstract. Motivated by recent discussions concerning differences of convective dynamics in polluted and pristine environments, the so-called convective invigoration in particular, this paper provides an analysis of factors affecting convective updraft buoyancy, such as the in-cloud supersaturation, condensate and precipitation loading, and entrainment. We use the deep convective period from simulations of daytime convection development over land discussed in our previous publications. An entraining parcel framework in used in the theoretical analysis. We show that for the specific case considered here finite (positive) supersaturation noticeably reduces pseudo-adiabatic parcel buoyancy and cumulative CAPE in the lower troposphere. This comes from keeping a small fraction of the water vapor in a supersaturated state and thus reducing the latent heating. Such a lower-tropospheric impact is comparable to the effects of the condensate loading and entrainment in the idealized parcel framework. For the entire tropospheric depth, loading and entrainment have a much more significant impact on the total CAPE. For instance, an increase in the fractional entrainment rate from 0.05 km−1 to 0.3 km−1 reduces the theoretical level of neutral buoyancy from the upper to the middle troposphere and CAPE by a factor of 4. For the cloud model results, we compare ensemble simulations applying either a bulk microphysics scheme with saturation adjustment or a more comprehensive double-moment scheme with supersaturation prediction. The diagnosed bulk fractional entrainment rate, independent of the microphysics scheme applied in the simulations, is either 0.13 or 0.20 km−1 depending on whether we consider profiles of the upper end of the percentile range or of the mean in-cloud equivalent potential temperature. We compare deep convective updrafts, buoyancies, and supersaturations from all ensembles. In agreement with the parcel analysis, the saturation adjustment scheme provides noticeably stronger updrafts in the lower troposphere. For the simulations predicting supersaturation, there are small differences between pristine and polluted conditions below the freezing level that are difficult to explain by standard analysis of the in-cloud buoyancy components. By applying the piggybacking technique, we show that the lower-tropospheric buoyancy differences between pristine and polluted simulations come from a combination of temperature (i.e., latent heating) and condensate loading differences that work together to make polluted buoyancies and updraft velocities slightly larger when compared to their pristine analogues. Overall, the effects are rather small and contradict previous claims of a significant invigoration of deep convection in polluted environments.

Using Digital Cloud Photogrammetry to Characterize the Onset and Transition from Shallow to Deep Convection over Orography

2006 ◽  
Vol 134 (9) ◽  
pp. 2527-2546 ◽  
Author(s):  
Joseph A. Zehnder ◽  
Liyan Zhang ◽  
Dianne Hansford ◽  
Anshuman Radzan ◽  
Nancy Selover ◽  
...  
Keyword(s):  
Deep Convection ◽  
Potential Temperature ◽  
Threshold Value ◽  
Surface Fluxes ◽  
Cloud Base ◽  
Equivalent Potential Temperature ◽  
Shallow Convection ◽  
Thermodynamic Structure ◽  
Automated Method ◽  
Equivalent Potential

Abstract An automated method for segmenting digital images of orographic cumulus and a simple metric for characterizing the transition from shallow to deep convection are presented. The analysis is motivated by the hypothesis that shallow convection conditions the atmosphere for further deep convection by moistening it and preventing the evaporation of convective turrets through the entrainment of dry air. Time series of convective development are compared with sounding and surface data for 6 days during the summer of 2003. The observations suggest the existence of a threshold for the initiation of shallow convection based on the surface equivalent potential temperature and the saturated equivalent potential temperature above the cloud base. This criterion is similar to that controlling deep convection over the tropical oceans. The subsequent evolution of the convection depends on details of the environment. Surface fluxes of sensible and latent heat, along with the transport of boundary layer air by upslope flow, increase the surface equivalent potential temperature and once the threshold value is exceeded, shallow convection begins. The duration of the shallow convection period and growth rate of the deep convection are determined by the kinematic and thermodynamic structure of the mid- and upper troposphere.

scholarly journals Latent Heating and Mixing due to Entrainment in Tropical Deep Convection

2014 ◽  
Vol 71 (2) ◽  
pp. 816-832 ◽  
Author(s):  
Clayton J. McGee ◽  
Susan C. van den Heever
Keyword(s):  
High Resolution ◽  
Deep Convection ◽  
Potential Temperature ◽  
Latent Heating ◽  
Freezing Level ◽  
Tropical Deep Convection ◽  
Cloud Resolving Model ◽  
Nested Grid ◽  
Environmental Air ◽  
Convective Cells

Abstract Recent studies have noted the role of latent heating above the freezing level in reconciling Riehl and Malkus' hot tower hypothesis (HTH) with evidence of diluted tropical deep convective cores. This study evaluates recent modifications to the HTH through Lagrangian trajectory analysis of deep convective cores in an idealized, high-resolution cloud-resolving model (CRM) simulation that uses a sophisticated two-moment microphysical scheme. A line of tropical convective cells develops within a finer nested grid whose boundary conditions are obtained from a large-domain CRM simulation approaching radiative convective equilibrium (RCE). Microphysical impacts on latent heating and equivalent potential temperature (θe) are analyzed along trajectories ascending within convective regions of the high-resolution nested grid. Changes in θe along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. The simulations demonstrate that mixing with dry environmental air decreases θe along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase θe above the freezing level. Latent heating contributions along trajectories from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation are also quantified. Finally, the source regions of trajectories reaching the upper troposphere are identified. Much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, but the strongest updrafts are composed of air from closer to the surface. The importance of both boundary layer and midlevel inflow in moist environments is underscored in this study.

scholarly journals The Vertical City Weather Generator (VCWG v1.3.2)

2021 ◽  
Vol 14 (2) ◽  
pp. 961-984
Author(s):  
Mohsen Moradi ◽  
Benjamin Dyer ◽  
Amir Nazem ◽  
Manoj K. Nambiar ◽  
M. Rafsan Nahian ◽  
...  
Keyword(s):  
Wind Speed ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Potential Temperature ◽  
Building Energy ◽  
Weather Generator ◽  
Specific Humidity ◽  
Vertical Profiles ◽  
Vertical Diffusion ◽  
Average Bias

Abstract. The Vertical City Weather Generator (VCWG) is a computationally efficient urban microclimate model developed to predict temporal and vertical variation of potential temperature, wind speed, specific humidity, and turbulent kinetic energy. It is composed of various sub-models: a rural model, an urban vertical diffusion model, a radiation model, and a building energy model. Forced with weather data from a nearby rural site, the rural model is used to solve for the vertical profiles of potential temperature, specific humidity, and friction velocity at 10 m a.g.l. The rural model also calculates a horizontal pressure gradient. The rural model outputs are applied to a vertical diffusion urban microclimate model that solves vertical transport equations for potential temperature, momentum, specific humidity, and turbulent kinetic energy. The urban vertical diffusion model is also coupled to the radiation and building energy models using two-way interaction. The aerodynamic and thermal effects of urban elements, surface vegetation, and trees are considered. The predictions of the VCWG model are compared to observations of the Basel UrBan Boundary Layer Experiment (BUBBLE) microclimate field campaign for 8 months from December 2001 to July 2002. The model evaluation indicates that the VCWG predicts vertical profiles of meteorological variables in reasonable agreement with the field measurements. The average bias, root mean square error (RMSE), and R2 for potential temperature are 0.25 K, 1.41 K, and 0.82, respectively. The average bias, RMSE, and R2 for wind speed are 0.67 m s−1, 1.06 m s−1, and 0.41, respectively. The average bias, RMSE, and R2 for specific humidity are 0.00057 kg kg−1, 0.0010 kg kg−1, and 0.85, respectively. In addition, the average bias, RMSE, and R2 for the urban heat island (UHI) are 0.36 K, 1.2 K, and 0.35, respectively. Based on the evaluation, the model performance is comparable to the performance of similar models. The performance of the model is further explored to investigate the effects of urban configurations such as plan and frontal area densities, varying levels of vegetation, building energy configuration, radiation configuration, seasonal variations, and different climate zones on the model predictions. The results obtained from the explorations are reasonably consistent with previous studies in the literature, justifying the reliability and computational efficiency of VCWG for operational urban development projects.

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