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.

scholarly journals Supersaturation, buoyancy, and deep convection dynamics

2021 ◽  
Vol 21 (18) ◽  
pp. 13997-14018
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Cloud Model ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Lower Troposphere ◽  
Latent Heating ◽  
Microphysics Scheme ◽  
Factors Affecting ◽  
Freezing Level ◽  
Double Moment ◽  
Adjustment Scheme

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 is 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 convective available potential energy (cCAPE) 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 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 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. We compare deep convective updraft velocities, 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.

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.

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 A Cumulus Parameterization with State-Dependent Entrainment Rate. Part I: Description and Sensitivity to Temperature and Humidity Profiles

2010 ◽  
Vol 67 (7) ◽  
pp. 2171-2193 ◽  
Author(s):  
Minoru Chikira ◽  
Masahiro Sugiyama
Keyword(s):  
General Circulation ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Lower Troposphere ◽  
South Pacific Convergence Zone ◽  
Cumulus Parameterization ◽  
Cloud Base ◽  
Convergence Zone ◽  
Entrainment Rate ◽  
Surrounding Environment

Abstract A new cumulus parameterization is developed for which an entraining plume model is adopted. The lateral entrainment rate varies vertically depending on the surrounding environment. Two different formulations are examined for the rate. The cumulus ensemble is spectrally represented according to the updraft velocity at cloud base. Cloud-base mass flux is determined with prognostic convective kinetic energy closure. The entrainment rate tends to be large near cloud base because of the small updraft velocity near that level. Deep convection tends to be suppressed when convective available potential energy is small because of upward reduction of in-cloud moist static energy. Dry environmental air significantly reduces in-cloud humidity mainly because of the large entrainment rate in the lower troposphere, which leads to suppression of deep convection, consistent with observations and previous results of cloud-resolving models. The change in entrainment rate has the potential to influence cumulus convection through many feedbacks. The results of an atmospheric general circulation model are improved in both climatology and variability. A representation of the South Pacific convergence zone and the double intertropical convergence zone is improved. The moist Kelvin waves are represented without empirical triggering schemes with a reasonable equivalent depth. A spectral analysis shows a strong signal of the Madden–Julian oscillation. The scheme provides new insights and better understanding of the interaction between cumuli and the surrounding environment.

Life of a Six-Hour Hurricane

2009 ◽  
Vol 137 (1) ◽  
pp. 51-67 ◽  
Author(s):  
Kay L. Shelton ◽  
John Molinari
Keyword(s):  
Deep Convection ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Vertical Shear ◽  
Vortex Center ◽  
Dry Air ◽  
Sheared Flow ◽  
The Core ◽  
Equivalent Potential

Abstract Hurricane Claudette developed from a weak vortex in 6 h as deep convection shifted from downshear into the vortex center, despite ambient vertical wind shear exceeding 10 m s−1. Six hours later it weakened to a tropical storm, and 12 h after the hurricane stage a circulation center could not be found at 850 hPa by aircraft reconnaissance. At hurricane strength the vortex contained classic structure seen in intensifying hurricanes, with the exception of 7°–12°C dewpoint depressions in the lower troposphere upshear of the center. These extended from the 100-km radius to immediately adjacent to the eyewall, where equivalent potential temperature gradients reached 6 K km−1. The dry air was not present prior to intensification, suggesting that it was associated with vertical shear–induced subsidence upshear of the developing storm. It is argued that weakening of the vortex was driven by cooling associated with the mixing of dry air into the core, and subsequent evaporation and cold downdrafts. Evidence suggests that this mixing might have been enhanced by eyewall instabilities after the period of rapid deepening. The existence of a fragile, small, but genuinely hurricane-strength vortex at the surface for 6 h presents difficult problems for forecasters. Such a “temporary hurricane” in strongly sheared flow might require a different warning protocol than longer-lasting hurricane vortices in weaker shear.

scholarly journals The Atmospheric Overturning Induced by Hector the Convector

2017 ◽  
Vol 74 (10) ◽  
pp. 3271-3284 ◽  
Author(s):  
Thibaut Dauhut ◽  
Jean-Pierre Chaboureau ◽  
Patrick Mascart ◽  
Olivier Pauluis
Keyword(s):  
Gravity Waves ◽  
Lower Stratosphere ◽  
Deep Convection ◽  
Lower Troposphere ◽  
Convective System ◽  
Entrainment Rate ◽  
Northern Australia ◽  
Upper Troposphere ◽  
Eddy Simulation ◽  
Large Eddy

Abstract The overturning of Hector the Convector, a tropical multicellular convective system of northern Australia that regularly overshoots into the stratosphere, is synthesized at the scale of a large-eddy simulation. The isentropic analysis offers the advantage of filtering out the reversible motions due to gravity waves and taking into account the turbulent fluxes that contribute to the vertical transport. Two key circulations are characterized: the troposphere deep overturning and the mass exchange due to the overshoots into the stratosphere. The transition from deep to very deep convection is associated with a change in the diabatic tendency inside the tallest updrafts: the latent heat release due to the formation of a large amount of icy hydrometeors exceeds the loss of energy due to mixing with the drier, colder air of the environment. In agreement with a previous study of Hector examining the properties of its two tallest updrafts, the entrainment rate exhibits a minimum during the very deep convection phase as low as 0.04 km−1. The overturning intensity corroborates the Eulerian computation of the vertical mass flux in the midtroposphere and in the lower stratosphere. It however gives a lower estimate of the flux in the upper troposphere, filtering out the reversible motions, and a larger estimate in the lower troposphere and at the tropopause, where slow vertical motions contribute significantly to the transport.

scholarly journals Factors Affecting the Evolution of Hurricane Erin (2001) and the Distributions of Hydrometeors: Role of Microphysical Processes

2006 ◽  
Vol 63 (1) ◽  
pp. 127-150 ◽  
Author(s):  
Greg M. McFarquhar ◽  
Henian Zhang ◽  
Gerald Heymsfield ◽  
Jeffrey B. Halverson ◽  
Robbie Hood ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Doppler Radar ◽  
Mesoscale Model ◽  
Potential Temperature ◽  
Vertical Motion ◽  
Rain Area ◽  
Microphysics Scheme ◽  
Factors Affecting ◽  
Mixing Ratios ◽  
Microphysical Processes

Abstract Fine-resolution simulations of Hurricane Erin are conducted using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to investigate roles of thermodynamic, boundary layer, and microphysical processes on Erin’s structure and evolution. Choice of boundary layer scheme has the biggest impact on simulations, with the minimum surface pressure (Pmin) averaged over the last 18 h (when Erin is relatively mature) varying by over 20 hPa. Over the same period, coefficients used to describe graupel fall speeds (Vg) affect Pmin by up to 7 hPa, almost equivalent to the maximum 9-hPa difference between microphysical parameterization schemes; faster Vg and schemes with more hydrometeor categories generally give lower Pmin. Compared to radar reflectivity factor (Z) observed by the NOAA P-3 lower fuselage radar and the NASA ER-2 Doppler radar (EDOP) in Erin, all simulations overpredict the normalized frequency of occurrence of Z larger than 40 dBZ and underpredict that between 20 and 40 dBZ near the surface; simulations overpredict Z larger than 25 to 30 dBZ and underpredict that between 15 and 25 or 30 dBZ near the melting layer, the upper limit depending on altitude. Brightness temperatures (Tb) computed from modeled fields at 37.1- and 85.5-GHz channels that respond to scattering by graupel-size ice show enhanced scattering, mainly due to graupel, compared to observations. Simulated graupel mixing ratios are about 10 times larger than values observed in other hurricanes. For the control run at 6.5 km averaged over the last 18 simulated hours, Doppler velocities computed from modeled fields (Vdop) greater than 5 m s−1 make up 12% of Erin’s simulated area for the base simulation but less than 2% of the observed area. In the eyewall, 5% of model updrafts above 9 km are stronger than 10 m s−1, whereas statistics from other hurricanes show that 5% of updrafts are stronger than only 5 m s−1. Variations in distributions of Z, vertical motion, and graupel mixing ratios between schemes are not sufficient to explain systematic offsets between observations and models. A new iterative condensation scheme, used with the Reisner mixed-phase microphysics scheme, limits unphysical increases of equivalent potential temperature associated with many condensation schemes and reduces the frequency of Z larger than 50 dBZ, but has minimal effect on Z below 50 dBZ, which represent 95% of the modeled hurricane rain area. However, the new scheme changes the Erin simulations in that 95% of the updrafts are weaker than 5 m s−1 and Pmin is up to 12 hPa higher over the last 18 simulated hours.

Reply to “Comments on ‘Do Ultrafine Cloud Condensation Nuclei Invigorate Deep Convection?’”

2021 ◽  
Vol 78 (1) ◽  
pp. 341-350
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Negative Impact ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Diagnostic Technique ◽  
Cloud Water ◽  
Latent Heating ◽  
Condensation Nuclei ◽  
Modeling Framework ◽  
Microphysics Scheme ◽  
The Impact

AbstractThis is a rebuttal of Fan and Khain’s comments (hereafter FK21) on a 2020 paper by Grabowski and Morrison (hereafter GM20) that questions the impact of ultrafine cloud condensation nuclei (CCN) on deep convection. GM20 argues that “cold invigoration,” an increase of the updraft speed from lofting and freezing of additional cloud water in polluted environments, is unlikely because the latent heating from freezing of this cloud water approximately recovers the negative impact on the buoyancy from the weight of this water. FK21 suggest a variety of processes that could invalidate our claim. We maintain that our argument is valid and invite the authors to compare their microphysics scheme with ours in the same simplified modeling framework. However, pollution does affect the partitioning of latent heating within the column and likely leads to convection changes beyond a single diurnal cycle through larger-scale circulation changes. This argument explains impacts seen in our idealized mesoscale simulations and in convective–radiative equilibrium simulations by others. We agree with FK21 on the existence of a “warm invigoration” mechanism but question its interpretation. Consistent with the simulations in GM20, we argue that changes in the buoyancy can be explained by the response of the supersaturation to droplet microphysical changes induced by pollution. The buoyancy change is determined by supersaturation differences between pristine and polluted conditions, while condensation rate responds to these supersaturation changes. Finally, we agree with FK21 that the piggybacking modeling technique cannot prove or disprove invigoration; rather, it is a diagnostic technique that can be used to understand mechanisms driving simulation differences.

scholarly journals Untangling Microphysical Impacts on Deep Convection Applying a Novel Modeling Methodology. Part II: Double-Moment Microphysics

2016 ◽  
Vol 73 (9) ◽  
pp. 3749-3770 ◽  
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Large Scale ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Cloud Droplet ◽  
Cloud Base ◽  
Microphysics Scheme ◽  
Freezing Level ◽  
Modeling Methodology ◽  
Double Moment ◽  
The Impact

Abstract The suggested impact of pollution on deep convection dynamics, referred to as the convective invigoration, is investigated in simulations applying microphysical piggybacking and a comprehensive double-moment bulk microphysics scheme. The setup follows the case of daytime convective development over land based on observations during the Large-Scale Biosphere–Atmosphere (LBA) experiment in Amazonia. In contrast to previous simulations with single-moment microphysics schemes and in agreement with results from bin microphysics simulations by others, the impact of pollution simulated by the double-moment scheme is large for the upper-tropospheric convective anvils that feature higher cloud fractions in polluted conditions. The increase comes from purely microphysical considerations: namely, the increased cloud droplet concentrations in polluted conditions leading to the increased ice crystal concentrations and, consequently, smaller fall velocities and longer residence times. There is no impact on convective dynamics above the freezing level and thus no convective invigoration. Polluted deep convective clouds precipitate about 10% more than their pristine counterparts. The small enhancement comes from smaller supersaturations below the freezing level and higher buoyancies inside polluted convective updrafts with velocities between 5 and 10 m s−1. The simulated supersaturations are large, up to several percent in both pristine and polluted conditions, and they call into question results from deep convection simulations applying microphysical schemes with saturation adjustment. Sensitivity simulations show that the maximum supersaturations and the upper-tropospheric anvil cloud fractions strongly depend on the details of small cloud condensation nuclei (CCN) that can be activated in strong updrafts above the cloud base.

The Influence of Time-Dependent Melting on the Dynamics and Precipitation Production in Maritime and Continental Storm Clouds

2007 ◽  
Vol 64 (2) ◽  
pp. 338-359 ◽  
Author(s):  
Vaughan T. J. Phillips ◽  
Andrei Pokrovsky ◽  
Alexander Khain
Keyword(s):  
Short Distance ◽  
Large Scale ◽  
Cloud Model ◽  
Deep Convection ◽  
Sedimentation Velocity ◽  
Water Fraction ◽  
Atmospheric Research ◽  
Freezing Level ◽  
Hebrew University ◽  
Bin Microphysics

Abstract Simulations of one maritime and four continental observed cases of deep convection are performed with the Hebrew University Cloud Model that has spectral bin microphysics. The maritime case is from observations made on 18 September 1974 during the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE). The continental storm cases are those of summertime Texas clouds observed on 13 August 1999, and green-ocean, smoky, and pyro-clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia–Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign on 1–4 October 2002. Simulations have been performed for these cases with a detailed melting scheme. This scheme allows calculation of liquid water fraction within each mass bin for the melting of graupel, hail, snowflakes, and crystals, as well as alteration of the sedimentation velocity of ice particles in the course of their melting. The results obtained with the detailed melting scheme are compared with corresponding results from simulations involving instantaneous melting at the freezing (0°C) level. The detailed melting scheme allows penetration of ice from the freezing level down into the boundary layer by distances ranging from a few hundred meters for the numerous, smaller particles to ∼1.5 km for the largest particles, which are much scarcer. In these simulations, most of the mass of ice falling out melts over this short distance of a few hundred meters. The deepening and intensification of the layer of latent cooling enhances the convective destabilization of the troposphere. This effect is especially pronounced under continental conditions, causing significant changes in the accumulated rain amount.

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