scholarly journals A study of the effect of overshooting deep convection on the water content of the TTL and lower stratosphere from Cloud Resolving Model simulations

2007 ◽  
Vol 7 (3) ◽  
pp. 7277-7346 ◽  
Author(s):  
D. P. Grosvenor ◽  
T. W. Choularton ◽  
H. Coe ◽  
G. Held
Keyword(s):  
Boundary Layer ◽  
Water Content ◽  
Lower Stratosphere ◽  
Deep Convection ◽  
Cold Trap ◽  
Cloud Base ◽  
Anthropogenic Aerosols ◽  
Height Range ◽  
Cloud Resolving Model ◽  
The Impact

Abstract. Simulations of overshooting, tropical deep convection using a Cloud Resolving Model with bulk microphysics are presented in order to examine the effect on the water content of the TTL (Tropical Tropopause Layer) and lower stratosphere. This case study is a subproject of the HIBISUCS (Impact of tropical convection on the upper troposphere and lower stratosphere at global scale) campaign, which took place in Bauru, Brazil (22° S), from the end of January to early March 2004. Comparisons between 2-D and 3-D simulations suggest that the use of 3-D dynamics is vital in order to capture the mixing between the overshoot and the stratospheric air, which caused evaporation of ice and resulted in an overall moistening of the lower stratosphere. In contrast, a dehydrating effect was predicted by the 2-D simulation due to the extra time, allowed by the lack of mixing, for the ice transported to the region to precipitate out of the overshoot air. Three different strengths of convection are simulated in 3-D by applying successively lower heating rates (used to initiate the convection) in the boundary layer. Moistening is produced in all cases, indicating that convective vigour is not a factor in whether moistening or dehydration is predicted, since the weakest case only just penetrated the tropopause. An estimate of the moistening effect of these clouds on an air parcel traversing a convective region is made based on the domain mean simulated moistening and the frequency of convective events observed by the IPMet (Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista) radar to have the same 10 dBZ echo top height as those simulated. These suggest a fairly significant mean moistening of 0.26, 0.13 and 0.05 ppmv in the strongest, medium and weakest cases, respectively, for heights between 16 and 17 km. Since the tropopause in this region is thought to lie at ~15.9 km, this is likely to represent direct stratospheric moistening. Much more moistening is predicted for the 15–16 km height range with increases of 0.85–2.8 ppmv predicted. However, it would be required that this air is lofted through the tropopause via the Brewer Dobson circulation in order for it to have a stratospheric effect. Whether this is likely is uncertain and, in addition, the dehydration of air as it passes through the cold trap and the number of times that trajectories sample convective regions needs to be taken into account to gauge the overall stratospheric effect. Nevertheless, the results suggest a potentially significant role for convection in determining the stratospheric water content. Sensitivity tests exploring the impact of increased aerosol numbers in the boundary layer suggest that a corresponding rise in cloud droplet numbers at cloud base would increase the number concentrations of the ice crystals transported to the TTL, which had the effect of reducing the fall speeds of the ice and causing a ~13% rise in the mean vapour increase in both the 15–16 and 16–17 km height ranges, respectively, when compared to the control case. Increases in the total water were much larger, being 34% and 132% higher for the same height ranges, but it is unclear whether the extra ice will be able to evaporate before precipitating from the region. These results suggest a possible impact of natural and anthropogenic aerosols on how convective clouds affect stratospheric moisture levels.

scholarly journals A study of the effect of overshooting deep convection on the water content of the TTL and lower stratosphere from Cloud Resolving Model simulations

2007 ◽  
Vol 7 (18) ◽  
pp. 4977-5002 ◽  
Author(s):  
D. P. Grosvenor ◽  
T. W. Choularton ◽  
H. Coe ◽  
G. Held
Keyword(s):  
Boundary Layer ◽  
Water Content ◽  
Lower Stratosphere ◽  
Deep Convection ◽  
Cold Trap ◽  
Cloud Base ◽  
Anthropogenic Aerosols ◽  
Height Range ◽  
Cloud Resolving Model ◽  
The Impact

Abstract. Simulations of overshooting, tropical deep convection using a Cloud Resolving Model with bulk microphysics are presented in order to examine the effect on the water content of the TTL (Tropical Tropopause Layer) and lower stratosphere. This case study is a subproject of the HIBISCUS (Impact of tropical convection on the upper troposphere and lower stratosphere at global scale) campaign, which took place in Bauru, Brazil (22° S, 49° W), from the end of January to early March 2004. Comparisons between 2-D and 3-D simulations suggest that the use of 3-D dynamics is vital in order to capture the mixing between the overshoot and the stratospheric air, which caused evaporation of ice and resulted in an overall moistening of the lower stratosphere. In contrast, a dehydrating effect was predicted by the 2-D simulation due to the extra time, allowed by the lack of mixing, for the ice transported to the region to precipitate out of the overshoot air. Three different strengths of convection are simulated in 3-D by applying successively lower heating rates (used to initiate the convection) in the boundary layer. Moistening is produced in all cases, indicating that convective vigour is not a factor in whether moistening or dehydration is produced by clouds that penetrate the tropopause, since the weakest case only just did so. An estimate of the moistening effect of these clouds on an air parcel traversing a convective region is made based on the domain mean simulated moistening and the frequency of convective events observed by the IPMet (Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista) radar (S-band type at 2.8 Ghz) to have the same 10 dBZ echo top height as those simulated. These suggest a fairly significant mean moistening of 0.26, 0.13 and 0.05 ppmv in the strongest, medium and weakest cases, respectively, for heights between 16 and 17 km. Since the cold point and WMO (World Meteorological Organization) tropopause in this region lies at ~15.9 km, this is likely to represent direct stratospheric moistening. Much more moistening is predicted for the 15–16 km height range with increases of 0.85–2.8 ppmv predicted. However, it would be required that this air is lofted through the tropopause via the Brewer Dobson circulation in order for it to have a stratospheric effect. Whether this is likely is uncertain and, in addition, the dehydration of air as it passes through the cold trap and the number of times that trajectories sample convective regions needs to be taken into account to gauge the overall stratospheric effect. Nevertheless, the results suggest a potentially significant role for convection in determining the stratospheric water content. Sensitivity tests exploring the impact of increased aerosol numbers in the boundary layer suggest that a corresponding rise in cloud droplet numbers at cloud base would increase the number concentrations of the ice crystals transported to the TTL, which had the effect of reducing the fall speeds of the ice and causing a ~13% rise in the mean vapour increase in both the 15–16 and 16–17 km height ranges, respectively, when compared to the control case. Increases in the total water were much larger, being 34% and 132% higher for the same height ranges, but it is unclear whether the extra ice will be able to evaporate before precipitating from the region. These results suggest a possible impact of natural and anthropogenic aerosols on how convective clouds affect stratospheric moisture levels.

scholarly journals Evaluating Boundary Layer–Based Mass Flux Closures Using Cloud-Resolving Model Simulations of Deep Convection

2010 ◽  
Vol 67 (7) ◽  
pp. 2212-2225 ◽  
Author(s):  
Jennifer K. Fletcher ◽  
Christopher S. Bretherton
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Large Scale ◽  
Deep Convection ◽  
Cloud Base ◽  
Cumulus Convection ◽  
Model Simulations ◽  
Shallow Cumulus ◽  
Wide Range ◽  
Cloud Resolving Model

Abstract High-resolution three-dimensional cloud resolving model simulations of deep cumulus convection under a wide range of large-scale forcings are used to evaluate a mass flux closure based on boundary layer convective inhibition (CIN) that has previously been applied in parameterizations of shallow cumulus convection. With minor modifications, it is also found to perform well for deep oceanic and continental cumulus convection, and it matches simulated cloud-base mass flux much better than a closure based only on the boundary layer convective velocity scale. CIN closure maintains an important feedback among cumulus base mass flux, compensating subsidence, and CIN that keeps the boundary layer top close to cloud base. For deep convection, the proposed CIN closure requires prediction of a boundary layer mean turbulent kinetic energy (TKE) and a horizontal moisture variance, both of which are strongly correlated with precipitation. For our cases, CIN closure predicts cloud-base mass flux in deep convective environments as well as the best possible bulk entraining CAPE closure, but unlike the latter, CIN closure also works well for shallow cumulus convection without retuning of parameters.

scholarly journals The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)

2015 ◽  
Vol 15 (11) ◽  
pp. 6467-6486 ◽  
Author(s):  
W. Frey ◽  
R. Schofield ◽  
P. Hoor ◽  
D. Kunkel ◽  
F. Ravegnani ◽  
...  
Keyword(s):  
Lower Stratosphere ◽  
Deep Convection ◽  
Horizontal Resolution ◽  
Transport Processes ◽  
Convective System ◽  
Upper Troposphere ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Transport And Mixing ◽  
The Impact

Abstract. In this study we examine the simulated downward transport and mixing of stratospheric air into the upper tropical troposphere as observed on a research flight during the SCOUT-O3 campaign in connection with a deep convective system. We use the Advanced Research Weather and Research Forecasting (WRF-ARW) model with a horizontal resolution of 333 m to examine this downward transport. The simulation reproduces the deep convective system, its timing and overshooting altitudes reasonably well compared to radar and aircraft observations. Passive tracers initialised at pre-storm times indicate the downward transport of air from the stratosphere to the upper troposphere as well as upward transport from the boundary layer into the cloud anvils and overshooting tops. For example, a passive ozone tracer (i.e. a tracer not undergoing chemical processing) shows an enhancement in the upper troposphere of up to about 30 ppbv locally in the cloud, while the in situ measurements show an increase of 50 ppbv. However, the passive carbon monoxide tracer exhibits an increase, while the observations show a decrease of about 10 ppbv, indicative of an erroneous model representation of the transport processes in the tropical tropopause layer. Furthermore, it could point to insufficient entrainment and detrainment in the model. The simulation shows a general moistening of air in the lower stratosphere, but it also exhibits local dehydration features. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.

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.

Drizzle in Stratiform Boundary Layer Clouds. Part I: Vertical and Horizontal Structure

2005 ◽  
Vol 62 (9) ◽  
pp. 3011-3033 ◽  
Author(s):  
R. Wood
Keyword(s):  
Boundary Layer ◽  
Water Content ◽  
Liquid Water ◽  
Liquid Water Content ◽  
Precipitation Rate ◽  
Cloud Base ◽  
Specific Reference ◽  
Horizontal Structure ◽  
Boundary Layer Clouds ◽  
Droplet Concentration

Abstract Detailed observations of stratiform boundary layer clouds on 12 days are examined with specific reference to drizzle formation processes. The clouds differ considerably in mean thickness, liquid water path (LWP), and droplet concentration. Cloud-base precipitation rates differ by a factor of 20 between cases. The lowest precipitation rate is found in the case with the highest droplet concentration even though this case had by far the highest LWP, suggesting that drizzle can be severely suppressed in polluted clouds. The vertical and horizontal structure of cloud and drizzle liquid water and bulk microphysical parameters are examined in detail. In general, the highest concentration of r > 20 μm drizzle drops is found toward the top of the cloud, and the mean volume radius of the drizzle drops increases monotonically from cloud top to base. The resulting precipitation rates are largest at the cloud base but decrease markedly only in the upper third of the cloud. Below cloud, precipitation rates decrease markedly with distance below base due to evaporation, and are broadly consistent in most cases with the results from a simple sedimentation–evaporation model. Evidence is presented that suggests evaporating drizzle is cooling regions of the subcloud layer, which could result in dynamical feedbacks. A composite power spectrum of the horizontal spatial series of precipitation rate is found to exhibit a power-law scaling from the smallest observable scales to close to the maximum observable scale (∼30 km). The exponent is considerably lower (1.1–1.2) than corresponding exponents for LWP variability obtained in other studies (∼1.5–2), demonstrating that there is relatively more variability of drizzle on small scales. Singular measures analysis shows that drizzle fields are much more intermittent than the cloud liquid water content fields, consistent with a drizzle production process that depends strongly upon liquid water content. The adiabaticity of the clouds, which can be modeled as a simple balance between drizzle loss and turbulent replenishment, is found to decrease if the time scale for drizzle loss is shorter than roughly 5–10 eddy turnover time scales. Finally, the data are compared with three simple scalings derived from recent observations of drizzle in subtropical stratocumulus clouds.

scholarly journals Evidence of the impact of deep convection on reactive Volatile Organic Compounds in the upper tropical troposphere during the AMMA experiment in West Africa

2010 ◽  
Vol 10 (21) ◽  
pp. 10321-10334 ◽  
Author(s):  
J. Bechara ◽  
A. Borbon ◽  
C. Jambert ◽  
A. Colomb ◽  
P. E. Perros
Keyword(s):  
Volatile Organic Compounds ◽  
West Africa ◽  
Organic Compounds ◽  
Deep Convection ◽  
Cloud Base ◽  
Convective Conditions ◽  
Air Masses ◽  
Convective Systems ◽  
Volatile Organic ◽  
The Impact

Abstract. A large dataset of reactive trace gases was collected for the first time over West Africa during the African Monsoon Multidisciplinary Analysis (AMMA) field experiment in August 2006. Volatile Organic Compounds (VOC from C5–C9) were measured onboard the two French aircrafts the ATR-42 and the Falcon-20 by a new instrument AMOVOC (Airborne Measurement Of Volatile Organic Compounds). The goal of this study is (i) to characterize VOC distribution in the tropical region of West Africa (ii) to determine the impact of deep convection on VOC distribution and chemistry in the tropical upper troposphere (UT) and (iii) to characterize its spatial and temporal extensions. Experimental strategy consisted in sampling at altitudes between 0 and 12 km downwind of Mesoscale Convective Systems (MCS) and at cloud base. Biogenic and anthropogenic VOC distribution in West Africa is clearly affected by North to South emission gradient. Isoprene, the most abundant VOC, is at maximum level over the forest (1.26 ppb) while benzene reaches its maximum over the urban areas (0.11 ppb). First, a multiple physical and chemical tracers approach using CO, O3 and relative humidity was implemented to distinguish between convective and non-convective air masses. Then, additional tools based on VOC observations (tracer ratios, proxy of emissions and photochemical clocks) were adapted to characterize deep convection on a chemical, spatial and temporal basis. VOC vertical profiles show a "C-shaped" trend indicating that VOC-rich air masses are transported from the surface to the UT by deep convective systems. VOC mixing ratios in convective outflow are up to two times higher than background levels even for reactive and short-lived VOC (e.g. isoprene up to 0.19 ppb at 12 km-altitude) and are dependent on surface emission type. As a consequence, UT air mass reactivity increases from 0.52 s−1 in non-convective conditions to 0.95 s−1 in convective conditions. Fractions of boundary layer air contained in convective outflow are estimated to be 40 ± 15%. Vertical transport timescale is calculated to be 25 ± 10 min between 0 to 12 km altitude. These results characterize deep convection occurring over West Africa and provide relevant information for tropical convection parameterization in regional/global models.

scholarly journals An overview of the HIBISCUS campaign

2007 ◽  
Vol 7 (1) ◽  
pp. 2389-2475 ◽  
Author(s):  
J.-P. Pommereau ◽  
A. Garnier ◽  
G. Held ◽  
A.-M. Gomes ◽  
F. Goutail ◽  
...  
Keyword(s):  
Lower Stratosphere ◽  
Deep Convection ◽  
Global Scale ◽  
South Atlantic Convergence Zone ◽  
Lapse Rate ◽  
Tropical Tropopause Layer ◽  
Convective Systems ◽  
Major Mechanism ◽  
Tropical Tropopause ◽  
The Impact

Abstract. HIBISCUS was a field campaign for investigating the impact of deep convection on the Tropical Tropopause Layer (TTL) and the Lower Stratosphere, which took place during the Southern Hemisphere summer in February–March 2004 in the State of São Paulo, Brazil. Its objective was to provide a set of new observational data on meteorology, tracers of horizontal and vertical transport, water vapour, clouds, and chemistry in the tropical UT/LS from balloon observations at local scale over a land convective area, as well as at global scale using circumnavigating long-duration balloons. Overall, the composition of the TTL, the region between 14 and 19 km of intermediate lapse rate between the almost adiabatic upper troposphere and the stable stratosphere, appears highly variable. Tracers and ozone measurements performed at both the local and the global scale indicate a strong quasi-horizontal isentropic exchange with the lowermost mid-latitude stratosphere suggesting that the barrier associated to the tropical jet is highly permeable at these levels in summer. But the project also provides clear indications of strong episodic updraught of cold air, short-lived tracers, low ozone, humidity and ice particles across the lapse rate tropopause at about 15 km, up to 18 or 19 km at 420–440 K potential levels in the lower stratosphere, suggesting that, in contrast to oceanic convection penetrating little the stratosphere, fast daytime developing land convective systems could be a major mechanism in the troposphere-stratosphere exchange at the global scale. The present overview is meant to provide the background of the project, as well as overall information on the instrumental tools available, on the way they have been used within the highly convective context of the South Atlantic Convergence Zone, and a brief summary of the results, which will be detailed in several other papers of this special issue.

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 Microphysical Structure of the Marine Boundary Layer under Strong Wind and Spray Formation as Seen from Simulations Using a 2D Explicit Microphysical Model. Part I: The Impact of Large Eddies

2011 ◽  
Vol 68 (10) ◽  
pp. 2366-2384 ◽  
Author(s):  
J. Shpund ◽  
M. Pinsky ◽  
A. Khain
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Surface Fluxes ◽  
Cloud Base ◽  
Small Scale ◽  
Sea Spray ◽  
Strong Wind ◽  
Spray Droplets ◽  
The Impact ◽  
Large Eddies

Abstract The effects of large eddies (LE) on the marine boundary layer (MBL) microphysics and thermodynamics is investigated using a 2D Lagrangian model with spectral bin microphysics including effects of sea spray. The 600 m × 400 m MBL computational area is covered by 3750 adjacent interacting Lagrangian parcels moving in a turbulent-like flow. A turbulent-like velocity field is designed as a sum of a high number of harmonics with random time-dependent amplitudes and different wavelengths including large eddies with scales of several hundred meters. The model explicitly calculates diffusion growth/evaporation, collisions, and sedimentation of droplets forming both as sea spray droplets and background aerosols, as well as aerosol masses within droplets. The turbulent mixing between parcels is explicitly taken into account. Sea spray generation is determined by a source function depending on the background wind speed assumed in the simulations to be equal to 20 m s−1. The results of simulations obtained by taking into account the effects of LE are compared to those obtained under the assumption that the vertical transport of droplets and passive scalars is caused by small-scale turbulent diffusion. Small-scale turbulence diffusion taken alone leads to an unrealistic MBL structure. Nonlocal mixing of the MBL caused by LE leads to the formation of a well-mixed MBL with a vertical structure close to the observed one. LE lead to an increase in the sensible and latent heat surface fluxes by 50%–100% and transport a significant amount of large spray droplets upward. Microphysical processes lead to formation of spray-induced drizzling clouds with cloud base near the 200-m level.

An Evaluation of Mass Flux Closures for Diurnal Cycles of Shallow Cumulus

2004 ◽  
Vol 132 (11) ◽  
pp. 2525-2538 ◽  
Author(s):  
R. A. J. Neggers ◽  
A. P. Siebesma ◽  
G. Lenderink ◽  
A. A. M. Holtslag
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Climate Model ◽  
Convective Available Potential Energy ◽  
Cloud Base ◽  
Quasi Equilibrium ◽  
Difficult Case ◽  
Diurnal Cycles ◽  
Shallow Cumulus ◽  
The Impact

Abstract Three closure methods for the mass flux at cloud base in shallow cumulus convection are critically examined for the difficult case of a diurnal cycle over land. The closure methods are first evaluated against large-eddy simulations (LESs) by diagnosing all parameters appearing in the closure equations during simulations of two different observed diurnal cycles of shallow cumulus. This reveals the characteristic behavior of each closure mechanism purely as a result of its core structure. With these results in hand the impact of each closure on the development of the cloudy boundary layer is then studied by its implementation in an offline single-column model of a regional atmospheric climate model. The LES results show that the boundary layer quasi-equilibrium closure typically overestimates the cloud-base mass flux after cloud onset, due to the neglect of significant moisture and temperature tendencies in the subcloud layer. The convective available potential energy (CAPE) adjustment closure is compromised by its limitation to compensating subsidence as the only CAPE breakdown mechanism and the use of a constant adjustment time scale. The closure method using the subcloud convective vertical velocity scale gives the best results, as it catches the time development of the cloud-base mass flux as diagnosed in LES.

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