Influences of environmental relative humidity and horizontal scale of sub-cloud ascent on deep convective initiation

2021 ◽  
Author(s):  
Hugh Morrison ◽  
John M. Peters ◽  
Kamal Kant Chandakar ◽  
Steven C. Sherwood
Keyword(s):  
Relative Humidity ◽  
Deep Convection ◽  
Threshold Value ◽  
Surface Fluxes ◽  
Lapse Rate ◽  
Entrainment Rate ◽  
Moist Convection ◽  
Convective Initiation ◽  
Two Factors ◽  
Environmental Relative Humidity

AbstractThis study examines two factors impacting initiation of moist deep convection: free tropospheric environmental relative humidity (ϕE) and horizon scale of sub-cloud ascent (Rsub), the latter exerting a dominant control on cumulus cloud width. A simple theoretical model is used to formulate a “scale selection” hypothesis: that a minimum Rsub is required for moist convection to go deep, and that this minimum scale decreases with increasing ϕE. Specifically, the ratio of to saturation deficit (1–ϕE) must exceed a certain threshold value that depends on cloud-layer environmental lapse rate. Idealized, large-eddy simulations of moist convection forced by horizontally-varying surface fluxes show strong sensitivity of maximum cumulus height to both ϕE and Rsub consistent with the hypothesis. Increasing Rsub by only 300-400 m can lead to a large increase (> 5 km) in cloud height. A passive tracer analysis shows that the bulk fractional entrainment rate decreases rapidly with Rsub but depends little on ϕE. However, buoyancy dilution increases as either Rsub or ϕE decreases; buoyancy above the level of free convection is rapidly depleted in dry environments when Rsub is small. While deep convective initiation occurs with an increase in relative humidity of the near environment from moistening by earlier convection, the importance of this moisture preconditioning is inconclusive as it is accompanied by an increase in Rsub. Overall, it is concluded that small changes to Rsub driven by external forcing or by convection itself could be a dominant regulator of deep convective initiation.

scholarly journals Can the Impact of Aerosols on Deep Convection be Isolated from Meteorological Effects in Atmospheric Observations?

2018 ◽  
Vol 75 (10) ◽  
pp. 3347-3363 ◽  
Author(s):  
Wojciech W. Grabowski
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Surface Fluxes ◽  
Phase Iii ◽  
Moist Convection ◽  
Atmospheric Measurements ◽  
The Impact ◽  
Meteorological Effects

Influence of pollution on dynamics of deep convection continues to be a controversial topic. Arguably, only carefully designed numerical simulations can clearly separate the impact of aerosols from the effects of meteorological factors that affect moist convection. This paper argues that such a separation is virtually impossible using observations because of the insufficient accuracy of atmospheric measurements and the fundamental nature of the interaction between deep convection and its environment. To support this conjecture, results from numerical simulations are presented that apply modeling methodology previously developed by the author. The simulations consider small modifications, difficult to detect in observations, of the initial sounding, surface fluxes, and large-scale forcing tendencies. All these represent variations of meteorological conditions that affect deep convective dynamics independently of aerosols. The setup follows the case of daytime convective development over land based on observations during the Large-Scale Biosphere–Atmosphere (LBA) field project in Amazonia. The simulated observable macroscopic changes of convection, such as the surface precipitation and upper-tropospheric cloudiness, are similar to or larger than those resulting from changes of cloud condensation nuclei from pristine to polluted conditions studied previously using the same modeling case. Observations from Phase III of the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) are also used to support the argument concerning the impact of the large-scale forcing. The simulations suggest that the aerosol impacts on dynamics of deep convection cannot be isolated from meteorological effects, at least for the daytime development of unorganized deep convection considered in this study.

scholarly journals CAPE Variations in the Current Climate and in a Climate Change

1998 ◽  
Vol 11 (8) ◽  
pp. 1997-2015 ◽  
Author(s):  
Bing Ye ◽  
Anthony D. Del Genio ◽  
Kenneth K-W. Lo
Keyword(s):  
Climate Change ◽  
Relative Humidity ◽  
General Circulation ◽  
Large Scale ◽  
Tropical Pacific ◽  
Lapse Rate ◽  
Moist Convection ◽  
Upper Troposphere ◽  
Current Climate ◽  
The Tropical Pacific

Abstract Observed variations of convective available potential energy (CAPE) in the current climate provide one useful test of the performance of cumulus parameterizations used in general circulation models (GCMs). It is found that frequency distributions of tropical Pacific CAPE, as well as the dependence of CAPE on surface wet-bulb potential temperature (Θw) simulated by the Goddard Institute for Space Studies’s GCM, agree well with that observed during the Australian Monsoon Experiment period. CAPE variability in the current climate greatly overestimates climatic changes in basinwide CAPE in the tropical Pacific in response to a 2°C increase in sea surface temperature (SST) in the GCM because of the different physics involved. In the current climate, CAPE variations in space and time are dominated by regional changes in boundary layer temperature and moisture, which in turn are controlled by SST patterns and large-scale motions. Geographical thermodynamic structure variations in the middle and upper troposphere are smaller because of the canceling effects of adiabatic cooling and subsidence warming in the rising and sinking branches of the Walker and Hadley circulations. In a forced equilibrium global climate change, temperature change is fairly well constrained by the change in the moist adiabatic lapse rate and thus the upper troposphere warms to a greater extent than the surface. For this reason, climate change in CAPE is better predicted by assuming that relative humidity remains constant and that the temperature changes according to the moist adiabatic lapse rate change of a parcel with 80% relative humidity lifted from the surface. The moist adiabatic assumption is not symmetrically applicable to a warmer and colder climate: In a warmer regime moist convection determines the tropical temperature structure, but when the climate becomes colder the effect of moist convection diminishes and the large-scale dynamics and radiative processes become relatively important. Although a prediction based on the change in moist adiabat matches the GCM simulation of climate change averaged over the tropical Pacific basin, it does not match the simulation regionally because small changes in the general circulation change the local boundary layer relative humidity by 1%–2%. Thus, the prediction of regional climate change in CAPE is also dependent on subtle changes in the dynamics.

scholarly journals Thermal Chains and Entrainment in Cumulus Updrafts. Part II: Analysis of Idealized Simulations

2020 ◽  
Vol 77 (11) ◽  
pp. 3661-3681 ◽  
Author(s):  
John M. Peters ◽  
Hugh Morrison ◽  
Adam C. Varble ◽  
Walter M. Hannah ◽  
Scott E. Giangrande
Keyword(s):  
Vertical Velocity ◽  
Conceptual Models ◽  
Deep Convection ◽  
Three Dimensional ◽  
Entrainment Rate ◽  
Vertical Profiles ◽  
Passive Tracer ◽  
Tracer Concentration ◽  
Environmental Relative Humidity

AbstractResearch has suggested that the structure of deep convection often consists of a series of rising thermals, or “thermal chain,” which contrasts with existing conceptual models that are used to construct cumulus parameterizations. Simplified theoretical expressions for updraft properties obtained in Part I of this study are used to develop a hypothesis explaining why this structure occurs. In this hypothesis, cumulus updraft structure is strongly influenced by organized entrainment below the updraft’s vertical velocity maximum. In a dry environment, this enhanced entrainment can locally reduce condensation rates and increase evaporation, thus eroding buoyancy. For moderate-to-large initial cloud radius R, this breaks up the updraft into a succession of discrete pulses of rising motion (i.e., a thermal chain). For small R, this leads to the structure of a single, isolated rising thermal. In contrast, moist environments are hypothesized to favor plume-like updrafts for moderate-to-large R. In a series of axisymmetric numerical cloud simulations, R and environmental relative humidity (RH) are systematically varied to test this hypothesis. Vertical profiles of fractional entrainment rate, passive tracer concentration, buoyancy, and vertical velocity from these runs agree well with vertical profiles calculated from the theoretical expressions in Part I. Analysis of the simulations supports the hypothesized dependency of updraft structure on R and RH, that is, whether it consists of an isolated thermal, a thermal chain, or a plume, and the role of organized entrainment in driving this dependency. Additional three-dimensional (3D) turbulent cloud simulations are analyzed, and the behavior of these 3D runs is qualitatively consistent with the theoretical expressions and axisymmetric simulations.

A Lagrangian Perspective on Parameterizing Deep Convection

2019 ◽  
Vol 147 (11) ◽  
pp. 4127-4149 ◽  
Author(s):  
Ron McTaggart-Cowan ◽  
Paul A. Vaillancourt ◽  
Ayrton Zadra ◽  
Leo Separovic ◽  
Shawn Corvec ◽  
...  
Keyword(s):  
Numerical Models ◽  
Spatial Scales ◽  
Deep Convection ◽  
Convective Precipitation ◽  
Moist Convection ◽  
Gray Zone ◽  
Convective Initiation ◽  
Intermittent Behavior ◽  
Cloud Properties ◽  
Deep Moist Convection

Abstract The parameterization of deep moist convection as a subgrid-scale process in numerical models of the atmosphere is required at resolutions that extend well into the convective “gray zone,” the range of grid spacings over which such convection is partially resolved. However, as model resolution approaches the gray zone, the assumptions upon which most existing convective parameterizations are based begin to break down. We focus here on one aspect of this problem that emerges as the temporal and spatial scales of the model become similar to those of deep convection itself. The common practice of static tendency application over a prescribed adjustment period leads to logical inconsistencies at resolutions approaching the gray zone, while more frequent refreshment of the convective calculations can lead to undesirable intermittent behavior. A proposed parcel-based treatment of convective initiation introduces memory into the system in a manner that is consistent with the underlying physical principles of convective triggering, thus reducing the prevalence of unrealistic gradients in convective activity in an operational model running with a 10 km grid spacing. The subsequent introduction of a framework that considers convective clouds as persistent objects, each possessing unique attributes that describe physically relevant cloud properties, appears to improve convective precipitation patterns by depicting realistic cloud memory, movement, and decay. Combined, this Lagrangian view of convection addresses one aspect of the convective gray zone problem and lays a foundation for more realistic treatments of the convective life cycle in parameterization schemes.

scholarly journals The Sensitivity of Convective Initiation to the Lapse Rate of the Active Cloud-Bearing Layer

2007 ◽  
Vol 135 (9) ◽  
pp. 3013-3032 ◽  
Author(s):  
Adam L. Houston ◽  
Dev Niyogi
Keyword(s):  
Thermal Instability ◽  
Deep Convection ◽  
Lapse Rate ◽  
Lagrangian Model ◽  
Subcritical State ◽  
State Variables ◽  
Convective Initiation ◽  
Rate Of Increase ◽  
Lapse Rates ◽  
Bearing Layer

Abstract Numerical experiments are conducted using an idealized cloud-resolving model to explore the sensitivity of deep convective initiation (DCI) to the lapse rate of the active cloud-bearing layer [ACBL; the atmospheric layer above the level of free convection (LFC)]. Clouds are initiated using a new technique that involves a preexisting airmass boundary initialized such that the (unrealistic) adjustment of the model state variables to the imposed boundary is disassociated from the simulation of convection. Reference state environments used in the experiment suite have identical mixed layer values of convective inhibition, CAPE, and LFC as well as identical profiles of relative humidity and wind. Of the six simulations conducted for the experiment set, only the three environments with the largest ACBL lapse rates support DCI. The simulated deep convection is initiated from elevated sources (parcels in the convective clouds originate near 1300 m) despite the presence of a surface-based boundary. Thermal instability release is found to be more likely in the experiments with larger ACBL lapse rates because the forced ascent at the preexisting boundary is stronger (despite nearly identical boundary depths) and because the parcels’ LFCs are lower, irrespective of parcel dilution. In one experiment without deep convection, DCI failure occurs even though thermal instability is released. Results from this experiment along with the results from a heuristic Lagrangian model reveal the existence of two convective regimes dependent on the environmental lapse rate: a supercritical state capable of supporting DCI and a subcritical state that is unlikely to support DCI. Under supercritical conditions the rate of increase in buoyancy due to parcel ascent exceeds the reduction in buoyancy due to dilution. Under subcritical conditions, the rate of increase in buoyancy due to parcel ascent is outpaced by the rate of reduction in buoyancy from dilution. Overall, results demonstrate that the lapse rate of the ACBL is useful in diagnosing and/or predicting DCI.

scholarly journals Marine Boundary Layer Cloud Feedbacks in a Constant Relative Humidity Atmosphere

2012 ◽  
Vol 69 (8) ◽  
pp. 2538-2550 ◽  
Author(s):  
Malte Rieck ◽  
Louise Nuijens ◽  
Bjorn Stevens
Keyword(s):  
Boundary Layer ◽  
Relative Humidity ◽  
Liquid Water ◽  
Radiative Forcing ◽  
Marine Boundary Layer ◽  
Surface Fluxes ◽  
Lapse Rate ◽  
Specific Humidity ◽  
Trade Wind ◽  
Positive Temperature

Abstract The mechanisms that govern the response of shallow cumulus, such as found in the trade wind regions, to a warming of the atmosphere in which large-scale atmospheric processes act to keep relative humidity constant are explored. Two robust effects are identified. First, and as is well known, the liquid water lapse rate increases with temperature and tends to increase the amount of water in clouds, making clouds more reflective of solar radiation. Second, and less well appreciated, the surface fluxes increase with the saturation specific humidity, which itself is a strong function of temperature. Using large-eddy simulations it is shown that the liquid water lapse rate acts as a negative feedback: a positive temperature increase driven by radiative forcing is reduced by the increase in cloud water and hence cloud albedo. However, this effect is more than compensated by a reduction of cloudiness associated with the deepening and relative drying of the boundary layer, driven by larger surface moisture fluxes. Because they are so robust, these effects are thought to underlie changes in the structure of the marine boundary layer as a result of global warming.

scholarly journals Sensitivities of Land Cover–Precipitation Feedback to Convective Triggering

2017 ◽  
Vol 18 (8) ◽  
pp. 2265-2283 ◽  
Author(s):  
Liang Chen ◽  
Paul A. Dirmeyer ◽  
Ahmed Tawfik ◽  
David M. Lawrence
Keyword(s):  
Land Cover ◽  
Land Cover Change ◽  
Great Plains ◽  
Coupling Strength ◽  
Land Surface ◽  
Deep Convection ◽  
Summer Precipitation ◽  
Surface Fluxes ◽  
Earth System ◽  
Convective Initiation

Abstract The land surface state can be an important factor in the triggering of precipitation, whose depiction in Earth system models (ESMs) crucially relies on the representation of convective initiation. However, the sensitivity of land-cover change–precipitation feedbacks to different parameterized triggering criteria in ESMs has not been examined. In this study, a new triggering mechanism based on the heated condensation framework (HCF) is implemented in the Community Earth System Model (CESM). A set of land-cover change experiments with different convective triggering conditions are performed to evaluate the influence of convective triggering on land–atmosphere coupling strength and the response of summer afternoon precipitation to land-cover change over North America. Compared with the default parameterization, which depends on a CAPE threshold, the HCF trigger shows an improvement in the diurnal timing of summer precipitation but larger dry biases over much of the study area. With the HCF trigger, CESM exhibits weakened coupling strength between soil moisture and surface turbulent fluxes over the Great Plains. The surface temperature deficit, as an additional triggering criterion in HCF, is not significantly coupled with surface fluxes over the central Great Plains despite strong latent heat–CAPE coupling. In contrast to the CAPE-trigger simulations, which indicate increased precipitation over the Great Plains after agricultural expansion, the HCF-trigger simulations show significantly increased afternoon precipitation only over the northern plains, which is mainly associated with more frequent deep convection. The discrepancies suggest caveats when investigating the impacts of land-cover change on precipitation, because the magnitude and spatial patterns of precipitation change can be greatly affected by the treatment of convection in ESMs.

scholarly journals Preconditioning Deep Convection with Cumulus Congestus

2013 ◽  
Vol 70 (2) ◽  
pp. 448-464 ◽  
Author(s):  
Cathy Hohenegger ◽  
Bjorn Stevens
Keyword(s):  
Time Scale ◽  
Large Scale ◽  
Deep Convection ◽  
Surface Fluxes ◽  
Scale Analysis ◽  
Convective Initiation ◽  
Clear Sky ◽  
Probability Of Transition ◽  
Cumulus Congestus ◽  
Sky Conditions

Abstract Recent studies have pushed forward the idea that congestus clouds, through their moistening of the atmosphere, could promote deep convection. On the other hand, older studies have tended to relate convective initiation to the large-scale forcing. These two views are here contrasted by performing a time-scale analysis. The analysis combines ship observations, large-eddy simulations, and 1 month of brightness temperature measurements with a focus on the tropical Atlantic and adjacent land areas. The time-scale analysis suggests that previous work may have overstated the importance of congestus moistening in the preconditioning of deep convection. It is found that cumuli congestus transition very rapidly to deep convection, in 2 h over land and 4 h over ocean. This is much faster than the time needed (10 h and longer) by congestus clouds to sufficiently moisten the atmosphere. Moreover, the majority of congestus clouds seem unable to grow into cumulonimbus and the probability of transition does not increase with increasing congestus lifetime (i.e., more moistening). Finally, the presence of cumuli congestus over a given region generally does not enhance the likelihood for deep convection development, either with respect to other regions or to clear-sky conditions. Hence, the results do not support the view of an atmosphere slowly deepening by local moistening, but rather, they may be interpreted as reminiscent of an atmosphere marked by violent and sudden outbursts of convection forced by dynamical effects. This also implies that moisture convergence is more important than local surface fluxes to trigger deep convection over a certain region.

scholarly journals The importance of moisture distribution for the growth and energetics of mid-latitude systems

1999 ◽  
Vol 17 (2) ◽  
pp. 242-256 ◽  
Author(s):  
V. Pavan ◽  
N. Hall ◽  
P. Valdes ◽  
M. Blackburn
Keyword(s):  
Relative Humidity ◽  
Relative Abundance ◽  
Large Scale ◽  
Initial Conditions ◽  
Equation Model ◽  
Life Cycles ◽  
Surface Fluxes ◽  
Moisture Distribution ◽  
Height Distribution ◽  
Moist Convection

Abstract. A primitive equation model is used to study the sensitivity of baroclinic wave life cycles to the initial latitude-height distribution of humidity. Diabatic heating is parametrized only as a consequence of condensation in regions of large-scale ascent. Experiments are performed in which the initial relative humidity is a simple function of model level, and in some cases latitude bands are specified which are initially relatively dry. It is found that the presence of moisture can either increase or decrease the peak eddy kinetic energy of the developing wave, depending on the initial moisture distribution. A relative abundance of moisture at mid-latitudes tends to weaken the wave, while a relative abundance at low latitudes tends to strengthen it. This sensitivity exists because competing processes are at work. These processes are described in terms of energy box diagnostics. The most realistic case lies on the cusp of this sensitivity. Further physical parametrizations are then added, including surface fluxes and upright moist convection. These have the effect of increasing wave amplitude, but the sensitivity to initial conditions of relative humidity remains. Finally, 'control' and 'doubled CO2' life cycles are performed, with initial conditions taken from the time-mean zonal-mean output of equilibrium GCM experiments. The attenuation of the wave resulting from reduced baroclinicity is more pronounced than any effect due to changes in initial moisture.Key words. Meteorology and atmospheric dynamics (climatology; convective processes; synoptic-scale meteorology)

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