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 Isentropic Analysis of a Simulated Hurricane

2016 ◽  
Vol 73 (5) ◽  
pp. 1857-1870 ◽  
Author(s):  
Agnieszka A. Mrowiec ◽  
Olivier M. Pauluis ◽  
Fuqing Zhang
Keyword(s):  
Mass Transport ◽  
Potential Temperature ◽  
Wrf Model ◽  
Secondary Circulation ◽  
Equivalent Potential Temperature ◽  
Moist Convection ◽  
Shallow Convection ◽  
High Entropy ◽  
Equivalent Potential ◽  
Wide Range

Abstract Hurricanes, like many other atmospheric flows, are associated with turbulent motions over a wide range of scales. Here the authors adapt a new technique based on the isentropic analysis of convective motions to study the thermodynamic structure of the overturning circulation in hurricane simulations. This approach separates the vertical mass transport in terms of the equivalent potential temperature of air parcels. In doing so, one separates the rising air parcels at high entropy from the subsiding air at low entropy. This technique filters out oscillatory motions associated with gravity waves and separates convective overturning from the secondary circulation. This approach is applied here to study the flow of an idealized hurricane simulation with the Weather Research and Forecasting (WRF) Model. The isentropic circulation for a hurricane exhibits similar characteristics to that of moist convection, with a maximum mass transport near the surface associated with a shallow convection and entrainment. There are also important differences. For instance, ascent in the eyewall can be readily identified in the isentropic analysis as an upward mass flux of air with unusually high equivalent potential temperature. The isentropic circulation is further compared here to the Eulerian secondary circulation of the simulated hurricane to show that the mass transport in the isentropic circulation is much larger than the one in secondary circulation. This difference can be directly attributed to the mass transport by convection in the outer rainband and confirms that, even for a strongly organized flow like a hurricane, most of the atmospheric overturning is tied to the smaller scales.

Mesoscale Thermodynamic Influences on Convection Initiation near a Surface Dryline in a Convection-Permitting Ensemble

2015 ◽  
Vol 143 (9) ◽  
pp. 3726-3753 ◽  
Author(s):  
Stanley B. Trier ◽  
Glen S. Romine ◽  
David A. Ahijevych ◽  
Robert J. Trapp ◽  
Russ S. Schumacher ◽  
...  
Keyword(s):  
Deep Convection ◽  
Potential Temperature ◽  
Surface Fluxes ◽  
Physical Processes ◽  
Sheared Flow ◽  
Severe Convection ◽  
Equivalent Potential ◽  
Pbl Scheme ◽  
Convection Initiation ◽  
Negative Buoyancy

Abstract In this study, the authors examine initiation of severe convection along a daytime surface dryline in a 10-member ensemble of convection-permitting simulations. Results indicate that the minimum buoyancy Bmin of PBL air parcels must be small (Bmin > −0.5°C) for successful deep convection initiation (CI) to occur along the dryline. Comparing different ensemble members reveals that CAPE magnitudes (allowing for entrainment) and the width of the zone of negligible Bmin extending eastward from the dryline act together to influence CI. Since PBL updrafts that initiate along the dryline move rapidly northeast in the vertically sheared flow as they grow into the free troposphere, a wider zone of negligible Bmin helps ensure adequate time for incipient storms to mature, which, itself, is hastened by larger CAPE. Local Bmin budget calculations and trajectory analysis are used to quantify physical processes responsible for the reduction of negative buoyancy prior to CI. Here, the grid-resolved forcing and forcing from temperature and moisture tendencies in the PBL scheme (arising from surface fluxes) contribute about equally in ensemble composites. However, greater spatial variability in grid-resolved forcing focuses the location of the greatest net forcing along the dryline. The grid-resolved forcing is influenced by a thermally direct vertical circulation, where time-averaged ascent at the east edge of the dryline results in locally deeper moisture and cooler conditions near the PBL top. Horizontal temperature advection spreads the cooler air eastward above higher equivalent potential temperature air at source levels of convecting air parcels, resulting in a wider zone of negligible Bmin that facilitates sustained CI.

scholarly journals The Three Atmospheric Circulations over the Indian Ocean and the Maritime Continent and Their Modulation by the Passage of the MJO

2019 ◽  
Vol 76 (2) ◽  
pp. 517-531
Author(s):  
Daria Kuznetsova ◽  
Thibaut Dauhut ◽  
Jean-Pierre Chaboureau
Keyword(s):  
Indian Ocean ◽  
Deep Convection ◽  
Potential Temperature ◽  
Active Phase ◽  
Equivalent Potential Temperature ◽  
Maritime Continent ◽  
Deep Circulation ◽  
Tropical Tropopause ◽  
Equivalent Potential ◽  
The Indian Ocean

Abstract The passage of the Madden–Julian oscillation (MJO) over the Indian Ocean and the Maritime Continent is investigated during the episode of 23–30 November 2011. A Meso-NH convection-permitting simulation with a horizontal grid spacing of 4 km is examined. The simulation reproduces the MJO signal correctly, showing the eastward propagation of the primary rain activity. The atmospheric overturning is analyzed using the isentropic method, which separates the ascending air with high equivalent potential temperature from the subsiding air with low equivalent potential temperature. Three key circulations are found. The first two circulations are a tropospheric deep circulation spanning from the surface to an altitude of 14 km and an overshoot circulation within the tropical tropopause layer. As expected for circulations associated with deep convection, their intensities, as well as their diabatic tendencies, increase during the active phase of the MJO, while their entrainment rates decrease. The third circulation is characterized by a rising of air with low equivalent potential temperature in the lower free troposphere. The intensity of the circulation, as well as its depth, varies with the MJO activity. During the suppressed phase, this circulation is associated with a dry air intrusion from the subtropical region into the tropical band and shows a strong drying of the lower to middle troposphere.

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.

scholarly journals Summertime Post-Cold-Frontal Marine Stratocumulus Transition Processes over the Eastern North Atlantic

2020 ◽  
Vol 77 (6) ◽  
pp. 2011-2037 ◽  
Author(s):  
Melissa kazemirad ◽  
Mark A. Miller
Keyword(s):  
North Atlantic ◽  
Marine Boundary Layer ◽  
Potential Temperature ◽  
Wrf Model ◽  
Cloud Base ◽  
Equivalent Potential Temperature ◽  
Cumulus Clouds ◽  
Equivalent Potential ◽  
Atmospheric Radiation Measurement ◽  
Eastern North Atlantic

Abstract Marine boundary layer (MBL) cloud morphology associated with two summertime cold fronts over the eastern North Atlantic (ENA) is investigated using high-resolution simulations from the Weather Research and Forecasting (WRF) Model and observations from the Atmospheric Radiation Measurement (ARM) ENA Climate Research Facility. Lagrangian trajectories are used to study the evolution of post-cold-frontal MBL clouds from solid stratocumulus to broken cumulus. Clouds within specified domains in the vicinity of transitions are classified according to their degree of decoupling, and cloud-base and cloud-top breakup processes are evaluated. The Lagrangian derivative of the surface latent heat flux is found to be strongly correlated with that of the cloud fraction at cloud base in the simulations. Cloud-top entrainment instability (CTEI) is shown to operate only in the decoupled MBL. A new indicator of inversion strength at cloud top that employs the vertical gradients of equivalent potential temperature and saturation equivalent potential temperature, which can be computed directly from soundings, is proposed as an alternative to CTEI. Overall, results suggest that the deepening–warming hypothesis suggested by Bretherton and Wyant explains many of the characteristics of the summertime postfrontal MBL evolution of cloud structure over the ENA, thereby widening the phase space over which the hypothesis may be applied. A subset of the deepening–warming hypothesis involving warming initially dominating over moistening is proposed. It is postulated that changes in climate change–induced modifications in cold-frontal structure over the ENA may be accompanied by coincident changes in the location and timing of MBL cloud transitions in the post-cold-frontal environment.

Mesoscale Convective Vortices Observed during BAMEX. Part II: Influences on Secondary Deep Convection

2007 ◽  
Vol 135 (6) ◽  
pp. 2051-2075 ◽  
Author(s):  
Stanley B. Trier ◽  
Christopher A. Davis
Keyword(s):  
Deep Convection ◽  
Potential Temperature ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Large Temperature ◽  
Equivalent Potential Temperature ◽  
Equivalent Potential ◽  
Mesoscale Convective ◽  
Conditional Instability ◽  
Convective Vortices

Abstract Observations from the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment are used to examine the role of the five mesoscale convective vortices described in Part I on heavy precipitation during the daytime heating cycle. Persistent widespread stratiform rain without deep convection occurs for two strong MCVs in conditionally stable environments with strong vertical shear. Two other MCVs in moderate-to-strong vertical shear have localized redevelopment of deep convection (termed secondary convection) on their downshear side, where conditional instability exists. The strongest of the five MCVs occurs in weak vertical shear and has widespread secondary convection, which is most intense on its conditionally unstable southeast periphery. The two MCVs with only localized secondary convection have well-defined mesoscale vertical motion couplets with downshear ascent and upshear descent above the planetary boundary layer (PBL). Although the amplitude is significantly greater, the kinematically derived vertical motion dipole resembles that implied by steady, vortex-relative isentropic flow, consistent with previous idealized (dry) simulations and diagnoses based on operational model analyses. In the other three cases with either widespread precipitation or weak environmental vertical shear, the kinematic and isentropic vertical motion patterns are poorly correlated. Vertical motions above the PBL provide a focus for secondary convection through adiabatic cooling downshear and adiabatic warming upshear of the MCV center. The MCVs occur within surface frontal zones with large temperature and moisture gradients across the environmental vertical shear vector (Part I). Thus, the effect of vertical motions on conditional instability is reinforced by horizontal advections of high equivalent potential temperature air downshear, and low equivalent potential temperature air upshear within the PBL. On average, the quadrant immediately right of downshear (typically southeast of the MCV center) best supports deep convection because of the juxtaposition of greatest mesoscale ascent, high equivalent potential temperature PBL air, and MCV-induced enhancement of the vertical shear.

What are the Best Thermodynamic Quantity and Function to Define a Front in Gridded Model Output?

2019 ◽  
Vol 100 (5) ◽  
pp. 873-895 ◽  
Author(s):  
Carl M. Thomas ◽  
David M. Schultz
Keyword(s):  
Real Time ◽  
Potential Temperature ◽  
Thermodynamic Quantity ◽  
Kinematics And Dynamics ◽  
Horizontal Gradient ◽  
Model Output ◽  
Equivalent Potential Temperature ◽  
Thermal Front ◽  
Equivalent Potential ◽  
Definition Of

AbstractFronts can be computed from gridded datasets such as numerical model output and reanalyses, resulting in automated surface frontal charts and climatologies. Defining automated fronts requires quantities (e.g., potential temperature, equivalent potential temperature, wind shifts) and kinematic functions (e.g., gradient, thermal front parameter, and frontogenesis). Which are the most appropriate to use in different applications remains an open question. This question is investigated using two quantities (potential temperature and equivalent potential temperature) and three functions (magnitude of the horizontal gradient, thermal front parameter, and frontogenesis) from both the context of real-time surface analysis and climatologies from 38 years of reanalyses. The strengths of potential temperature to identify fronts are that it represents the thermal gradients and its direct association with the kinematics and dynamics of fronts. Although climatologies using potential temperature show features associated with extratropical cyclones in the storm tracks, climatologies using equivalent potential temperature include moisture gradients within air masses, most notably at low latitudes that are unrelated to the traditional definition of a front, but may be representative of a broader definition of an airmass boundary. These results help to explain previously published frontal climatologies featuring maxima of fronts in the subtropics and tropics. The best function depends upon the purpose of the analysis, but Petterssen frontogenesis is attractive, both for real-time analysis and long-term climatologies, in part because of its link to the kinematics and dynamics of fronts. Finally, this study challenges the conventional definition of a front as an airmass boundary and suggests that a new, dynamically based definition would be useful for some applications.

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