Diurnal Preconditioning of Subtropical Coastal Convective Storm Environments

Boundary layer evolution in response to diurnal forcing is manifested at the mesobeta and smaller scales of the atmosphere. Because this variability resides on subsynoptic scales, the potential influence upon convective storm environments is often not captured in coarse observational and modeling datasets, particularly for complex physical settings such as coastal regions. A detailed observational analysis of diurnally forced preconditioning for convective storm environments of South East Queensland, Australia (SEQ), during the Coastal Convective Interactions Experiment (2013–15) is presented. The observations used include surface-based measurements, aerological soundings, and dual-polarization Doppler radar. The sea-breeze circulation was found to be the dominant influence; however, profile modification by the coastward advection of the continental boundary layer was found to be an essential mechanism for favorable preconditioning of deep convection. This includes 1) enhanced moisture in the city of Brisbane, potentiality due to an urban heat island–enhanced land–sea thermal contrast, 2) significant afternoon warming and moistening above the sea breeze resulting from the advection of the inland convective boundary layer coastward under prevailing westerly flow coupled with the sea-breeze return flow, and 3) substantial variations in near-surface moisture likely associated with topography and land use. For the 27 November 2014 Brisbane hailstorm, which caused damages exceeding $1.5 billion Australian dollars (AUD), the three introduced diurnal preconditioning processes are shown to favor a mesoscale convective environment supportive of large hailstone growth. The hybrid high-precipitation supercell storm mode noted for this event and previous similar events in SEQ is hypothesized to be more sensitive to variations in near-surface and boundary layer instability in contrast to contemporary supercell storms.

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The Dryline on 22 May 2002 during IHOP_2002: Convective-Scale Measurements at the Profiling Site

Monthly Weather Review ◽

10.1175/mwr3054.1 ◽

2006 ◽

Vol 134 (1) ◽

pp. 294-310 ◽

Cited By ~ 34

Author(s):

Belay Demoz ◽

Cyrille Flamant ◽

Tammy Weckwerth ◽

David Whiteman ◽

Keith Evans ◽

...

Keyword(s):

Boundary Layer ◽

Continuous Wave ◽

Doppler Radar ◽

Layer Structure ◽

Deep Convection ◽

Radiosonde Data ◽

Small Scale ◽

Convective Initiation ◽

Near Surface ◽

Fine Line

Abstract A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment. Moisture and temperature jumps of about 3 g kg−1 and 1–2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1–2 g kg−1) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s−1 and variable horizontal widths of 2–5 km, although some were as wide as 7–8 km. These updrafts were up to 1.5 g kg−1 moister than the surrounding environment. Although models suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms over the intensive observation region (IOR). Possible reasons for this lack of convection are discussed. Strong capping inversion and moisture detrainment between the lifting condensation level and the level of free convection related to an overriding drier air, together with the relatively small near-surface moisture values (less than 10 g kg−1), were detrimental to CI in this case.

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A Numerical Study of the Evolving Convective Boundary Layer and Orographic Circulation around the Santa Catalina Mountains in Arizona. Part I: Circulation without Deep Convection

Monthly Weather Review ◽

10.1175/2009mwr3098.1 ◽

2010 ◽

Vol 138 (5) ◽

pp. 1902-1922 ◽

Cited By ~ 12

Author(s):

J. Cory Demko ◽

Bart Geerts

Keyword(s):

Boundary Layer ◽

Numerical Study ◽

Deep Convection ◽

Weather Research And Forecasting ◽

Return Flow ◽

Convective Boundary ◽

Warm Anomaly ◽

Orographic Convection ◽

Upper Level ◽

Santa Catalina Mountains

Abstract The daytime evolution of the thermally forced boundary layer (BL) circulation over an isolated mountain, about 30 km in diameter and 2 km high, is examined by means of numerical simulations validated with data collected in the Cumulus Photogrammetric, In Situ, and Doppler Observations (CuPIDO) field campaign. Two cases are presented, one remains cloud free in the simulations, and the second produces orographic convection just deep enough to yield a trace of precipitation. The Weather Research and Forecasting version 3 simulations, at a resolution of 1 km, compare well with CuPIDO observations. The simulations reveal a solenoidal circulation mostly contained within the convective BL, but this circulation and especially its upper-level return flow branch are not immediately apparent since they are overwhelmed by BL thermals. A warm anomaly forms over the high terrain during the day, but it is rather shallow and does not extend over the depth of the convective BL, which bulges over the mountain. Low-level mountain-scale convergence (MSC), driven by an anabatic pressure gradient, deepens during the day. Even relatively shallow and relatively small cumulus convection can temporarily overwhelm surface MSC by cloud shading and convective downdraft dynamics. In the evening drainage flow develops near the surface before the anabatic forcing ceases, and anabatic flow is still present in the residual mixed layer, decoupled from the surface. The interaction of the boundary layer circulation with deep orographic convection is examined in Part II of this study.

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Dynamics of the Shallow Meridional Circulation around Intertropical Convergence Zones

Journal of the Atmospheric Sciences ◽

10.1175/jas3964.1 ◽

2007 ◽

Vol 64 (7) ◽

pp. 2262-2285 ◽

Cited By ~ 39

Author(s):

David S. Nolan ◽

Chidong Zhang ◽

Shu-hua Chen

Keyword(s):

Boundary Layer ◽

Pressure Gradient ◽

Meridional Circulation ◽

Deep Convection ◽

Hadley Circulation ◽

Sea Breeze ◽

Return Flow ◽

Convective Heating ◽

Moist Convection ◽

East Pacific

Abstract The generally accepted view of the meridional circulation in the tropical east Pacific is that of a single deep overturning cell driven by deep convective heating in the intertropical convergence zone (ITCZ), similar to the zonal mean Hadley circulation. However, recent observations of the atmosphere from the tropical eastern Pacific have called this view into question. In several independent datasets, significant meridional return flows out of the ITCZ region were observed, not only at high altitudes, but also at low altitudes, just above the atmospheric boundary layer. This paper presents a theory and idealized simulations to understand the causes and dynamics of this shallow meridional circulation (SMC). Fundamentally, the SMC can be seen as a large-scale sea-breeze circulation driven by sea surface temperature gradients when deep convection is absent in the ITCZ region. A simple model of this circulation is presented. Using observed values, the sea-breeze model shows that the pressure gradient above the boundary can indeed reverse, leading to the pressure force that drives the shallow return flow out of the ITCZ. The Weather Research and Forecast Model (WRF) is used to simulate an idealized Hadley circulation driven by moist convection in a tropical channel. The SMC is reproduced, with reasonable similarity to the circulation observed in the east Pacific. The simulations confirm that the SMC is driven by a reversal of the pressure gradient above the boundary layer, and that the return flow is strongest when deep convection is absent in the ITCZ, and weakest when deep convection is active. The model also shows that moisture transport out of the ITCZ region is far greater in the low-level shallow return flow than in the high-altitude return flow associated with the deep overturning, and that a budget for water transport in and out of the ITCZ region is grossly incomplete without it. Much of the moisture carried in the shallow return flow is recycled into the boundary layer, but does not appear to contribute to enhanced cloudiness in the subtropical stratocumulus poleward of the ITCZ.

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Unusually Long Duration, Multiple-Doppler Radar Observations of a Front in a Convective Boundary Layer

Monthly Weather Review ◽

10.1175/mwr3261.1 ◽

2007 ◽

Vol 135 (1) ◽

pp. 93-117 ◽

Cited By ~ 12

Author(s):

John R. Stonitsch ◽

Paul M. Markowski

Keyword(s):

Boundary Layer ◽

Density Gradient ◽

Doppler Radar ◽

Deep Convection ◽

Three Dimensional ◽

Horizontal Wind ◽

Normal Density ◽

Wind Fields ◽

Convective Boundary ◽

Convection Initiation

Abstract Dual-Doppler observations acquired by a network of mobile radars deployed in the Oklahoma panhandle on 3 June 2002 are used to document the kinematic structure and evolution of a front. The data were collected during the International H2O Project on a mission to study the initiation of deep convection. Synchronized scanning allowed for the synthesis of three-dimensional wind fields for nearly 5.5 h of the 1557–0000 UTC period. The front initially moved southward as a cold front, stalled, and later retreated northward as a warm front. Deep convection failed to be initiated along the front. In situ thermodynamic measurements obtained by a mobile mesonet were used to document changes in the density gradient at the surface. This paper examines the relationships among the changes in baroclinity, the thermally direct frontal circulation, updraft intensity, alongfront updraft variability, and the intensity of vortices along the front. Increases in the front-normal density gradient tended to be associated with increases in the thermally direct frontal circulation, as expected. Increases in the front-normal density gradient were also associated with an increase in the tilt of the frontal updraft as well as an increase in the contiguity of the updraft along the front, termed the “slabularity.” During periods when the front-normal density gradient and associated thermally direct frontal circulation were weak, the kinematic fields were dominated by boundary layer convection and the slabularity of the front was reduced. Intensification of the front-normal density gradient was accompanied by an increase in the horizontal wind shear and the intensity of vortices that were observed along the front. The vortices modulated the vertical velocity field along the front and therefore the slabularity, too. Thus, although the slabularity was a strong function of the strength of the thermally direct frontal circulation, the slabularity appeared to be modified by vortices in complex ways. Possible implications of the observations for convection initiation are also discussed, particularly with respect to updraft tilt and slabularity.

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Diurnal cycle of precipitation and near-surface atmospheric conditions over the maritime continent: land–sea contrast and impacts of ambient winds in cloud-permitting simulations

Climate Dynamics ◽

10.1007/s00382-021-06012-3 ◽

2021 ◽

Author(s):

Yuntao Wei ◽

Zhaoxia Pu

Keyword(s):

Diurnal Cycle ◽

Large Scale ◽

Deep Convection ◽

Vertical Motion ◽

Sea Breeze ◽

Return Flow ◽

Atmospheric Conditions ◽

Maritime Continent ◽

Regional Variability ◽

Near Surface

AbstractA set of cloud-permitting-scale numerical simulations during January–February 2018 is used to examine the diurnal cycle (DC) of precipitation and near-surface variables (e.g., 2 m temperature, 10 m wind and convergence) over the Indo-Pacific Maritime Continent under the impacts of shore-orthogonal ambient winds (SOAWs). It is found that the DC of these variables and their variabilities of daily maxima, minima, and diurnal amplitudes vary over land, sea, and coastal regions. Among all variables, the DC of precipitation has the highest linear correlation with near-surface convergence (near-surface temperature) over coastal (noncoastal) regions. The correlations among the DCs of precipitation, wind, and heating are greater over the ocean than over land. Sine curves can model accurately the DCs of most variables over the ocean, but not over land. SOAWs act to influence the DC mainly by affecting the diurnal amplitude of the considered variables, with DC being stronger under more strengthened offshore SOAWs, though variable dependence and regional variability exist. Composite analysis over Sumatra reveals that under weak SOAWs, shallow clouds are dominant and cause a pre-moistening effect, supporting shallow-to-deep convection transition. A sea breeze circulation (SBC) with return flow aloft can develop rapidly. Cold pools are better able to trigger new updrafts and contribute to the upscale growth and inland migration of deep convection. In addition, warm gravity waves can propagate upward throughout the troposphere, thereby supporting a strong DC. In contrast, under strong SOAWs, both shallow and middle-high clouds prevail and persist throughout the day. The evolution of moistening and SBC is reduced, leading to weak variation in vertical motion and rainwater confined to the boundary layer. Large-scale winds, moisture, and convection are discussed to interpret how strong SOAWs affect the DC of Sumatra.

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Stochastic Backscatter for Cloud-Resolving Models. Part I: Implementation and Testing in a Dry Convective Boundary Layer

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2166.1 ◽

2008 ◽

Vol 65 (1) ◽

pp. 123-139 ◽

Cited By ~ 11

Author(s):

Sonja Weinbrecht ◽

Paul J. Mason

Keyword(s):

Boundary Layer ◽

Free Convection ◽

Convective Boundary Layer ◽

Deep Convection ◽

Time Development ◽

Convective Boundary ◽

Near Surface ◽

Complex Issue ◽

Marked Influence ◽

Mesh Refinements

Abstract In simulations of deep convection with cloud-resolving models the turbulence is often rather poorly resolved, and the influence of the subfilter-scale parameterization used in such circumstances is probably greater than in better-resolved simulations. Therefore a study to investigate the influence of stochastic backscatter was performed and presented in two papers. This first paper focuses on a description of the stochastic backscatter model and its effect on a neutral and a dry convective boundary layer. The second paper then deals with two cases of deep convection. The dry convective boundary layer is typical of the subcloud layer in deep convection and this study allows for influences on this layer to be investigated separately. As a simple case of convection it also allows for general effects to be identified. The implementation of stochastic backscatter was improved to ensure an appropriate scale of backscatter that is independent of any mesh refinements and always spatially isotropic. It can also be applied in a deep atmosphere with use of the anelastic approximation. Generally the backscatter is found to affect the velocity and scalar variances as well as the spatial structure and time development of the simulations. An additional issue is the marked influence of both varying resolution and backscatter on the near-surface temperature differences. This is an element of the complex issue of surface transfers under free convection conditions. Overall, the investigations show that the backscatter gives some beneficial changes to the simulations, which tend to keep results in less well- resolved cases closer to those in better-resolved simulations.

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Dynamics of the Stratiform Sector of a Tropical Cyclone Rainband

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0245.1 ◽

2013 ◽

Vol 70 (7) ◽

pp. 1891-1911 ◽

Cited By ~ 38

Author(s):

Anthony C. Didlake ◽

Robert A. Houze

Keyword(s):

Boundary Layer ◽

Doppler Radar ◽

Mesoscale Convective System ◽

Convective System ◽

Stratiform Region ◽

Mesoscale Convective ◽

Upward Transport ◽

Radial Outflow ◽

Convective Cells ◽

Low Entropy

Abstract Airborne Doppler radar documented the stratiform sector of a rainband within the stationary rainband complex of Hurricane Rita. The stratiform rainband sector is a mesoscale feature consisting of nearly uniform precipitation and weak vertical velocities from collapsing convective cells. Upward transport and associated latent heating occur within the stratiform cloud layer in the form of rising radial outflow. Beneath, downward transport is organized into descending radial inflow in response to two regions of latent cooling. In the outer, upper regions of the rainband, sublimational cooling introduces horizontal buoyancy gradients, which produce horizontal vorticity and descending inflow similar to that of the trailing-stratiform region of a mesoscale convective system. Within the zone of heavier stratiform precipitation, melting cooling along the outer rainband edge creates a midlevel horizontal buoyancy gradient across the rainband that drives air farther inward beneath the brightband. The organization of this transport initially is robust but fades downwind as the convection dissipates. The stratiform-induced secondary circulation results in convergence of angular momentum above the boundary layer and broadening of the storm's rotational wind field. At the radial location where inflow suddenly converges, a midlevel tangential jet develops and extends into the downwind end of the rainband complex. This circulation may contribute to ventilation of the eyewall as inflow of low-entropy air continues past the rainband in both the boundary layer and midlevels. Given the expanse of the stratiform rainband region, its thermodynamic and kinematic impacts likely help to modify the structure and intensity of the total vortex.

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A Numerical Study of the Evolving Convective Boundary Layer and Orographic Circulation around the Santa Catalina Mountains in Arizona. Part II: Interaction with Deep Convection

Monthly Weather Review ◽

10.1175/2010mwr3318.1 ◽

2010 ◽

Vol 138 (9) ◽

pp. 3603-3622 ◽

Cited By ~ 11

Author(s):

J. Cory Demko ◽

Bart Geerts

Keyword(s):

Boundary Layer ◽

Numerical Study ◽

Deep Convection ◽

Horizontal Resolution ◽

Surface Flow ◽

Convective Boundary ◽

Convergence Line ◽

Orographic Convection ◽

Present Part ◽

Santa Catalina Mountains

Abstract This is the second part of a study that examines the daytime evolution of the thermally forced boundary layer (BL) circulation over a relatively isolated mountain, about 30 km in diameter and 2 km high, and its interaction with locally initiated deep convection by means of numerical simulations validated with data collected in the 2006 Cumulus Photogrammetric, In Situ, and Doppler Observations (CuPIDO) field campaign in southeastern Arizona. Part I examined the BL circulation in cases with, at most, rather shallow orographic cumulus (Cu) convection; the present part addresses deep convection. The results are based on output from version 3 of the Weather Research and Forecasting model run at a horizontal resolution of 1 km. The model output verifies well against CuPIDO observations. In the absence of Cu convection, the thermally forced (solenoidal) circulation is largely contained within the BL over the mountain. Thunderstorm development deepens this BL circulation with inflow over the depth of the BL and outflow in the free troposphere aloft. Primary deep convection results from destabilization over elevated terrain and tends to be triggered along a convergence line, which arises from the solenoidal circulation but may drift downwind of the terrain crest. While the solenoidal anabatic flow converges moisture over the mountain, it also cools the air. Thus, a period of suppressed anabatic flow following a convective episode, at a time when surface heating is still intense, can trigger new and possibly deeper convection. The growth of deep convection may require enhanced convergent flow in the BL, but this is less apparent in the mountain-scale surface flow signal than the decay of orographic convection. A budget study over the mountain suggests that the precipitation efficiency of the afternoon convection is quite low, ~10% in this case.

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A Thermal Plume Model for the Convective Boundary Layer: Representation of Cumulus Clouds

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2256.1 ◽

2008 ◽

Vol 65 (2) ◽

pp. 407-425 ◽

Cited By ~ 73

Author(s):

Catherine Rio ◽

Frédéric Hourdin

Keyword(s):

Boundary Layer ◽

Mass Flux ◽

Convective Boundary Layer ◽

Turbulent Transport ◽

Thermal Plume ◽

Cumulus Clouds ◽

Convective Boundary ◽

Near Surface ◽

Plume Model ◽

Surface Drying

Abstract The “thermal plume model,” a mass-flux scheme combined with a classical diffusive approach, originally developed to represent turbulent transport in the dry convective boundary layer, is extended here to the representation of cloud processes. The modified parameterization is validated in a 1D configuration against results of large eddy simulations (LES), as well as in a 3D configuration against in situ measurements, for a series of cases of dry and cloudy convective boundary layers. Accounting for coherent structures of the mixed layer with the mass-flux scheme improves the representation of the diurnal cycle of the boundary layer, particularly its progressive deepening during the day and the associated near-surface drying. Results also underline the role of the prescription of the mixing of air between the plume and its environment, and of submean-plume fluctuations.

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