Precipitation Enhancement in Squall Lines Moving Over Mountainous Coastal Regions

2021 ◽  
Author(s):  
Fan Wu ◽  
Kelly Lombardo
Keyword(s):  
Convective Available Potential Energy ◽  
Leading Edge ◽  
Sea Breeze ◽  
Cold Pool ◽  
Pressure Perturbation ◽  
Coastal Regions ◽  
Low Level ◽  
Squall Lines ◽  
Sloping Terrain ◽  
Precipitation Enhancement

AbstractA mechanism for precipitation enhancement in squall lines moving over mountainous coastal regions is quantified through idealized numerical simulations. Storm intensity and precipitation peak over the sloping terrain as storms descend from an elevated plateau toward the coastline and encounter the marine atmospheric boundary layer (MABL). Storms are most intense as they encounter the deepest MABLs. As the descending storm outflow collides with a moving MABL (sea breeze), surface and low-level air parcels initially accelerate upward, though their ultimate trajectory is governed by the magnitude of the negative non-hydrostatic inertial pressure perturbation behind the cold pool leading edge. For shallow MABLs, the baroclinic gradient across the gust front generates large horizontal vorticity, a low-level negative pressure perturbation, and thus a downward acceleration of air parcels following their initial ascent. A deep MABL reduces the baroclinically-generated vorticity, leading to a weaker pressure perturbation and minimal downward acceleration, allowing air to accelerate into a storm’s updraft.Once storms move away from the terrain base and over the full depth of the MABLs, storms over the deepest MABLs decay most rapidly, while those over the shallowest MABLs initially intensify. Though elevated ascent exists above all MABLs, the deepest MABLs substantially reduce the depth of the high-θe layer above the MABLs and limit instability. This relationship is insensitive to MABL temperature, even though surface-based ascent is present for the less cold MABLs, the MABL thermal deficit is smaller, and convective available potential energy (CAPE) is higher.

How does vertical wind shear influence entrainment in squall lines?

2021 ◽  
Author(s):  
Jake P. Mulholland ◽  
John M. Peters ◽  
Hugh Morrison
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Boundary Shear ◽  
Squall Lines ◽  
Outflow Boundary

AbstractThe influence of vertical wind shear on updraft entrainment in squall lines is not well understood. To address this knowledge gap, a suite of high-resolution idealized numerical model simulations of squall lines were run in various vertical wind shear (hereafter “shear”) environments to study the effects of shear on entrainment in deep convective updrafts. Low-level horizontal mass flux into the leading edge of the cold pool was strongest in the simulations with the strongest low-level shear. These simulations consequently displayed wider updrafts, less entrainment-driven dilution, and larger buoyancy than the simulations with comparatively weak low-level shear. An analysis of vertical accelerations along trajectories that passed through updrafts showed larger net accelerations from buoyancy in the simulations with stronger low-level shear, which demonstrates how less entrainment-driven dilution equated to stronger updrafts. The effects of upper-level shear on entrainment and updraft vertical velocities were generally less pronounced than the effects of low-level shear. We argue that in addition to the outflow boundary-shear interactions and their effect on updraft tilt established by previous authors, decreased entrainment-driven dilution is yet another beneficial effect of strong low-level shear on squall line updraft intensity.

scholarly journals Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008)

2014 ◽  
Vol 142 (8) ◽  
pp. 2838-2859 ◽  
Author(s):  
Buo-Fu Chen ◽  
Russell L. Elsberry ◽  
Cheng-Shang Lee
Keyword(s):  
Inner Core ◽  
Outer Region ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Cold Pool ◽  
Convective System ◽  
Rain Area ◽  
Characteristic Structure ◽  
Low Level ◽  
Mesoscale Convective

Abstract Outer mesoscale convective systems (OMCSs) are long-lasting, heavy rainfall events separate from the inner-core rainfall that have previously been shown to occur in 22% of western North Pacific tropical cyclones (TCs). Environmental conditions accompanying the development of 62 OMCSs are contrasted with the conditions in TCs that do not include an OMCS. The development, kinematic structure, and maintenance mechanisms of an OMCS that occurred to the southwest of Typhoon Fengshen (2008) are studied with Weather Research and Forecasting Model simulations. Quick Scatterometer (QuikSCAT) observations and the simulations indicate the low-level TC circulation was deflected around the Luzon terrain and caused an elongated, north–south moisture band to be displaced to the west such that the OMCS develops in the outer region of Fengshen rather than spiraling into the center. Strong northeasterly vertical wind shear contributed to frictional convergence in the boundary layer, and then the large moisture flux convergence in this moisture band led to the downstream development of the OMCS when the band interacted with the monsoon flow. As the OMCS developed in the region of low-level monsoon westerlies and midlevel northerlies associated with the outer circulation of Fengshen, the characteristic structure of a rear-fed inflow with a leading stratiform rain area in the cross-line direction (toward the south) was established. A cold pool (Δθ < −3 K) associated with the large stratiform precipitation region led to continuous formation of new cells at the leading edge of the cold pool, which contributed to the long duration of the OMCS.

Impact of Environmental Variations on Simulated Squall Lines Interacting with Terrain

2011 ◽  
Vol 139 (10) ◽  
pp. 3163-3183 ◽  
Author(s):  
Casey E. Letkewicz ◽  
Matthew D. Parker
Keyword(s):  
Wind Profile ◽  
Windward Side ◽  
Squall Line ◽  
Cold Pool ◽  
Convective System ◽  
Low Level ◽  
Convective Systems ◽  
Squall Lines ◽  
Strong Convection ◽  
The Mean

Abstract The complex evolution of convective systems crossing (or attempting to cross) mountainous terrain represents a substantial forecasting challenge. This study examines the processes associated with environments of “crossing” squall lines (which were able to redevelop strong convection in the lee of a mountain barrier) and “noncrossing” squall lines (which were not able to redevelop strong convection downstream of the barrier). In particular, numerical simulations of mature convective systems crossing idealized terrain roughly approximating the Appalachian Mountains were used to test the first-order impact of variations in the vertical wind profile upon system maintenance. By itself, the wind profile showed no ability to uniquely discriminate between simulated crossing and noncrossing squall lines; each test revealed a similar pattern of orographic enhancement, suppression, and lee reinvigoration in which a hydraulic jump deepened the system’s cold pool and renewed the low-level lifting. Increasing the mean wind led to greater enhancement of vertical velocities on the windward side of the barrier and greater suppression on the lee side. Variations in the low-level shear influenced the temperature and depth of the outflow, which in turn altered the lifting along the system’s gust front. However, in all of the wind profile tests, convection redeveloped in the lee. Additional simulations explored more marginal environments in which idealized low-level cooling or drying stabilized the downstream environment. In most such tests, the systems weakened but the presence of CAPE aloft still enabled the systems to survive in the lee. However, the combination of a stronger mean wind with diminished CAPE and increased convective inhibition (CIN) was ultimately found to eliminate downstream redevelopment and produce a noncrossing mesoscale convective system (MCS). Within these experiments, the ability of a squall line to cross a barrier similar to the Appalachians is primarily tied to the characteristics of the downstream thermodynamic environment; however, as the lee thermodynamic environment becomes less favorable, the mean wind exerts a greater influence on system intensity and redevelopment.

A Squall-Line-Like Principal Rainband in Typhoon Hagupit (2008) Observed by Airborne Doppler Radar

2014 ◽  
Vol 71 (7) ◽  
pp. 2733-2746 ◽  
Author(s):  
Xiaowen Tang ◽  
Wen-Chau Lee ◽  
Michael Bell
Keyword(s):  
Wind Shear ◽  
Doppler Radar ◽  
Structural Characteristics ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Squall Lines ◽  
Convective Cells

Abstract This study examines the structure and dynamics of Typhoon Hagupit’s (2008) principal rainband using airborne radar and dropsonde observations. The convection in Hagupit’s principal rainband was organized into a well-defined line with trailing stratiform precipitation on the inner side. Individual convective cells had intense updrafts and downdrafts and were aligned in a wavelike pattern along the line. The line-averaged vertical cross section possessed a slightly inward-tilting convective core and two branches of low-level inflow feeding the convection. The result of a thermodynamic retrieval showed a pronounced cold pool behind the convective line. The horizontal and vertical structures of this principal rainband show characteristics that are different than the existing conceptual model and are more similar to squall lines and outer rainbands. The unique convective structure of Hagupit’s principal rainband was associated with veering low-level vertical wind shear and large convective instability in the environment. A quantitative assessment of the cold pool strength showed that it was quasi balanced with that of the low-level vertical wind shear. The balanced state and the structural characteristics of convection in Hagupit’s principal rainband were dynamically consistent with the theory of cold pool dynamics widely applied to strong and long-lived squall lines. The analyses suggest that cold pool dynamics played a role in determining the principal rainband structure in addition to storm-scale vortex dynamics.

scholarly journals Tropical Cyclogenesis Associated with Premonsoon Climatological Dryline over the Bay of Bengal

2021 ◽  
Author(s):  
Nasreen Akter
Keyword(s):  
Bay Of Bengal ◽  
Convective Available Potential Energy ◽  
Leading Edge ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Rear Side ◽  
Convective System ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Stratiform Region

Abstract Tropical cyclones of the Bay of Bengal (BoB) that formed near the synoptic-scale dryline usually intensified over a short distance (~600-800 km) within 3 days and caused severe destruction after landfall. High-resolution simulations of very severe cyclonic storms in association with dryline indicate that the meridional shear aids in the development of a group of linear convective cells that mature as an east-west oriented quasi-linear convective system (QLCS) within the boundary between the dry-moist air masses. The leading edge deep convections are supported by low-level moist southwesterly inflow; however, the typical mid-level mesoscale convective vortex (MCV) associated with these QLCS is unremarkable due to a very narrow trailing stratiform region within the QLCS. Supercells are likely to be organized within the QLCS due to extremely unstable atmospheric conditions resulting from a strong vertical shear of 27-39 m s−1 between 0-6 km and large convective available potential energy of >3000 J kg−1. The vertical shear veering with height causes several numbers of low-level mesovortices having diameters less than 10 km at the leading edge in the different convective stages of the QLCS. The dryline aloft in the BoB produces horizontal positive shear vorticity of the order 10–5 s−1 with higher values in the levels 850-600 hPa. The advection of intense cloud-scale cyclonic vortices (~10–3 s−1) assists and enhances a cyclonic vortex to the rear side of the QLCS that performs as an MCV for cyclogenesis over the BoB.

scholarly journals On the Squall Lines Preceding Landfalling Tropical Cyclones in China

2012 ◽  
Vol 140 (2) ◽  
pp. 445-470 ◽  
Author(s):  
Zhiyong Meng ◽  
Yunji Zhang
Keyword(s):  
Tropical Cyclones ◽  
Radar Reflectivity ◽  
Vertical Shear ◽  
Cold Pool ◽  
Low Level ◽  
Squall Lines ◽  
Suboptimal Condition ◽  
Moist Environment ◽  
Conditional Instability ◽  
Landfalling Tropical Cyclones

Based on a 3-yr (2007–09) mosaic of radar reflectivity and conventional surface and synoptic radiosonde observations, the general features of squall lines preceding landfalling tropical cyclones (TCs) (pre-TC) in China are examined and compared with their midlatitude and subtropical counterparts. The results show that about 40% of landfalling TCs are associated with pre-TC squall lines with high-occurring frequency in August and from late afternoon to midnight. Most pre-TC squall lines form in a broken-line mode with a trailing-stratiform organization. On average, they occur about 600 km from the TC center in the front-right quadrant with a maximum length of 220 km, a maximum radar reflectivity of 57–62 dBZ, a life span of 4 h, and a moving speed of 12.5 m s−1. Pre-TC squall lines are generally shorter in lifetime and length than typical midlatitude squall lines. Pre-TC squall lines tend to form in the transition area between the parent TC and subtropical high in a moist environment and with a weaker cold pool than their midlatitude counterparts. The environmental 0–3-km vertical shear is around 10 m s−1 and generally normal to the orientation of the squall lines. This weak shear makes pre-TC squall lines in a suboptimal condition according to the Rottuno–Klemp–Weisman (RKW) theory. Convection is likely initiated by low-level mesoscale frontogenesis, convergence, and/or confluence instead of synoptic-scale forcing. The parent TC may contribute to (i) the development of convection by enhancing conditional instability and low-level moisture supply, and (ii) the linear organization of discrete convection through the interaction between the TC and the neighboring environmental system.

Numerical Simulations of the Effects of Coastlines on the Evolution of Strong, Long-Lived Squall Lines

2007 ◽  
Vol 135 (5) ◽  
pp. 1710-1731 ◽  
Author(s):  
Todd P. Lericos ◽  
Henry E. Fuelberg ◽  
Morris L. Weisman ◽  
Andrew I. Watson
Keyword(s):  
Numerical Simulations ◽  
Land Surface ◽  
Leading Edge ◽  
Squall Line ◽  
Cold Pool ◽  
Radiation Physics ◽  
Advanced Regional Prediction System ◽  
Squall Lines ◽  
Regional Prediction ◽  
The Right

Abstract This study develops conceptual models of how a land–water interface affects the strength and structure of squall lines. Two-dimensional numerical simulations using the Advanced Regional Prediction System model are employed. Five sets of simulations are performed, each testing eight wind shear profiles of varying strength and depth. The first set of simulations contains a squall line but no surface or radiation physics. The second and third sets do not contain a squall line but include surface and radiation physics with a land surface on the right and a water surface on the left of the domain. The land is either warmer or cooler than the sea surface. These three simulations provide a control for later simulations. Finally, the remaining two simulation sets examine squall-line interaction with a relatively cool or warm land surface. The simulations document the thermodynamic and shear characteristics of squall lines interacting with the coastline. Results show that the inclusion of a land surface did not sufficiently affect the thermodynamic properties ahead of the squall line to change its overall structure. Investigation of ambient shear ahead of the squall line revealed that the addition of either warm or cool land reduced the strength of the net circulation in the inflow layer as measured by ambient shear. The amount of reduction in shear was found to be directly proportional to the depth and strength of the original shear layer. For stronger and deeper shears, the reduction in shear is sufficiently great that the buoyancy gradient circulation at the leading edge of the cold pool is no longer in balance with the shear circulation leading to changes in squall-line updraft structure. The authors hypothesize two ways by which a squall line might respond to passing from water to land. The weaker and more shallow the ambient shear, the greater likelihood that the squall-line structure remains unaffected by this transition. Conversely, the stronger and deeper the shear, the greater likelihood that the squall line changes updraft structure from upright/downshear to upshear tilted.

Dynamical Effects of Aerosol Perturbations on Simulated Idealized Squall Lines

2014 ◽  
Vol 142 (3) ◽  
pp. 991-1009 ◽  
Author(s):  
Zachary J. Lebo ◽  
Hugh Morrison
Keyword(s):  
Mass Flux ◽  
Wind Shear ◽  
Cold Pool ◽  
Strong Wind ◽  
Low Level ◽  
Aerosol Loading ◽  
Squall Lines ◽  
Wide Range ◽  
Wind Shears ◽  
Convective Mass Flux

Abstract The dynamical effects of increased aerosol loading on the strength and structure of numerically simulated squall lines are explored. Results are explained in the context of Rotunno–Klemp–Weisman (RKW) theory. Changes in aerosol loading lead to changes in raindrop size and number that ultimately affect the strength of the cold pool via changes in evaporation. Thus, the balance between cold pool and low-level wind shear–induced vorticities can be changed by an aerosol perturbation. Simulations covering a wide range of low-level wind shears are performed to study the sensitivity to aerosols in different environments and provide more general conclusions. Simulations with relatively weak low-level environmental wind shear (0.0024 s−1) have a relatively strong cold pool circulation compared to the environmental shear. An increase in aerosol loading leads to a weakening of the cold pool and, hence, a more optimal balance between the cold pool– and environmental shear–induced circulations according to RKW theory. Consequently, there is an increase in the convective mass flux of nearly 20% in polluted conditions relative to pristine. This strengthening coincides with more upright convective updrafts and a significant increase (nearly 20%) in cumulative precipitation. An increase in aerosol loading in a strong wind shear environment (0.0064 s−1) leads to less optimal storms and a suppression of the convective mass flux and precipitation. This occurs because the cold pool circulation is weak relative to the environmental shear when the shear is strong, and further weakening of the cold pool with high aerosol loading leads to an even less optimal storm structure (i.e., convective updrafts begin to tilt downshear).

scholarly journals Low-Level Convergence Lines over Northeastern Australia. Part I: The North Australian Cloud Line

2006 ◽  
Vol 134 (11) ◽  
pp. 3092-3108 ◽  
Author(s):  
Robert Goler ◽  
Michael J. Reeder ◽  
Roger K. Smith ◽  
Harald Richter ◽  
Sarah Arnup ◽  
...  
Keyword(s):  
West Coast ◽  
East Coast ◽  
Gravity Current ◽  
Leading Edge ◽  
Sea Breeze ◽  
Land Breeze ◽  
The West ◽  
Sea Breezes ◽  
Low Level ◽  
Cape York

Abstract Observations of dry-season north Australian cloud lines (NACLs) that form in the Gulf of Carpentaria region of northern Australia and the sea-breeze circulations that initiate them are described. The observations were made during the 2002 Gulf Lines Experiment (GLEX) and include measurements made by an instrumented research aircraft. The observations are compared with numerical simulations made from a two-dimensional cloud-scale model. Particular emphasis is placed on the interaction between the east coast and west coast sea breezes near the west coast of Cape York Peninsula. The sea breezes are highly asymmetric due to the low-level easterly synoptic flow over the peninsula. The west coast sea breeze is well defined with a sharp leading edge since the opposing flow limits its inland penetration, keeping it close to its source of cold air. In contrast, the east coast sea breeze is poorly defined since it is aided by the easterly flow and becomes highly modified by daytime convective mixing as it crosses over the peninsula. Both the observations and the numerical model show that, in the early morning hours, the mature NACL forms at the leading edge of a gravity current. The numerical model simulations show that this gravity current arises as a westward-moving land breeze from Cape York Peninsula. Convergence at the leading edge of this land breeze is accompanied by ascent, which when strong enough produces cloud. Observations show that the decay of the NACL is associated with a decline in the low-level convergence and a weakening of the ascent.

scholarly journals The Behavior of Squall Lines in Horizontally Heterogeneous Coastal Environments

2018 ◽  
Vol 75 (4) ◽  
pp. 1243-1269 ◽  
Author(s):  
Kelly Lombardo ◽  
Tristan Kading
Keyword(s):  
Deep Convection ◽  
Internal Gravity Wave ◽  
Sea Breeze ◽  
Squall Line ◽  
Cold Pool ◽  
Stable Layer ◽  
Squall Lines ◽  
Unstable Layer ◽  
Storm Characteristics ◽  
Marine Layer

Abstract Inland squall lines respond to the stable marine atmospheric boundary layer (MABL) as they move toward a coastline and offshore. As a storm’s cold pool collides with the marine layer, characteristics of both determine the resulting convective forcing mechanism over the stable layer and storm characteristics. Idealized numerical experiments exploring a parameter space of MABL characteristics show that the postcollision forcing mechanism is determined by the buoyancy of the cold pool relative to the MABL. When the outflow is less buoyant, storms are forced by a cold pool within the marine environment. When the buoyancies are equivalent, a hybrid cold pool–internal gravity wave develops after the collision. The collision between a cold pool and less buoyant MABL initiates internal waves along the stable layer, regardless of MABL depth. These waves are inefficient at lifting air into the storm, and ascent from the trailing cold pool is needed to support deep convection. Storm intensity decreases with deeper and less buoyant MABLs, in part due to the reduction in elevated instability. Precipitation is enhanced just prior to the collision between a storm and the deepest marine layers. Storms modify their environment downstream, leading to the development of a moist adiabatic unstable layer and a lowering of the level of free convection (LFC) to below the top of the deepest marine layer. An MABL moving as a sea breeze into the storm-modified air successfully lifts parcels to the new LFC, generating convective towers ahead of the squall line. This mechanism may contribute to increased coastal flash flooding risks during observed events.

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