Quasi-idealized numerical simulations of processes involved in orogenic convection initiation over the Sierras de Córdoba mountains

Abstract The Sierras de Córdoba (SDC) mountain range in Argentina is a hotspot of deep moist convection initiation (CI). Radar climatology indicates that 44% of daytime CI events that occur near the SDC in spring and summer seasons and that are not associated with the passage of a cold front or an outflow boundary involve a northerly LLJ, and these events tend to preferentially occur over the southeast quadrant of the main ridge of the SDC. To investigate the physical mechanisms acting to cause CI, idealized convection-permitting numerical simulations with a horizontal grid spacing of 1 km were conducted using CM1. The sounding used for initializing the model featured a strong northerly LLJ, with synoptic conditions resembling those in a previously postulated conceptual model of CI over the region, making it a canonical case study. Differential heating of the mountain caused by solar insolation in conjunction with the low-level northerly flow sets up a convergence line on the eastern slopes of the SDC. The southern portion of this line experiences significant reduction in convective inhibition, and CI occurs over the SDC southeast quadrant. Thesimulated storm soon acquires supercellular characteristics, as observed. Additional simulations with varying LLJ strength also show CI over the southeast quadrant. A simulation without background flow generated convergence over the ridgeline, with widespread CI across the entire ridgeline. A simulation with mid- and upper-tropospheric westerlies removed indicates that CI is minimally influenced by gravity waves. We conclude that the low-level jet is sufficient to focus convection initiation over the southeast quadrant of the ridge.

Low-level Mesoscale and Cloud-scale Interactions Promoting Deep Convection Initiation

10.1175/mwr-d-20-0391.1 ◽

2021 ◽

Author(s):

James N. Marquis ◽

Adam C. Varble ◽

Paul Robinson ◽

T. Connor. Nelson ◽

Katja Friedrich

Keyword(s):

Boundary Layer ◽

Deep Convection ◽

Convective Available Potential Energy ◽

Surface Wind ◽

Horizontal Wind ◽

Moist Convection ◽

Surface Precipitation ◽

Low Level ◽

Deep Moist Convection ◽

AbstractData from scanning radars, radiosondes, and vertical profilers deployed during three field campaigns are analyzed to study interactions between cloud-scale updrafts associated with initiating deep moist convection and the surrounding environment. Three cases are analyzed in which the radar networks permitted dual-Doppler wind retrievals in clear air preceding and during the onset of surface precipitation. These observations capture the evolution of: i) the mesoscale and boundary layer flow, and ii) low-level updrafts associated with deep moist convection initiation (CI) events yielding sustained or short-lived precipitating storms.The elimination of convective inhibition did not distinguish between sustained and unsustained CI events, though the vertical distribution of convective available potential energy may have played a role. The clearest signal differentiating the initiation of sustained versus unsustained precipitating deep convection was the depth of the low-level horizontal wind convergence associated with the mesoscale flow feature triggering CI, a sharp surface wind shift boundary or orographic upslope flow. The depth of the boundary layer relative to the height of the LFC failed to be a consistent indicator of CI potential. Widths of the earliest detectable low-level updrafts associated with sustained precipitating deep convection were ~3-5 km, larger than updrafts associated with surrounding boundary layer turbulence (~1-3-km wide). It is hypothesized that updrafts of this larger size are important for initiating cells to survive the destructive effects of buoyancy dilution via entrainment.

Image Processing of Radar Mosaics for the Climatology of Convection Initiation in South China

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-19-0081.1 ◽

2020 ◽

Vol 59 (1) ◽

pp. 65-81 ◽

Cited By ~ 3

Author(s):

Lanqiang Bai ◽

Guixing Chen ◽

Ling Huang

Keyword(s):

South China ◽

Numerical Models ◽

Early Morning ◽

Moist Convection ◽

Late Night ◽

Morning Peak ◽

Spatial Modes ◽

Triggering Mechanisms ◽

Deep Moist Convection ◽

AbstractA dataset of convection initiation (CI) is of great value in studying the triggering mechanisms of deep moist convection and evaluating the performances of numerical models. In recent years, the data quality of the operationally generated radar mosaics over China has been greatly improved, which provides an opportunity to retrieve a CI dataset from that region. In this work, an attempt is made to reveal the potential of applying a simple framework of objective CI detection for the study of CI climatology in China. The framework was tested using radar mosaic maps in South China that were accessible online. The identified CI events were validated in both direct and indirect ways. On the basis of a direct manual check, nearly all of the identified CI cells had an organized motion. The precipitation echoes of the cells had a median duration of approximately 2.5 h. The CI occurrences were further compared with rainfall estimates to ensure physical consistency. The diurnal cycle of CI occurrence exhibits three major modes: a late-night-to-morning peak at the windward coasts and offshore, a noon-to-late-afternoon peak on the coastal land, and an evening-to-early-morning peak over the northwestern highland. These spatial modes agree well with those of rainfall, indirectly suggesting the reliability of the CI statistics. By processing radar mosaic maps, such a framework could be applied for studying CI climatology over China and other regions.

Analysis of Convergence Boundaries Observed during IHOP_2002

10.1175/2010mwr3266.1 ◽

2010 ◽

Vol 138 (7) ◽

pp. 2737-2760 ◽

Cited By ~ 9

Author(s):

Roger M. Wakimoto ◽

Hanne V. Murphey

Keyword(s):

Doppler Radar ◽

The Other ◽

Horizontal Gradient ◽

Mobile Platforms ◽

Thermodynamic Structure ◽

Low Level ◽

Axis Parallel ◽

Using Data ◽

Outflow Boundary ◽

Abstract An analysis of six convergence boundaries observed during the International H2O Project (IHOP_2002) is presented. The detailed kinematic and thermodynamic structure of these boundaries was examined using data collected by an airborne Doppler radar and a series of dropsondes released by a jet flying at ∼500 mb. The former and latter platforms were able to resolve the meso-γ- and meso-β-scale circulations, respectively. Convection initiated on three of the days while no storms developed in the regions targeted by the mobile platforms on the other days (referred to as null cases). The airborne radar resolved the finescale structure of four drylines, a cold front, and an outflow boundary on the six days. Horizontal profiles through radar-detected thin lines revealed “bell-shaped distributions” and there appeared to be a seasonal dependence of the peak values of radar reflectivity. The echo profiles through the fine line in May were, in general, greater than those plotted for the June cases. There was no apparent relationship between the intensity of the low-level updraft and convection initiation. The strongest updraft resolved in the dual-Doppler wind synthesis was associated with a null case. There was also no relationship between the strength of the moisture discontinuity across the boundaries and convection initiation. The three days during which the storms developed were all associated with two convergence boundaries that were adjacent to each other. The two boundaries collided on one of the days; however, the boundaries on the other two days were approximately parallel and remained separated by a distance of 5–15 km. The total derivative of the horizontal vorticity rotating along an axis parallel to the boundary was calculated using dropsonde data. The horizontal gradient of buoyancy was the largest contributor to the change in vorticity and revealed maximum and minimum values that would support the generation of counterrotating circulations, thus promoting vertically rising air parcels. These updrafts would be more conducive to convection initiation. The null cases were characterized by a low-level vorticity generation of only one sign. This pattern would support tilted updrafts. The results presented in this study suggest that it is not necessary for two boundaries to collide in order for thunderstorms to develop. Solenoidally generated horizontal circulations can produce conditions favorable for convection initiation even if the boundaries remain separate.

A Subtropical Oceanic Mesoscale Convective Vortex Observed during SoWMEX/TiMREX

10.1175/2010mwr3411.1 ◽

2011 ◽

Vol 139 (8) ◽

pp. 2367-2385 ◽

Cited By ~ 18

Author(s):

Hsiao-Wei Lai ◽

Christopher A. Davis ◽

Ben Jong-Dao Jou

Keyword(s):

Vertical Wind Shear ◽

Mesoscale Convective System ◽

Southwest Monsoon ◽

Convective System ◽

Vortex Center ◽

Moist Convection ◽

Low Level ◽

Mesoscale Convective ◽

Deep Moist Convection ◽

Mesoscale Convective Vortex

AbstractThis study examines a subtropical oceanic mesoscale convective vortex (MCV) that occurred from 1800 UTC 4 June to 1200 UTC 6 June 2008 during intensive observing period (IOP) 6 of the Southwest Monsoon Experiment (SoWMEX) and the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). A dissipating mesoscale convective system reorganized within a nearly barotropic vorticity strip, which formed as a southwesterly low-level jet developed to the south of subsiding easterly flow over the southern Taiwan Strait. A cyclonic circulation was revealed on the northern edge of the mesoscale rainband with a horizontal scale of 200 km. An inner subvortex, on a scale of 25–30 km with maximum shear vorticity of 3 × 10−3 s−1, was embedded in the stronger convection. The vortex-relative southerly flow helped create local potential instability favorable for downshear convection enhancement. Strong low-level convergence suggests that stretching occurred within the MCV. Higher θe air, associated with significant potential and conditional instability, and high reflectivity signatures near the vortex center suggest that deep moist convection was responsible for the vortex stretching. Dry rear inflow penetrated into the MCV and suppressed convection in the upshear direction. A mesolow was also roughly observed within the larger vortex. The presence of intense vertical wind shear in the higher troposphere limited the vortex vertical extent to about 6 km.

Environment Associated with Deep Moist Convection under SALLJ Conditions: A Case Study

Weather and Forecasting ◽

10.1175/2010waf2222352.1 ◽

2010 ◽

Vol 25 (3) ◽

pp. 970-984 ◽

Cited By ~ 8

Author(s):

Paloma Borque ◽

Paola Salio ◽

Matilde Nicolini ◽

Yanina García Skabar

Keyword(s):

Frontal Zone ◽

Mesoscale Convective Systems ◽

South American ◽

Moist Convection ◽

Low Level ◽

Convective Systems ◽

Mesoscale Convective ◽

Low Level Jet ◽

Deep Moist Convection

Abstract The present work focuses on the study of the environmental conditions preceding the development of a group of subtropical mesoscale convective systems over central and northern Argentina on 6–7 February 2003 during the South American Low Level Jet Experiment. This period was characterized by an extreme northerly low-level flow along the eastern Andes foothills [South American low-level jet (SALLJ)]. The entire studied episode was dominated by the presence of a very unstable air mass over northern Argentina and a frontal zone near 40°S. The SALLJ generated an important destabilization of the atmosphere due to the strong humidity and differential temperature advection. Orography provided an extra lifting motion to the configuration of the regional wind field, which was efficient in forcing the initiation of convection. Once convection developed, it moved and regenerated in regions where the convective instability was horizontally homogeneous and stronger.

High Resolution HIRLAM Simulations of the Role of Low-Level Sea-Breeze Convergence in Initiating Deep Moist Convection in the Eastern Iberian Peninsula

Boundary-Layer Meteorology ◽

10.1007/s10546-014-9961-z ◽

2014 ◽

Vol 154 (1) ◽

pp. 81-100 ◽

Cited By ~ 6

Author(s):

Cesar Azorin-Molina ◽

Sander Tijm ◽

Elizabeth E. Ebert ◽

Sergio-M. Vicente-Serrano ◽

Maria-Jose Estrela

Keyword(s):

High Resolution ◽

Iberian Peninsula ◽

Sea Breeze ◽

Moist Convection ◽

Low Level ◽

Deep Moist Convection

Practical Predictability of the 20 May 2013 Tornadic Thunderstorm Event in Oklahoma: Sensitivity to Synoptic Timing and Topographical Influence

10.1175/mwr-d-14-00394.1 ◽

2015 ◽

Vol 143 (8) ◽

pp. 2973-2997 ◽

Cited By ~ 17

Author(s):

Yunji Zhang ◽

Fuqing Zhang ◽

David J. Stensrud ◽

Zhiyong Meng

Keyword(s):

Boundary Layer ◽

Vertical Mixing ◽

Vertical Wind Shear ◽

Mesoscale Convective System ◽

Convective System ◽

Moist Convection ◽

Atmospheric Conditions ◽

Low Level ◽

Deep Moist Convection ◽

Simulation Time

Abstract The practical predictability of severe convective thunderstorms during the 20 May 2013 severe weather event that produced the catastrophic enhanced Fujita scale 5 (EF-5) tornado in Moore, Oklahoma, was explored using ensembles of convective-permitting model simulations. The sensitivity of initiation and the subsequent organization and intensity of the thunderstorms to small yet realistic uncertainties in boundary layer and topographical influence within a few hours preceding the thunderstorm event was examined. It was found that small shifts in either simulation time or terrain configuration led to considerable differences in the atmospheric conditions within the boundary layer. Small shifts in simulation time led to changes in low-level moisture and instability, primarily through the vertical distribution of moisture within the boundary layer due to vertical mixing during the diurnal cycle as well as advection by low-level jets, thereby influencing convection initiation. Small shifts in terrain led to changes in the wind field, low-level vertical wind shear, and storm-relative environmental helicity, altering locally enhanced convergence that may trigger convection. After initiation, an upscale growth of errors resulting from deep moist convection led to large forecast uncertainties in the timing, intensity, structure, and organization of the developing mesoscale convective system and its embedded supercells.

Radiosonde observations of environments supporting deep moist convection initiation during RELAMPAGO-CACTI

10.1175/mwr-d-20-0148.1 ◽

2020 ◽

Author(s):

T. Connor Nelson ◽

James Marquis ◽

Adam Varble ◽

Katja Friedrich

Keyword(s):

Complex Terrain ◽

Deep Convection ◽

Convective Available Potential Energy ◽

Environmental Parameters ◽

Convective Precipitation ◽

Moist Convection ◽

Spatiotemporal Resolution ◽

Close Proximity ◽

Deep Moist Convection ◽

AbstractThe Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) and Cloud, Aerosol, and Complex Terrain Interactions (CACTI) projects deployed a high-spatiotemporal-resolution radiosonde network to examine environments supporting deep convection in the complex terrain of central Argentina. This study aims to characterize atmospheric profiles most representative of the near-cloud environment (in time and space) to identify the mesoscale ingredients affecting storm initiation and growth. Spatiotemporal autocorrelation analysis of the soundings reveals that there is considerable environmental heterogeneity, with boundary layer thermodynamic and kinematic fields becoming statistically uncorrelated on scales of 1–2 hr and 30 km. Using this as guidance, we examine a variety of environmental parameters derived from soundings collected within close proximity (30 km and 30 min in space and time) of 44 events over 9 days where the atmosphere either: 1) supported the initiation of sustained precipitating convection, 2) yielded weak and short-lived precipitating convection, or 3) produced no precipitating convection in disagreement with numerical forecasts from convection-allowing models (i.e., Null events). There are large statistical differences between the Null event environments and those supporting any convective precipitation. Null event profiles contained larger convective available potential energy, but had low free tropospheric relative humidity, higher freezing levels, and evidence of limited horizontal convergence near the terrain at low levels that likely suppressed deep convective growth. We also present evidence from the radiosonde and satellite measurements that flow-terrain interactions may yield gravity wave activity that affects CI outcome.

Diurnal Cycle of a Heavy Rainfall Corridor over East Asia

10.1175/mwr-d-16-0423.1 ◽

2017 ◽

Vol 145 (8) ◽

pp. 3365-3389 ◽

Cited By ~ 24

Author(s):

Guixing Chen ◽

Weiming Sha ◽

Toshiki Iwasaki ◽

Zhiping Wen

Keyword(s):

East Asia ◽

Heavy Rainfall ◽

Southern China ◽

Central China ◽

Moist Convection ◽

Low Level ◽

Effective Discharge ◽

Upward Transport ◽

Convection Initiation ◽

The Impact

Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.

Improving Operational Numerical Prediction of Afternoon Thunderstorms over Taiwan through Surface Data Assimilation

10.5194/egusphere-egu2020-7244 ◽

2020 ◽

Author(s):

I-Han Chen ◽

Jing-Shan Hong ◽

Ya-Ting Tsai ◽

Chin-Tzu Fong

Keyword(s):

Data Assimilation ◽

Radar Data ◽

Precipitation Forecast ◽

Cold Pool ◽

Moist Convection ◽

Convective Systems ◽

Forecast Lead Time ◽

Deep Moist Convection ◽

Outflow Boundary ◽

Surface Observations

<p>Taiwan, a subtropical island with steep mountains, is influenced by diverse weather systems, including typhoons, monsoons, frontal, and convective systems. Of these, the prediction of deep, moist convection here is particularly challenging due to complex topography and apparent landsea contrast. This study explored the benefits of assimilating surface observations on prediction of afternoon thunderstorms using a 2-km resolution WRF and WRFDA model system with rapid update cycles. Consecutive afternoon thunderstorm events during 30 June to 08 July 2017 are selected. Five experiments, consisting of 240 continuous cycles are designed to evaluate the data assimilation strategy and observation impact. Statistical results show that assimilating surface observations systematically improves the accuracy of wind and temperature prediction near the surface. Also, assimilating surface observations alone in one-hour intervals improves model quantitative precipitation forecast (QPF) skill, extending the forecast lead time in the morning. Furthermore, radar data assimilation can benefit by the additional assimilation of surface observations, particularly for improving the model QPF skill for large rainfall thresholds. An afternoon thunderstorm event that occurred on 06 July 2017 is further examined. By assimilating surface and radar observations, the model is able to capture the timing and location of the convection. Consequently, the accuracy of the predicted cold pool and outflow boundary is improved, when compared to the surface observations.</p>