Evaluation and Modification of Microphysics Schemes on the Cold Pool Evolution for A Simulated Bow Echo in Southeast China

Abstract The effect of changes in microphysical cooling rates on bow echo development and longevity are examined through changes to graupel parameterization in the Advanced Research Weather Research and Forecasting Model (ARW-WRF). Multiple simulations are performed that test the sensitivity to different graupel size distributions as well as the complete removal of graupel. It is found that size distributions with larger and denser, but fewer, graupel hydrometeors result in a weaker cold pool due to reduced microphysical cooling rates. This yields weaker midlevel (3–6 km) buoyancy and pressure perturbations, a later onset of more elevated rear inflow, and a weaker convective updraft. The convective updraft is also slower to tilt rearward, and thus bowing occurs later. Graupel size distributions with more numerous, smaller, and lighter hydrometeors result in larger microphysical cooling rates, stronger cold pools, more intense midlevel buoyancy and pressure gradients, and earlier onset of surface-based rear inflow; these systems develop bowing segments earlier. A sensitivity test with fast-falling but small graupel hydrometeors revealed that small mean size and slow fall speed both contribute to the strong cooling rates. Simulations entirely without graupel are initially weaker, because of limited contributions from cooling by melting of the slowly falling snow. However, over the next hour increased rates of melting snow result in an increasingly more intense system with new bowing. Results of the study indicate that the development of a bow echo is highly sensitive to microphysical processes, which presents a challenge to the prediction of these severe weather phenomena.

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Practical and Intrinsic Predictability of Severe and Convective Weather at the Mesoscales

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0315.1 ◽

2012 ◽

Vol 69 (11) ◽

pp. 3350-3371 ◽

Cited By ~ 72

Author(s):

Christopher Melhauser ◽

Fuqing Zhang

Keyword(s):

Weather Prediction ◽

Squall Line ◽

Cold Pool ◽

Initial Condition ◽

Large Variability ◽

Forecast Performance ◽

Sensitivity Experiments ◽

Bow Echo ◽

Upper Level ◽

Convective Weather

Abstract This study explores both the practical and intrinsic predictability of severe convective weather at the mesoscales using convection-permitting ensemble simulations of a squall line and bow echo event during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX) on 9–10 June 2003. Although most ensemble members—initialized with realistic initial condition uncertainties smaller than the NCEP Global Forecast System Final Analysis (GFS FNL) using an ensemble Kalman filter—forecast broad areas of severe convection, there is a large variability of forecast performance among different members, highlighting the limit of practical predictability. In general, the best-performing members tend to have a stronger upper-level trough and associated surface low, producing a more conducive environment for strong long-lived squall lines and bow echoes, once triggered. The divergence in development is a combination of a dislocation of the upper-level trough, surface low with corresponding marginal environmental differences between developing and nondeveloping members, and cold pool evolution by deep convection prior to squall line formation. To further explore the intrinsic predictability of the storm, a sequence of sensitivity experiments was performed with the initial condition differences decreased to nearly an order of magnitude smaller than typical analysis and observation errors. The ensemble forecast and additional sensitivity experiments demonstrate that this storm has a limited practical predictability, which may be further improved with more accurate initial conditions. However, it is possible that the true storm could be near the point of bifurcation, where predictability is intrinsically limited. The limits of both practical and intrinsic predictability highlight the need for probabilistic and ensemble forecasts for severe weather prediction.

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Bow Echo Sensitivity to Ambient Moisture and Cold Pool Strength

Monthly Weather Review ◽

10.1175/mwr3109.1 ◽

2006 ◽

Vol 134 (3) ◽

pp. 950-964 ◽

Cited By ~ 29

Author(s):

Richard P. James ◽

Paul M. Markowski ◽

J. Michael Fritsch

Keyword(s):

Convective Available Potential Energy ◽

Three Dimensional ◽

Cold Pool ◽

Convective System ◽

Cold Air ◽

Convective Systems ◽

Cold Pools ◽

Bow Echo ◽

Dry Conditions ◽

Water Vapor Mixing Ratio

Abstract Bow echo development within quasi-linear convective systems is investigated using a storm-scale numerical model. A strong sensitivity to the ambient water vapor mixing ratio is demonstrated. Relatively dry conditions at low and midlevels favor intense cold-air production and strong cold pool development, leading to upshear-tilted, “slab-like” convection for various magnitudes of convective available potential energy (CAPE) and low-level shear. High relative humidity in the environment tends to reduce the rate of production of cold air, leading to weak cold pools and downshear-tilted convective systems, with primarily cell-scale three-dimensionality in the convective region. At intermediate moisture contents, long-lived, coherent bowing segments are generated within the convective line. In general, the scale of the coherent three-dimensional structures increases with increasing cold pool strength. The bowing lines are characterized in their developing and mature stages by segments of the convective line measuring 15–40 km in length over which the cold pool is much stronger than at other locations along the line. The growth of bow echo structures within a linear convective system appears to depend critically on the local strengthening of the cold pool to the extent that the convection becomes locally upshear tilted. A positive feedback process is thereby initiated, allowing the intensification of the bow echo. If the environment favors an excessively strong cold pool, however, the entire line becomes uniformly upshear tilted relatively quickly, and the along-line heterogeneity of the bowing line is lost.

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Numerical Simulations of Bow Echo Formation Following a Squall Line–Supercell Merger

Monthly Weather Review ◽

10.1175/mwr-d-13-00356.1 ◽

2014 ◽

Vol 142 (12) ◽

pp. 4791-4822 ◽

Cited By ~ 8

Author(s):

Adam J. French ◽

Matthew D. Parker

Keyword(s):

Numerical Simulations ◽

Vertical Wind Shear ◽

Leading Edge ◽

Direct Result ◽

Squall Line ◽

Cold Pool ◽

Surface Winds ◽

Sensitivity Tests ◽

Bow Echo ◽

Inflow Jet

Abstract Output from idealized numerical simulations is used to investigate the storm-scale processes responsible for squall-line evolution following a merger with an isolated supercell. A simulation including a squall line–supercell merger is compared to one using the same initial squall line and background environment without the merger. These simulations reveal that while bow echo formation is favored by the strongly sheared background environment, the merger produces a more compact bowing structure owing to a locally enhanced rear-inflow jet. The merger also represents a favored location for severe weather production relative to other portions of the squall line, with surface winds, vertical vorticity, and rainfall all being maximized in the vicinity of the merger. An analysis of storm-scale processes reveals that the premerger squall line weakens as it encounters outflow from the preline supercell, and the supercell becomes the leading edge of the merged system. Subsequent localized strengthening of the cold pool and rear-inflow jet produce a compact, intense bow echo local to the merger, with a descending rear-inflow jet creating a broad swath of damaging surface winds. These features, common to severe bow echoes, are shown to be a direct result of the merger in the present simulations, and are diminished or absent in the no-merger simulation. Sensitivity tests reveal that mergers in a weaker vertical wind shear environment do not produce an enhanced bow echo structure, and only produce a localized region of marginally enhanced surface winds. Additional tests demonstrate that the details of postmerger evolution vary with merger location along the line.

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The Vertical Vorticity Structure within a Squall Line Observed during BAMEX: Banded Vorticity Features and the Evolution of a Bowing Segment

Monthly Weather Review ◽

10.1175/mwr-d-14-00246.1 ◽

2015 ◽

Vol 143 (1) ◽

pp. 341-362 ◽

Cited By ~ 7

Author(s):

Roger M. Wakimoto ◽

Phillip Stauffer ◽

Wen-Chau Lee

Keyword(s):

Squall Line ◽

Cold Pool ◽

Vertical Vorticity ◽

Stratiform Region ◽

Circulation Patterns ◽

Bow Echo ◽

Mesoscale Convective ◽

Oriented Parallel ◽

Mesoscale Convective Vortex ◽

Inflow Jet

Abstract A quasi-linear convective line with a trailing stratiform region developed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) while being sampled by two airborne Doppler radars. The finescale reflectivity and Doppler velocities recorded by the radars documented the evolution of the convective line. Bands of positive and negative vertical vorticity oriented parallel to the convective line were resolved in the analysis. This type of structure has rarely been reported in the literature and appears to be a result of the tilting and subsequent stretching of ambient horizontal vorticity produced by the low-level wind shear vector with a significant along-line component. The radar analysis also documented the evolution of an embedded bow echo within the convective line. Embedded bow echoes have been documented for a number of years; however, a detailed analysis of their kinematic structure has not been previously reported in the literature. The counterrotating circulation patterns that are characteristic of bow echoes appeared to be a result of tilting and stretching of the horizontal vorticity produced in the cold pool. The analysis suggests that the location along the convective line where embedded bow echoes form depends on the local depth of the cold pool. The rear-inflow jet is largely driven by the combined effects of the counterrotating vortices and the upshear-tilted updraft.

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Examination of Gravity Waves Associated with the 13 March 2003 Bow Echo

Monthly Weather Review ◽

10.1175/mwr-d-12-00343.1 ◽

2013 ◽

Vol 141 (11) ◽

pp. 3735-3756 ◽

Cited By ~ 24

Author(s):

Rebecca D. Adams-Selin ◽

Richard H. Johnson

Keyword(s):

Gravity Wave ◽

Gravity Waves ◽

Vertical Motion ◽

Cold Pool ◽

Positive Pressure ◽

Vertical Mode ◽

Low Level ◽

Oklahoma Mesonet ◽

Bow Echo ◽

Pressure Surge

Abstract Numerical simulations of the 13 March 2003 bow echo over Oklahoma are used to evaluate bow echo development and its relationship with gravity wave generation. Multiple fast-moving (with speeds of 30–35 m s−1) gravity waves are generated in association with fluctuations in the first vertical mode of heating in the convective line. The surface impacts of four such waves are observed in Oklahoma Mesonet data during this case. Observations of surface pressure surges ahead of convective lines prior to the bowing process are reproduced; a slower gravity wave produced in the simulation is responsible for a prebowing pressure surge. This slower gravity wave, moving at approximately 11 m s−1, is generated by an increase in low-level microphysical cooling associated with an increase in rear-to-front flow and low-level downdrafts shortly before bowing. The wave moves ahead of the convective line and is manifested at the surface by a positive pressure surge. The pattern of low-level vertical motion associated with this wave, in conjunction with higher-frequency gravity waves generated by multicellularity of the convective line, increases the immediate presystem CAPE by approximately 250 J kg−1 just ahead of the bowing segment of the convective line. Increased presystem CAPE aids convective updraft strength in that segment despite amplified updraft tilt due to a stronger cold pool and surface-based rear-to-front flow, compared to updraft strength in other, nonbowing segments of the convective line.

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Synoptic and Mesoscale Analysis of a High-Latitude Derecho–Severe Thunderstorm Outbreak in Finland on 5 July 2002

Weather and Forecasting ◽

10.1175/waf953.1 ◽

2006 ◽

Vol 21 (5) ◽

pp. 752-763 ◽

Cited By ~ 24

Author(s):

Ari-Juhani Punkka ◽

Jenni Teittinen ◽

Robert H. Johns

Keyword(s):

Potential Temperature ◽

Leading Edge ◽

The United States ◽

Wind Damage ◽

Cold Pool ◽

Convective System ◽

Synoptic Situation ◽

Large Area ◽

Synoptic Pattern ◽

Bow Echo

Abstract On 5 July 2002, a rapidly propagating bow echo formed over eastern Finland causing severe wind damage in an exceptionally large area. The Ministry of the Interior’s Emergency Response Centers received nearly 400 thunderstorm-related wind damage reports. The 5 July 2002 case is the highest-latitude derecho that has ever been documented. The bow echo developed ahead of a northeastward-moving 500-hPa trough inside of the warm sector of a secondary low and moved north-northwestward on the eastern (warm) side of the quasi-stationary front. The leading edge of the bow echo was oriented perpendicular to the low-level southerly wind shear and the convective system propagated along the 850-hPa equivalent potential temperature ridge with a speed that was close to the maximum wind throughout the troposphere. It is particularly noteworthy that the synoptic pattern was oriented about 90° counterclockwise when compared with the typical synoptic pattern associated with warm season derechos in the United States. This kind of synoptic situation associated along with the derecho mesoscale convective system’s (MCS’s) motion toward the north-northwest has not been mentioned in literature before. The MCS started as a cluster of thunderstorms and became a bow echo a few hours later. The leading edge of the bow echo had a strong reflectivity gradient and the region of stratiform precipitation was behind the strongest echoes. At the most intense stage, a rear-inflow notch was visible both in radar and satellite pictures. It was in good accordance with the location of an area of the most severe damage. Moreover, the storm-relative winds derived from the proximity sounding in the wake of the system showed the existence of rear-to-front flow above 850 hPa. The downdraft air appeared to originate from 4 km ASL, where the relative humidity was less than 50%. This probably led to enhanced evaporative cooling and the intense cold pool, which propagated faster than the mean wind. In the mesoscale, the 5 July 2002 derecho had many similarities to other derecho MCSs that have been described in the literature.

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Long-Term Morphological Changes in Simulated Supercells Following Mergers with Nascent Supercells in Directionally Varying Shear

Monthly Weather Review ◽

10.1175/mwr-d-15-0193.1 ◽

2016 ◽

Vol 144 (2) ◽

pp. 471-499 ◽

Cited By ~ 9

Author(s):

Ryan Hastings ◽

Yvette Richardson

Keyword(s):

Conceptual Models ◽

Morphological Changes ◽

Cell System ◽

Cold Pool ◽

Long Term Outcomes ◽

Severe Thunderstorm ◽

Bow Echo ◽

High Precipitation ◽

Original Cell

Abstract Mergers involving supercells remain a challenge for severe thunderstorm forecasting. In this study, mergers between supercells and ordinary cells (e.g., cells forming in a similar environment but too young to be fully developed supercells) are investigated. A series of numerical experiments are performed using an idealized, homogenous environment supportive of cyclonically rotating, right-moving supercells. Warm bubbles are introduced at different times, resulting in two storms of different maturity; their placement is used to control the location of the merger and the relative maturity of the second storm. Simplified conceptual models for the long-term outcomes of mergers are developed. In the simplest mode of merger, outflow from the new cell cuts off inflow to the original. If the new cell’s cold pool is not sufficiently strong to cut off the inflow to the original cell, the minimum separation of the updraft maxima during the merger becomes a key controlling factor in the outcome. If it is less than 10 km, an updraft collision occurs, resulting in a classic supercell. If it is greater than 20 km and the new cell merges into the original cell’s forward flank, a dual-cell system results. If it is between 10 and 20 km, the enhanced precipitation produced during the merger leads to a cold pool surge and an updraft bridge, joining the original updrafts and developing into either a small bow echo (with forward-flank mergers) or a supercell on the classic high-precipitation spectrum (with rear-flank mergers), depending on the distribution of precipitation in the merging system.

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Thermodynamic Properties of Mesoscale Convective Systems Observed during BAMEX

Monthly Weather Review ◽

10.1175/2008mwr2284.1 ◽

2008 ◽

Vol 136 (11) ◽

pp. 4242-4271 ◽

Cited By ~ 3

Author(s):

James Correia ◽

Raymond W. Arritt

Keyword(s):

Potential Temperature ◽

Mesoscale Convective Systems ◽

Lapse Rate ◽

Spatiotemporal Variability ◽

Cold Pool ◽

Convective Systems ◽

Stratiform Region ◽

Bow Echo ◽

Mesoscale Convective ◽

Lapse Rates

Abstract Dropsonde observations from the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) are used to document the spatiotemporal variability of temperature, moisture, and wind within mesoscale convective systems (MCSs). Onion-type sounding structures are found throughout the stratiform region of MCSs, but the temperature and moisture variability is large. Composite soundings were constructed and statistics of thermodynamic variability were generated within each subregion of the MCS. The calculated air vertical velocity helped identify subsaturated downdrafts. It was found that lapse rates within the cold pool varied markedly throughout the MCS. Layered wet-bulb potential temperature profiles seem to indicate that air within the lowest several kilometers comes from a variety of source regions. It was also found that lapse-rate transitions across the 0°C level were more common than isothermal, melting layers. The authors discuss the implications these findings have and how they can be used to validate future high-resolution numerical simulations of MCSs.

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Bow Echo Mesovortices. Part I: Processes That Influence Their Damaging Potential

Monthly Weather Review ◽

10.1175/2008mwr2649.1 ◽

2009 ◽

Vol 137 (5) ◽

pp. 1497-1513 ◽

Cited By ~ 46

Author(s):

Nolan T. Atkins ◽

Michael St. Laurent

Keyword(s):

Cold Pool ◽

Horizontal Shear ◽

Saint Louis ◽

Low Level ◽

Wind Speeds ◽

Cold Pools ◽

Bow Echo ◽

Mesoscale Convective ◽

The Impact ◽

Relative Wind

Abstract This two-part study examines the damaging potential and genesis of low-level, meso-γ-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). This bow echo produced both damaging and nondamaging mesovortices. A series of sensitivity simulations were performed to assess the impact of low- and midlevel shear, cold-pool strength, and Coriolis forcing on mesovortex strength. By analyzing the amount of circulation, maximum vertical vorticity, and number of mesovortices produced at the lowest grid level, it was observed that more numerous and stronger mesovortices were formed when the low-level environmental shear nearly balanced the horizontal shear produced by the cold pool. As the magnitude of deeper layer shear increased, the number and strength of mesovortices increased. Larger Coriolis forcing and stronger cold pools also produced stronger mesovortices. Variability of ground-relative wind speeds produced by mesovortices was noted in many of the experiments. It was observed that the strongest ground-relative wind speeds were produced by mesovortices that formed near the descending rear-inflow jet (RIJ). The strongest surface winds were located on the southern periphery of the mesovortex and were created by the superposition of the RIJ and mesovortex flows. Mesovortices formed prior to RIJ genesis or north and south of the RIJ core produced weaker ground-relative wind speeds. The forecast implications of these results are discussed. The genesis of the mesovortices is discussed in Part II.

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