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Severe Quasi Linear Convec;Ve Systems (QLCS)

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Severe Quasi Linear Convec;Ve Systems (QLCS) Severe Quasi Linear Convec0ve Systems (QLCS) “Derechos and Bow Echoes” Notes from SPC, various journal ar5cles and Markowski & Richardson Venn Diagram of MCS Subclassificaons Bow Echoes vs. Derechos • Derechos typically are associated with bands of showers or thunderstorms (collec5vely referred to as "convec5on") that assume a curved or bowed shape. • The bow-shaped storms are called bow echoes. • A derecho may be associated with a single bow echo or with mul5ple bows. Bow echoes themselves may consist of an individual storm, or may be comprised of a series of adjacent storms (i.e., a "squall line" or "quasi-linear convec5ve system") that together take on a larger scale bow shape. • Bow echoes may dissipate and subsequently redevelop during the course of given derecho. • Derecho winds occasionally are enhanced when a rotang band of storms called a "bookend vortex" develops on the poleward side of the bow echo storm system. • Derecho winds also may be enhanced on a smaller scale by the presence of embedded supercells in the derecho-producing convec5ve system. Bow Echoes • The figure is a prototype of the evolu5on of a "bow echo” Dr. T. Theodore Fujita, coined the term "bow echo" in the late 1970s. • The terminology was based on how bands of rain showers or thunderstorms "bow out" when strong winds associated with the storms reach the surface and spread out like pancake baer. • The bowed convec5ve band is near the leading edge of the damaging winds. The strongest wind is usually observed at the apex of the bow structure. • Downbursts are associated with Bow Echoes and Line Echo Wave Paern (LEWP). • Tornadoes can occur on the northern por5on of the bow echo. Stages of an Idealized Bow Echo • Stage A – The ini5al stage of a bow echo is a tall, strong thunderstorms with a gust front located along the leading edge of the line echo. • Stage B – A downburst associated with the thunderstorm descends and distorts the line echo such that a part of the line accelerates, and a part or all the line assumes a concave shape. The downburst intensifies where the bow becomes more pronounced. • Stage C – The downburst reaches the mature stage, and the bow echo assumes the shape of a spearhead. Frequent weak echo channels (WEC) occur along the center axis of the bow and indicate where the downburst winds are expected to be strongest. Gustnadoes may occur (short-lived and weak) in the vicinity of the bow where the near –surface shear is high. • Stage D – As the downburst weakens, aer pushing ahead of the parent bow echo, a mesocyclone may develop on the northern end of the bow (a hook-like echo may develop). It is not unusual to have tornadoes occur within this area. • Stage E – The bow evolves into a radar paern called the comma echo. Occasionally, tornadogenesis con5nues on the northern end of the comma where a mesocyclone is most likely to exist . However, when the echo weakens and/or becomes fragmented, the downburst weakens and ends. In addi5on, tornadogenesis ends on the northern periphery of the comma echo. Bow Echo vs. LEWP 19-20 April 2011 Bow Echo 19-20 April 2011 LEWP Images Courtesy of IWX FO. Long-lived Bow Echoes Weisman (1993), J. Atm. Sci. • Approximately 60-100 km long forming from an isolated cell or a part of a large-scale squall line. • Basic necessary parameters: Surface Td > 20°C; LI values ≥ -9°C; low-mid level wind shear larger than other typical severe weather environments • Elevated Rear Inflow Jet (RIJ) can increase the longevity of a MCS. • An elevated RIJ can be depicted on a radar as a weak echo channel at the back of a bow echo. • Bookend vor5ces act to increase the flow in between the vor5ces and therefore amplifies the bowing. • Rain-cooled downdras along with bookend vortex rotaons produce “lower” pressure locally. • The spacing between the bookend vor5ces are func5ons of the strength of the RIJ and bookend vor5ces. Long-lived Bow Echoes • An elevated RIJ can be depicted on a radar as a weak echo channel at the back of a bow echo. Base reflec5vity (leZ) and storm-relave map velocity (SRM; right) images of a bow echo. Wind damage is maximized along the bulged out cold/gust front, especially when a weak echo channel (WEC; leZ) is present behind the leading line associated with a strong rear inflow jet (RIJ; right). In this case, a tornado occurred near the triple point within the frontal structure just north of the bow apex. The black circle in SRM data iden5fies the mesovortex that produced the tornado. Long-lived Bow Echoes Weisman (1993), J. Atm. Sci. • RIJ ≠ Downburst but can contribute to gusty surface wind condi5ons. • The strongest liZing of near-surface air is produced when there is strong low-level shear to balance the cold pool generated circulaon. • If the contribu5ons to the generaon of the RIJ due to the warm pool of air aloZ are similar in magnitude to the contribu5ons due to the cold pool near the surface, then the RIJ may remain elevated to near the leading edge of the system. • Bow Echo formaon is a func5on of CAPE and low- to mid-level (2.5 and 5 km) wind shear. • As shear increases, convec5on becomes more organized with bow echo forming in high instability environment. • In general. bow echoes occur over a shear magnitude of 20 ms-1 over a prescribed depth and CAPE values of at least 2000 J Kg-1 but > 3000 J kg-1 is preferred. What is a derecho? • Simply is defined as a widespread, long-lived windstorm that is associated with a band of rapidly moving convec5ve storms. • Derecho word origin is a Spanish word meaning “direct” or “straight ahead.” • The term Derecho was first used by Dr. Gustavus Hinrichs in 1888 in the American Meteorological Journal. Derechos • The bow echoes may vary in scale, but typically go through an evolu5on that has at least some of the aspects of the prototype shown in the figure from Fujita (1978). • Derechos typically are associated with a long lived bow echo or a series of bow echoes. Derecho Criteria Johns and Hirt (1987), WAF • There must be a concentrated area of reports consis5ng of convec5vely induced wind damage and/or convec5ve gusts > 26 ms-1 (50 kt). This area must have a major axis length of at least 400 km. • The reports within this area must also exhibits a nonrandom paern of occurrence. That is, the reports must show a paern of chronological progression, either a singular swath (progressive) or as a series of swaths (serial). • Within the area there must be at least three reports, separated by 64 km or more, of either F1 (EF1) damage and/or convec5ve gusts ≥ 33 ms-1 (65 kt). • No more than 3 h can elapse between successive wind damage (gust) events. An isochrone analysis of the affected area of a derecho event on 5 July 1980. Official convec5ve wind gusts are indicated by wind barbs (kt). Dots are personal injuries (67) and X marks fatali5es (6). Derechos • Occurs in Moderate–Strong Shear Environment • High instability is necessary • Wind reports can exceed 100 mph (128 mph report with a 31 May 1998 event) • Types of Derechos • Progressive (Warm Season) • Serial (Cool Season) • Hybrid (Cross between the two) • Tornadoes can occur but more oZen with the serial type Progressive characterized by a short curved squall line (with a single or mulple bows) oriented nearly perpendicular to the mean wind direc0on with a bulge in the general direc0on of the mean flow. – Warm season events – late Spring/Summer – Low-level WAA paern – Pronounced Elevated RIJ – High convec5ve instability – Relavely strong mid-level winds » 500 mb wind from W to NW and > 40 kt – LLJ veers as it approaches the front – ULJ has a W-E orientaon – Mainly observe across South Dakota-Ohio – Typically begins in the aernoon and goes into the evening hours. – Several hours between ini5al convec5on and first wind damage reports – LI > -6°C but usually closer to > -8°C – Moves along the boundary and could deviate as much 15° to the right of the mean wind and end up in the warm sector – Speed of the system is faster than the mean wind due to discrete propagaon – Similar to the Maddox Frontal case setup – Harder to forecast of the derechos – More circular in appearance Serial consists of an extensive squall line which is oriented such that the angle between the mean wind direcon and the squall line axis is small (nearly parallel). A series of LEWPS and Bow echoes move along the line. – Cool season events – Fall/Winter/early Spring – Least common during mid-late Spring – Ahead of a strong progressive low – Moves ahead of a cold front or dry line – Typically associated with a strong shortwave – Dynamically driven – LLJ and ULJ more unidirec5onal – Warm sector marginally unstable – Low-mid Mississippi Valley to East Coast of US – Less diurnally dependent – Easier to forecast due to stronger dynamic forcing – Mid-level dry air intrusion is oZen observed(increase convec5ve instability) – Bows move rapidly along the line in the direc5on of the mean flow (SW to NE) with speed > 35 kt – More linear in appearance Derecho Climatology Coniglio and Stensrud (2004), WaF Approximate number of 4mes "moderate and high intensity" (MH; ou?low winds > 75 mph at several points along the damage path) derechos affected points in the United States during the years 1980 through 2001. Areas affected by 3 or more derecho events are shaded in yellow, orange, and red (modified from Coniglio and Stensrud 2004).
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