Title goes here for lessonFebruary 2002 Mesoscale Convective Systems book sources: Markowski and Richardson (2009), chapter 10 Houze 1993: Cloud Dynamics, chapter 9 MCS comet modules: MCSs: squall lines and bow echoes MCSs: squall lines and bow echoes (unnarrated, 1999) Severe convection II: MCSs Severe convection II: MCSs (narrated, 2002) Tropical MCSs Tropical MCSs (unnarrated, 2013)

Title goes here for lessonFebruary 2002 Overview Introduction to MCSs Squall lines definition types synoptic setting importance of shear conceptual model of SL with stratiform region pressure perturbations origin of rear inflow jet Bow echoes Tropical squall lines Mesoscale Convective Complexes 300 km

Title goes here for lessonFebruary 2002 Introduction

Title goes here for lessonFebruary 2002 Definition Mesoscale convective systems (MCSs) refer to all organized convective systems larger than ~50 km (long axis) Some classic convective system types include: squall lines bow echoes line echo wave patterns mesoscale convective complexes (MCCs) are large MCSs (Maddox 1980) definition is based on IR imagery: <241 K cloud shield at least 100,000 km 2 <221 K cloud shield at least 50,000 km 2 Eccentricity >0.7 Must last at least 6 hrs

Title goes here for lessonFebruary 2002 Where and when MCSs occur worldwide mainly in the warm season MCC global distribution: Laing and Fritsch (1993) TRMM data

Title goes here for lessonFebruary 2002 Compare this to the distribution of hail and tornadoes … Conclusion: in mid-latitudes the distribution is similar. MCSs and MCCs are surprisingly common in the tropics.

Title goes here for lessonFebruary 2002 US tornado tracks

Title goes here for lessonFebruary 2002 MCS examples warm ocean MCSDryline Squall line in Texas

Title goes here for lessonFebruary 2002 MCC examples 30 April 2004, 1:32 UTC

Title goes here for lessonFebruary 2002 MCS morphology linear symmetric asymmetric amorphous

Title goes here for lessonFebruary 2002 MCS morphology linear TS LS PS Fig. 9.11, adapted from Parker and Johnson (2000) TS PS

Title goes here for lessonFebruary 2002 Upscale growth of convection towards a squall line is accelerated when the deep layer (surface to cloud top) mean shear is close to aligned to the boundary along which cells first initiate. MCS morphology

Title goes here for lessonFebruary 2002 MCS morphology squall lines can be continuous or cellular

Title goes here for lessonFebruary 2002 Synoptic patterns Favorable conditions conducive to severe MCSs and MCCs often occur with identifiable synoptic patterns Synoptic forcing may be quite weak. Temperature (C, dashed) and dewpoint (C, solid)

Title goes here for lessonFebruary 2002 Squall Lines

Title goes here for lessonFebruary 2002 Squall line definition A squall line is any line of convective cells km long no strict size definition Usually trailed by stratiform precip (TS) Squall line animation 1 Squall line animation 2 (mov file) Squall line animation 2

Title goes here for lessonFebruary 2002 Initial organization Squall lines may either be triggered as a line along some boundary (e.g. dryline) or organize into a line from an amorphous cluster of cells Thus the majority of cases developed from pre-existing convergence lines pre-existing convergence line pre-existing convergence line

Title goes here for lessonFebruary 2002 Life cycle Little is known about the initial and dissipating stages. Mature structure is well known. It can be steady-state (long-lived MCS) or a brief transition (Leary and Houze 1979)

Title goes here for lessonFebruary 2002 Environmental factors Both CIN and CAPE impact MCS structure and evolution low-level shear deep shear

Title goes here for lessonFebruary 2002 Importance of shear For a given CAPE, the strength and longevity of an MCS increases with increasing depth and strength of the low-level shear S 0-3 (between 0-3 km AGL) For midlatitude environments, |S 0-3 | : weak |S 0-3 | <10 m/s moderate |S 0-3 | m/s strong |S 0-3 | >18 m/s A balance is needed between baroclinically generated hor. vorticity and ambient low-level hor. vorticity (RKW theory) Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A Theory for Strong, Long-Lived Squall Lines. J. Atmos. Sci., 45, 463–485. To continue initiating convection, the cold pool needs to lift air above the LFC. The higher the LFC (drier air), the deeper the layer in which the RKW condition needs to be assessed deeper ambient shear deeper cold pool

Title goes here for lessonFebruary 2002 Which low-level shear matters? It is the component of low-level vertical wind shear perpendicular to the line that is most critical for controlling squall line structure & evolution Note that the initial line orientation is often due to convergence lines (fine lines) independent of LL shear

Title goes here for lessonFebruary 2002 Impact of shear on longevity Short-lived squall lines tend to form under weak LL shear Long-lived squall lines have at least 10 m/s of line-normal wind shear in the lowest 3 km Severe squall lines have more CAPE and/or shear than non- severe ones

Title goes here for lessonFebruary 2002 Classic evolution with weak shear The characteristic squall line life cycle is to evolve from a narrow band of intense convective cells to a broader, weaker system over time

Title goes here for lessonFebruary 2002 Classic evolution with stronger shear Stronger shear environments produce stronger long-lived lines composed of strong leading line convective cells and even bow echoes.

Title goes here for lessonFebruary 2002 Later evolution in moderate-to-strong shear Several bow echoes may form Asymmetry (if present) explained by the Coriolis force

Title goes here for lessonFebruary 2002 Cold pool motion and shear Squall lines often have their ‘own’, intrinsic propagation speed, which may be different from the deep-layer mean wind This speed tends to be controlled by the speed of the system cold pool New cells are constantly triggered along its leading edge Speed is close to cold pool density current speed In mid-latitudes an "average" gust front speed is ~15 m/s. Longevity of a squall line depends on its ability to “keep up” with the gust front  LL shear  u needs to match gust front speed c.

Title goes here for lessonFebruary 2002 RKW theory: ambient shear and MCS longevity

Title goes here for lessonFebruary 2002 low-level wind shear & updraft slope note vertical aspect ratio height is exaggerated by ~5

Title goes here for lessonFebruary 2002 wind shear & updraft slope height Buoyancy tilting (by shear) produces a larger downward BPPGA, and thus a weaker net upward acceleration

Title goes here for lessonFebruary 2002 RKW’s optimal state implies that LL shear  u = c or  u = c for an erect updraft Density current plowing into westerly shear (RKW’s optimal state) The integration of along the control volume gives: The vertical vorticity flux, integrated along the top of the control box, cancels only under RKW’s optimal state Judiciously choose control volume such that u=0 at z=d. Then S

Title goes here for lessonFebruary 2002 LL shear  u vs. c, and storm evolution Squall lines may move thru the optimal state over time. c tends to increase as cold pool deepens / strengthens

Title goes here for lessonFebruary 2002 The RKW theory for squall line maintenance is not without controversy … Fig. 9.20: the cold pool strength usually exceeds the low level (0-3 km) shear.

Title goes here for lessonFebruary 2002 deep shear may matter as well cold

Title goes here for lessonFebruary 2002 Squall line motion squall lines sometimes re-orient normal to the low-level shear

Title goes here for lessonFebruary 2002 Conceptual model of the mature stage of a symmetric squall line (textbook Fig 9.7, adapted from Houze et al. 1989)

Title goes here for lessonFebruary 2002 Conceptual model of the mature stage of a symmetric squall line (textbook Fig 9.7, adapted from Houze et al. 1989) Fig. 9.8

Title goes here for lessonFebruary 2002 Conceptual model of the mature stage of a symmetric squall line LL conv UL div Fig. 9.3 model output (Rotunno et al. 1988)

Title goes here for lessonFebruary 2002 LL shear deep shear HL updraft

Title goes here for lessonFebruary 2002

Title goes here for lessonFebruary 2002 Trailing stratiform region sustained by a mesoscale updraft above the freezing level this updraft probably is buoyancy-driven. buoyancy is due to latent heating (condensation/freezing)

Title goes here for lessonFebruary 2002 Surface pressure fields +3 mb -3 mb interpret mesohigh and wake low hydrostatically weaker shear stronger shear

Title goes here for lessonFebruary 2002 Vertical cross section of reflectivity and pressure perturbations mature stage pressure perturbations low under convective towers, near LFC, dominates due to buoyancy source

Title goes here for lessonFebruary 2002 Observed pressure perturbations low near LFC due to buoyancy source (base of CAPE layer) hydrostatic high below plus stagnation point high ahead of gust front S E W LeMone and Tarleton 1986, Jtech LeMone et al 1987, Mon Wea Rev

Title goes here for lessonFebruary 2002 Numerical simulation of a mature squall line (Fovell and Ogura 1988) Reflectivity Storm-relative airflow  e

Title goes here for lessonFebruary 2002 Numerical simulation of a mature squall line (Fovell and Ogura 1988) L H S Pressure perturbations Total Buoyancy source Dynamic source numerical simulation of a density current (Droegemeier and Wilhelmson 1987) p’ wind w S w

Title goes here for lessonFebruary 2002 The Rear-Inflow Jet (RIJ) down-gradient subsident, evaporatively-cooled may join gust front feeder flow

Title goes here for lessonFebruary 2002 Buoyancy distribution, the trailing stratiform region, and the rear-inflow jet The convective line is positively buoyant from the LFC to the LNB This B+ spreads into the TSR Decreasing B+ towards the rear implies a decreasing B- induced low below the B+ This implies a horizontal PGF towards the convective line This drives the rear inflow jet (RIJ)

Title goes here for lessonFebruary 2002 The RIJ is stronger in more unstable environments More CAPE implies more B in convective line  stronger B-induced low at its base  stronger PGF  stronger RIJ  possible straight-line wind damage & bow-echo formation Updraft should be more erect!! Fig. 9.22

Title goes here for lessonFebruary 2002 Bow Echoes

Title goes here for lessonFebruary 2002 Bow echo definition Bow echoes are relatively small ( km long), bow-shaped systems of convective cells noted for producing long swaths of damaging surface winds. bow echoes

Title goes here for lessonFebruary 2002 Bow echo environments deep shear: Bow echo and supercell shallow shear: bow echo only red line: downburst winds

Title goes here for lessonFebruary 2002 evolution of a bow echo S towards a balance between ambient, RIJ, and cold pool vorticities

Title goes here for lessonFebruary 2002 Bow echo evolution

Title goes here for lessonFebruary 2002 Reasons for bow echoes intensity Rear-inflow jet connects to gust front feeder flow

Title goes here for lessonFebruary 2002 Bow echoes and supercells Bow echoes represent mesoscale organization, supercells are individual storms. Both require strong shear. Supercells within squall lines tend to become bow echoes, but cells at the ends of lines can remain super- cellular for long periods of time.

Title goes here for lessonFebruary 2002 mature bow echo: RIJ Fig. 9.25

Title goes here for lessonFebruary 2002 mature bow echo: hook echo Fig Fig. 9.26: simulated vortex lines around the RIJ RIJ 3 km DD winds (NOAA + NRL P-3) BAMEX – Bow Echo and MCV experiment

Title goes here for lessonFebruary 2002 Rear-inflow notch The reflectivity notch, if sustained or growing, is an indication of a strong RIJ and possible damaging straight-line surface winds.

Title goes here for lessonFebruary 2002 The MARC signature MARC: Mid-altitude radial convergence (applied to the radial that is normal to the squall line) rear-inflow notch

Title goes here for lessonFebruary 2002 MARC characteristics: MARCs are locally enhanced convergent areas (velocity differentials), embedded within a larger region of convergence extending from 60 to 120 km in length width: 2-6 km height: mid-levels (~5 km AGL) depth: 3-9 km magnitude: ~25-30 m/s

Title goes here for lessonFebruary 2002 Derechoes: definition If the cumulative impact of the severe wind from one or more bow echoes covers a wide enough and long enough path, the event is referred to as a derecho. To be classified as a derecho, a single convective system must produce wind damage or gusts greater than 26 m/s within a concentrated area with a major axis length of at least 400 km. The severe wind reports must exhibit a chronological progression and there must be at least 3 reports of F1 damage and/or convective wind gusts of 33 m/s (65 kts) or greater separated by at least 64 km. Additionally, no more than 3 hours can elapse between successive wind damage or gust events.

Title goes here for lessonFebruary 2002 Derechoes cont’d A progressive derecho is a single bow- shaped system that typically propagates north of and parallel to a weak stationary boundary Serial derechos are most commonly a series of bow-echoes along a squall line (usually located within the warm sector of a cyclone)

Title goes here for lessonFebruary 2002 MCCs

Title goes here for lessonFebruary 2002 MCC Definition A mesoscale convective complex is defined via IR satellite imagery (Maddox 1980). <241 K (-32°C) cloud shield at least 100,000 km 2 <221 K (-52°C) cloud shield at least 50,000 km 2 Eccentricity >0.7 Must last at least 6 hrs

Title goes here for lessonFebruary 2002 Mesoscale convective vortices form in some MCSs

Title goes here for lessonFebruary 2002 MCCs may form mid-to-UL cyclonic vortices 1.These vortices may be large relative to L R ( Rossby radius of deformation ) 2.Mechanism the same as that for tropical cyclogenesis 3.Can be explained in terms of PV creation due to latent heating 7 July :30 UTC 7 July :30 UTC source: Fritsch et al 1994

Title goes here for lessonFebruary Squall line bows out, stratiform region increases in size. 2.Mesoscale Cyclonic Vortex (MCV) may develop (due to f) Odten long-lived squall lines in moderate-to-strong shear eventually become asymmetric, with an MCV

Title goes here for lessonFebruary 2002 MCV creation due to diabatic PV generation define IPV consider diabatic heating and cooling profiles in sustained deep convection infer isentrope distortions and PV anomalies

Title goes here for lessonFebruary 2002 MCV creation due to diabatic PV generation define IPV consider diabatic heating and cooling profiles in sustained mesoscale deep convection infer isentrope distortions and PV anomalies source: Fritsch et al 1994 SW NE

Title goes here for lessonFebruary 2002 Tropical squall lines

Title goes here for lessonFebruary 2002 Tropical squall lines Overall, squall lines in the tropics are structurally similar to midlatitude squall lines. Notable differences include: develop in lower shear, lower LFC, less-CAPE environments taller convective cells system cold pools are generally weaker less of a tendency toward asymmetric evolution AND most tropical squall lines move from east to west

Title goes here for lessonFebruary 2002 Tropical Squall Lines example: Arizona monsoon

Title goes here for lessonFebruary 2002 Typical evolution of tropical squall lines Examine divergence, B and p B at three stages: Convective stage Intermediary stage Stratiform stage Houze’s book con div

Title goes here for lessonFebruary 2002 Summary MCS structure and evolution depend on the characteristics of the environmental buoyancy and shear, as well as the details of the initial forcing mechanism. The strength and the degree of organization of most MCSs increases with increasing low-level vertical wind shear values. MCS evolution and motion is heavily controlled by the cold pool. MCS longevity depends on the interaction between cold pool (c) and low- level vertical wind shear (  u). Deeper shear may be important as well. The Coriolis effect significantly impacts long-lived MCSs (> 4 hrs).

Title goes here for lessonFebruary 2002 References Fritsch, J. M., Murphy, J. D., Kain, J. S : Warm Core Vortex Amplification over Land. J. Atmos. Sci., 51, 1780–1807. Hilgendorf, E.R. and R.H. Johnson, 1998: A study of the evolution of mesoscale convective systems using WSR-88D data. Wea. Forecasting, 13, Houze, R.A., 1977: Structure and Dynamics of a Tropical Squall-Line System. Mon. Wea. Rev., 105, Johns, R.H., 1993: Meteorological conditions associated with bow echo development in convective storms. Wea. Forecasting, 8, Johnson, R.H., and P.J. Hamilton, 1988: The relationship of surface pressure features to the precipitation and airflow structure of an intense midlatitude squall line. Mon. Wea. Rev., 116, Maddox, R. A., 1983: Large-Scale Meteorological Conditions Associated with Midlatitude, Mesoscale Convective Complexes. Mon. Wea. Rev., 111, Przybylinski, R.W., 1995: The bow echo: Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10,