Multi-scale MetUM simulations ofa M esoscaleconvective System Sam antha Clarke

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1 Multi-scale MetUM simulations ofa M esoscaleconvective System Sam antha Clarke Supervisors: Suzanne Gray (U niv.ofreading), NigelRoberts (M eto ffice@ Reading) 31 October 2014 Em ail:sam antha.clarke@ pgr.reading.ac.uk

2 What is an MCS? An am algam ation/ organisation ofm any individualthunderstorm s into a single cloud system with Definition (Gray and M arshall1998): W idespread and continuous lightning reports > 100km in any direction. Heavy and/or persistent precipitation (> 10m m ). Identification ofa cold cloud top anvil High surface tem peratures (>, proxy for high CAPE MCSs have associated PV anom alies: Static Stability Satellite im age 0216UTC 13 June 2014 Dundee satellite receiving station Vorticity 2

3 Why study European MCSs? M esoscaleconvective system s (M CSs) occur allover the globe but are relatively rare in W estern Europe in com parison to other places such as the USA an average of2 a year in UK (G ray and Marshall1998). The extrem e weather associated with MCSs makes them im portant to forecast accurately (G ray and Marshall1998) MCS form ation is not consistently predictableand can be hard to forecast in NW P m odels.the move to convection-perm itting m odels has im proved the representation ofmcss,but good forecasts can stillbe elusive -it is difficult to get them in the right place som etimes and the structure doesn't always look like 'reality'(radar). 3

4 Questions Here we explore the interactions betw een weather processes on different scalesspecifically in Western Europe (U K,France,Germ any and Spain). Key questions: 1. H ow is the forecast ofm CSs affected by the representation of convection in models;what are the associated potentialvorticity (PV ) structures? 2. W hat is the synoptic-scale im pact ofpv anom alies generated by MCSs? 3. How does the predictability ofmcss depend on the forecasting of synoptic-scale features. 4

PV representation in NW P models (Chagnon and Gray,2009) Differences in the representation of PV anom alies may im pact the associated circulation patterns and static stability distributions

5 PV representation in NW P models (Chagnon and Gray,2009) Differences in the representation of PV anom alies may im pact the associated circulation patterns and static stability distributions influencing the forecast dow nstream (Gray,2001). Horizontaldipoles ofpv are poorly represented in convection-param eterizing m odels and deeper,m ore intense PV anom alies are found in convection-perm itting m odels (Chagnon and Gray,2009). Accurate PV distributions at tropopause elevations are key to achieving a successful forecast in som e cases (Fehlmannand Davies,1997). 5

6 MCS 5 July 2012:Mature stage 18UTC Satellite im age 1826UTC (Dundee Satellites) Rainfallradar (M eteox.com ) Sferics(M et Office) 6

7 Met Office Unified Model Met Office operational forecast m odel. Operationalm odels: Global25km grid-spacing Euro4km 4.4km grid-spacing UKV 1.5km grid-spacing Globalm odel:convection param eterization schem e. Euro4km :Predom inantly convection resolving but uses a m odified convection param eterization schem e for weak or sm allstorm s which cannot be resolved by the m odelw hich restricts m ass flux in the schem e (Roberts,2003). UKV:Variable resolution,convection perm itting. 7

8 MetUM simulations:version 8.2 GlobalBCs every hour Output every 3 hours Globalstart dum ps 00UTC 5 th July Z 6 th UKV 1.5km Euro4.4 km France 1.5 km UKV:No data assim ilation used France (1.5km ):No variable resolution or data assimilation 8

9 Mature stage:rainfall Global Radar Euro4km France 1.5km UKV 9

10 Mature stage:pv at 250 hpa Global Euro4km France 1.5km UKV PVU^-1 10

11 Synoptic-scale influence ofmcss Is the representation ofthe PV anom alies different when the modeloutput is com pared at com parable resolution? Method: coarse grain data from the Global,Euro4km,France 1.5km and UKV models. Interpolate to a higher resolution grid before averaging out onto a low er resolution grid w here the data can be com pared. Data was averaged onto 25km (originalglobalgrid),100km and 150km grids. MCS covers around 200km therefore 150km may be too sm ooth. 25km is not representative ofthe processes occurring in the global m odeltherefore is not coarse enough to be com parable. Not just a simple interpolation due to the rotated grid ofthe convective resolving m odels com pared with low resolution grids. 11

12 Coarse Graining:Mature stage Rainfall Global UKV Euro4km France 1.5km 12

13 Coarse Graining :M ature stage PV UKV PVU Global Euro4km W W O SC August2014 France 1.5km em ail: sam antha.clarke@ pgr.reading.ac.uk 13

14 Difference grow th:w indspeed 06UTC 18UTC Euro4km France 1.5km UKV 14

15 Conclusions Large am plitude differences in rainfalland tropopause-levelpv and windspeed occur betw een convection-perm itting and convectionparam eterizing simulations ofan MCS even when considered at the sam e resolution. Larger am plitude negative PV anom alies exist at tropopause levelin the convection-perm itting sim ulations and these influence the circulation. Large-scale dynam ics are potentially affected by poor resolution ofmcss since these differences grow rapidly with time reaching large am plitude,even after only 12 hours. The dow nstream im pact ofthese differences might affect rainfallforecasts and cyclone developm ent as the divergence and convergence profiles willbe affected changing upper-levelm esoscale forcing. Ongoing w ork:investigate the im pact ofmcss on predictability through running an ensem ble with perturbations added to it which are produced using the coarse graining data. 15

16 M CC O ccurrences (Liang and Fritsch,1997) No globalstudy ofmcss has been conducted only regional studies. MCCs/MCSs occur allover the globe except for Antarctica. 16

17 Spanish plum e Spanish plum e creates the necessary ingredients (m oisture, potential instability and a lifting mechanism ) for deep,moist convection to occurleading to MCS form ation in W estern Europe. (Lew is and Gray,2010) 17

18 NW P ofmcss Reality Convection-param eterizing 25km Convection-resolving 4km,1.5km Initial storm MCS mature MCSs develop from a single thunderstorm or thunderstorm s m aking them unpredictable and hard to forecast. 18

19 PotentialVorticity (PV) Static Stability Tropopause Vorticity Conserved in adiabatic, frictionless flow In the atm osphere friction/ diabatic processes vary the value ofpv leading to changes in the environm entalflow. Clim atology PV and in NH winter (Hoskins,1990) 19

20 PV in MCSs MCS Initiation MCS Developing MCS Mature Height 250hPa -ve PV Tropopause Divergence -ve PV -ve PV 500hPa Heating +vepv Convergence Heating +vepv Heating +ve PV Time 900hPa Convection Convection Convection (Adapted from Fritsch et al.,1994) The mid-levelcyclonic circulation leads to the organisation ofthe system. Can persist beyond the decay ofthe convection from which they form ed and affect clouds and precipitation far dow nstream from their origin (D avis and Galarneau,2008). Typically km in diam eter and often span most ofthe troposphere (Fritsch et al.1994;davis and Trier,2007) so have a large-scale influence. 20

21 NW P modelling ofmcss Accurate representation ofmcss is know n to be sensitive to the m esoscalefeatures present in the initialconditions used in the m odelrun.ifthe initialconditions are incorrectly represented in term s oftiming and location in com parison to observations the simulation quickly diverges from observations when param eterized convection is used in the m odel(stensrud and Fritsch,1994). To represent an MCS wellthe convection perm itting m odels ideally should cover the area that initiation ofthe convection associated with the MCS form s.c urrently the highest resolution m odelused by the Met Office (U KV 1.5km ) does not include this initiation area as it would be too expensive. Ifthe initialconvection is forecast poorly,then the subsequent MCS is often poorly forecast,potentially leading to dow nstream forecast busts (Rodw ellet al.,2013). Errors at sm all-scales grow rapidly (Lorenz 1969) in regions w here convection occurs and eventually infect larger scales throughout the dom ain (Zhang et al.2003). The adjustm ent ofthe large-scale environm ent due to the heating in an MCS is rapid as the gravity waves move aw ay from the system at speeds of50m /s in response to the convective region heating profile (M apes,1993). 21

22 Resolution dependence ofmcs PV structure Convection param eterizing Convection perm itting 250hPa 500hPa 900hPa 22

Diabatic processes and the structure of the warm conveyor belt

2 nd European Windstorm Workshop Leeds, 3-4 September 2012 Diabatic processes and the structure of the warm conveyor belt Oscar Martínez-Alvarado J. Chagnon, S. Gray, R. Plant, J. Methven Department of