The primary initiation of deep convection from boundary-layer convection during CSIP School of Earth and Environment NATIONAL CENTRE FOR ATMOSPHERIC SCIENCE (NCAS) John Marsham 1, Lindsay Bennett 1, Alan Blyth 1, Keith Browning 1, Peter Clark 2,3, Qian Huang 1,4, Cyril Morcette 3, Doug Parker 1, Tammy Weckwerth 5. 1 NCAS, The University of Leeds, UK 2 The University of Reading, UK 3 The Met Office Joint Centre for Mesoscale Meteorology (JCMM) 4 Lanzhou University, China. 5 TheNational Center for Atmospheric Research, USA.

Introduction  Overcoming CIN is a necessary but not sufficient condition for initiation of deep convection (E.g. Ziegler and Rasmussen, 1998).  Dilution of initially buoyant parcel can restrict depths of clouds (E.g. Redelsperger et al, 2002, Chaboureau et al, 2004)  According to Neggers et al 2002 and Houston and Niyogi 2007, the dilution is greater for less buoyant parcels  Therefore lids (e.g. Bennett et al, 2009) should affect dilution.  Therefore to forecast initiation must predict:  Not only the synoptic and mesoscale and controls of “source air” and “profile” (lifting, warming/moistening)  but also CBL scale, entrainment and mixing.

(1)Interaction of synoptic and mesoscales E.g. CSIP IOP 1, Morcrette et al, km Meteosat (visible) Radar rainrate Meteosat: water vapour Chilbolton radar cross section

Height of the capping inversion (CSIP IOP1) 1.5 km UM20 radar cross sections  Lifting of lid well modelled (coastal convergence and PV anomaly) (Morcrette et al, MWR, 2007) 200 km  Initiation from coastal convergence is common, and often “well predicted”. E.g. This case, 10 th July 2004 (Morcrette et al, 2006), IOP7, IOP12 (Marsham et al 2008), IOP18 (Clark et al, 2009), Boscastle storm etc.

(1) Other sources of mesoscale convergence  Hills (IOP8, IOP12, IOP16 etc, also Tian et al 2003).  Secondary initiation (cold pools and waves, Marsham and Parker 2006, Clark et al 2009) Meteosat 12:00 UTCMeteosat 13:00 UTC  Cirrus shading (Marsham et al 2005a,b) (i) Cirrus reduced surface fluxes and CBL growth (ii) An internal BL formed under the cirrus (ii) Cirrus affects convergence at its edges

3) CBL scale variability  Rolls are ubiquitous. E.g. for 22 out of 44 days during TRMM-LBA the initial storms were initiated from rolls (Lima & Wilson, 2008). E.g. Weckwerth et al, 1996, CBL roll updrafts are: (i) 0.5 K warmer and up to 2.5 g/kg moister (ii) Consistent with the observed LCL. Weckwerth, 2000:  Can predict storms from soundings, only if BL variability is accounted for using aircraft data. Storms No storms CIN (J/kg) CAPE (J/kg)

CSIP IOP 12 : Observed initiation from BL rolls Meteosat Visible 10 UTC (almost no precipitation) 50 km Chilbolton radar Spacing ~ 6km Larkhill Swanage Radar cross- section (Extended abstract: Marsham et al, ICCP, Cancun, Mexico, 2008)

CSIP IOP 12: Multiple lids Lid 3 (CIN) Lid 2 (CIN) Lid 1 (CIN) 11:00 UTC

CSIP IOP 12: Observed initiation from BL rolls Meteosat Visible 12 UTC (significant precipitation) Spacing ~12 km Radar cross section  Apparent Doubling of spacing as clouds broke through from “Lid 2” to “Lid 3”.

LEM results: doubling of roll spacings Fourier analysis shows:  “Doubling” of cloud street spacings, as observed (~6 km to ~12 km).  BL rolls on smaller scales (not well observed).  In UM Δx=500m run captures 6km streets better than Δx= 1.5km Obs height (a2) Obs spacings (area 2) Obs height (a1) Obs spacings (area 1) Heights / Spacings Observed clouds Modelled clouds Modelled BL rolls Model spacings Model cloud height LEM: 200m grid- spacing, single initial profile, uniform fluxes

What limits the depth of convection in the LEM?  Variability in source air, not lifting, dominates variability in CIN (as in Huang et al, MWR, 2009)  When minimum CIN is zero, mixing of source air with lid limits convection.  Huang et al (MWR, 2009) also show that, for their case:  Variability in CIN from rolls, as compared with less organised convection, does not favour initiation. CIN calculated using: standard method mean source air mean profile Cloud-top height

CSIP IOP8: Larger CBL variations IOP 8 14:02 UTC Convection capped by dry lid Aircraft data: strong CBL thermals < (4 ms -1, 1 gkg -1, 1 K). Bennett, PhD, 2009.

Effects of moisture above the BL – IOP 8 Initial WVMR LWPCloud-top & base IOP 8: LEM initialised using 14:00 UTC radiosonde profile ( ), dry layer at 2 km to 3 km ( ) and 3 km to 4 km ( ). WVMR 500 m 1200 m CSIP IOP8: Dilution, the role of water vapour in the FT  Reduced humidity in lower FT, reduces cloud-top heights and LWP.

Conclusions  Initiation of convection during CSIP occurred as a result of a wide variety of synoptic, meoscale and CBL-scale processes.  Many papers have evaluated the sources, predictability and model representations of synoptic/mesoscale processes.  Work ongoing on the coupling of CBL (BL rolls and thermals) with the larger scales.  Main impact of BL convection is source air variability  Overcoming CIN is a necessary but not sufficient condition for initiating deep convection – entrainment can restrict otherwise buoyant clouds.  E.g. CSIP IOP8, half hour delay in LEM for a drier FT  The overall importance of accurately representing CBL processes and entrainment in the moist maritime environment of the UK is unclear.