Deep Convection Initiation: Source Air, Profile, and CBL Scale Controls

1. School of Earth and Environment NATIONAL CENTRE FOR ATMOSPHERIC SCIENCE (NCAS) • The primary initiation of deep convection from boundary-layer convection during CSIP John Marsham1, Lindsay Bennett1 , Alan Blyth1, Keith Browning1, Peter Clark2,3, Qian Huang1,4, Cyril Morcette3, Doug Parker1, Tammy Weckwerth5. 1NCAS, The University of Leeds, UK 2The University of Reading, UK 3The Met Office Joint Centre for Mesoscale Meteorology (JCMM) 4Lanzhou University, China. 5TheNational Center for Atmospheric Research, USA.

2. Introduction • Overcoming CIN is a necessarybut 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.

3. Chilbolton radar cross section Interaction of synoptic and mesoscalesE.g. CSIP IOP 1, Morcrette et al, 2007 Meteosat (visible) Meteosat: water vapour 200 km Radarrainrate 200 km 200 km

4. Height of the capping inversion (CSIP IOP1) (Morcrette et al, MWR, 2007) 200 km 1.5 km UM 20 radar cross sections • Initiation from coastal convergence is common, and often “well predicted”. E.g. This case, 10th July 2004 (Morcrette et al, 2006), IOP7, IOP12 (Marsham et al 2008), IOP18 (Clark et al, 2009), Boscastle storm etc. • Lifting of lid well modelled (coastal convergence and PV anomaly)

5. Meteosat 12:00 UTC Meteosat 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 (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)

6. Weckwerth, 2000: • Can predict storms from soundings, only if BL variability is accounted for using aircraft data. CAPE (J/kg) Storms No storms CIN (J/kg) 3) CBL scale variability 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. • Rolls are ubiquitous. E.g. for 22 out of 44 days during TRMM-LBA the initial storms were initiated from rolls (Lima & Wilson, 2008).

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

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

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

10. LEM results: doubling of roll spacings Obs spacings (area 2) Model 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 Heights /Spacings Observed clouds Modelled clouds Modelled BL rolls Obs spacings (area 1) Obs height (a2) Model cloud height LEM: 200m grid-spacing, single initial profile, uniform fluxes Obs height (a1)

11. What limits the depth of convection in the LEM? CIN calculated using: standard method mean source air mean profile Cloud-top height • 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.

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

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

14. 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.