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.
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, 2007
Meteosat (visible)
Meteosat: water vapour
200cross
km
Chilbolton radar
section
Radar rainrate
200 km
200 km
Height of the capping inversion (CSIP IOP1)
(Morcrette et al, MWR, 2007)
200 km
1.5 km UM
20 radar cross sections
 Initiation
coastal convergence
is common,
and often
Lifting of lidfrom
well modelled
(coastal convergence
and PV anomaly)
“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.
(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)
 Cirrus shading (Marsham et al 2005a,b)
Meteosat 12:00 UTC
Meteosat 13:00 UTC
(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
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:
CAPE (J/kg)
 Can predict storms from
soundings, only if BL variability is
accounted for using aircraft data.
Storms
No storms
CIN (J/kg)
 Rolls are ubiquitous. E.g. for 22 out of 44 days during TRMM-LBA the initial
storms were initiated from rolls (Lima & Wilson, 2008).
CSIP IOP 12: Observed initiation from BL rolls
10 UTC (almost no precipitation)
Meteosat Visible
Chilbolton radar
Radar crosssection
Larkhill
Swanage
Spacing ~ 6km
50 km
(Extended abstract: Marsham et al, ICCP, Cancun, Mexico, 2008)
CSIP IOP 12: Multiple lids
11:00 UTC
Lid 3 (CIN)
Lid 2 (CIN)
Lid 1 (CIN)
CSIP IOP 12: Observed initiation from BL rolls
12 UTC (significant precipitation)
Meteosat Visible
Radar cross section
Spacing ~12 km
Apparent Doubling of spacing as clouds broke through from
“Lid 2” to “Lid 3”.
LEM results: doubling of roll spacings
Heights / Spacings
Observed clouds
Modelled clouds
Modelled BL rolls
Obs spacings
(area 2)
Obs spacings
(area 1)
Obs height (a2)
LEM: 200m gridspacing, single initial
profile, uniform fluxes
Model
spacings
Obs height (a1)
Model
cloud
height
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
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.
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
CSIP IOP8: Dilution, the role of water vapour in the FT
Effects
moisture
above
BLUTC
– IOP
8
IOP 8: of
LEM
initialised
using the
14:00
radiosonde
profile (
to 3 km (
) and 3 km to 4 km (
).
), dry layer at 2 km
500 m
Initial WVMR
Cloud-top & base
1200 m
LWP
WVMR
 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.