Diurnal Variability of Deep Tropical
Convection
R. A. Houze
Lecture, Indian Institute of Tropical Meteorology, Pune, 12 August 2010
Clouds in Low Latitudes
Lecture Sequence
1.
2.
3.
4.
5.
6.
7.
Basic tropical cloud types
Severe convection & mesoscale systems
Tropical cloud population
Convective feedbacks to large-scales
Monsoon convection
Diurnal variability
Clouds in tropical cyclones
This talk will illustrate that…
Diurnal cycle of tropical convection depends on:
• the scale of the convective phenomenon—time scale
of large systems not separable from diurnal time
scale
• The location of the convection—open ocean, near
coastline, over mountains
A Purely Oceanic Environment
Convective systems over the West Pacific
Cloud shield <208 K
1
2
3
4
<80 km
80-170 km
170-300 km
>300 km
Chen &
Houze 1997
Convective systems over the West Pacific
Cloud systems tracked in time in IR satellite data
Small
Large
Convective systems
over the West Pacific
Chen &
Houze 1997
Convective systems over the West Pacific
Time needed for large systems to reach maximum size
Chen &
Houze 1997
Convective systems over the West Pacific
Relationship to surface
air temperature
Cooler than
Day 1
Chen &
Houze 1997
Convective systems over the West Pacific
“Diurnal Dancing”
Chen &
Houze 1997
Over Land Near Mountains
Diurnal cycles of
different types of
extreme
convective
systems in
mountainous
regions
Romatschke
et al. 2010
Wide convective
core occurrence
by time of day
during monsoon
season
This illustrates
the effect of
nocturnal
downslope flow
on the diurnal
cycle of wide
convective core
occurrence
Romatschke
et al. 2010
Pre-monsoon Season in South Asia
Mountains can lead to small and
medium sized systems having
different diurnal cycles—
mesoscale lifecycle effect
Δ Small
600-8,500 km2
 Medium 8,500-35,000 km2
CHF
MEP
Romatschke et al. 2010
A Coastal Environment
December
1978
January
1979
Johnson & Houze 1987
WINTER MONEX
Diurnal variation of high cloudiness near Borneo
December
1978
Mean
fractional
area
covered
high
clouds in
IR images
Bintulu
S. CHINA
SEA
BORNEO
08 LST
20 LST
14 LST
02 LST
Houze et al. 1981
WINTER MONEX
Diurnal variation of precipitation near Borneo
Bintulu
BORNEO
December
1978
Mean
fractional
area
covered
by radar
echo
.1
.5
Houze et al. 1981
Radar Obs. of WINTER MONEX Borneo cloud system
BORNEO
S. CHINA SEA
Bintulu
Stratiform
Precipitation
Houze et al.
1981
Diurnal gravity wave generation of mesoscale convection
over coastal South America
Andes
Pacific
South America
Andes
Pacific
South America
Mapes et al. 2003
Propagational Diurnal Cycle
Pre-monsoon Season in South Asia
Romatschke & Houze 2010
JASMINE
1999 Pre-monsoon
NOAA Ship
Ronald H. Brown
Bay of Bengal
Equator
60E
100E
Webster et al. 2002
Percent High Cloudiness in the Summer Monsoon
May-September 1999
< 235 K
850 mb wind
< 210 K
300 mb wind & sfc pressure
Zuidema 2002
Diurnal cycle, mean percent high cloudiness, 1999
Cloud Top < 210 K
Zuidema 2002
Location of cloud systems by horizontal dimension
May-September 1999
Cloud
Top
< 210 K
r < 85 km
r = 85-140 km
r = 140-210 km
r > 210 km
Zuidema 2002
JASMINE Mesoscale Convective Systems
Defined & tracked by 218 K infrared threshold
Zuidema 2002
Propagational diurnal cycle occurs over Bay
of Bengal
JASMINE 1999, Ship Track & Satellite Data
85-90 E
Ship
Track
Webster et al. 2002
IR Temperature
08:30 LST
IR Temperature
11:30 LST
IR Temperature
14:30 LST
IR Temperature
17:30 LST
IR Temperature
20:30 LST
IR Temperature
20:30 LST
Ship radar
JASMINE 1999
Ship Radar Data
2345 LST 22 May 99
0215 LST 23 May 99
0615 LST 23 May 99
JASMINE 1999
Ship Radar Data
Reflectivity
Reflectivity
Doppler Radial Velocity
22 May 1999
2300 LST
23 May 1999 0650 LST
JASMINE
Ship Radar
Data
TRMM
Precipitation
Radar Swath
TRMM PR shows extensive stratiform structure
23 May 1999 0650 LST
~270 km
Summary
• Over open tropical oceans:
– Small systems max in late afternoon
– Large MCSs max around dawn
– 2-day cycle at a given location (“diurnal dancing”)
• In mountainous regions:
– Isolated deep convective elements max in late afternoon
– Nocturnal downslope generates early morning max MCSs
– Max of small rain systems precedes max of medium systems
• In coastal regions:
– Large MCSs generated as response to afternoon heating over high
terrain near coasts
– Propagational diurnal cycles
Clouds in Low Latitudes
Lecture Sequence
1.
2.
3.
4.
5.
6.
7.
Basic tropical cloud types
Severe convection & mesoscale systems
Tropical cloud population
Convective feedbacks to large-scales
Monsoon convection
Diurnal variability
Clouds in tropical cyclones
Next
This research was supported by
NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G,
NSF grants, ATM-0743180, ATM-0820586,
DOE grant DE-SC0001164 / ER-6