Diurnal Variability of Deep Tropical Convection

R. A. HouzeLecture, Indian Institute of Tropical Meteorology, Pune, 12 August 2010

Clouds in Low Latitudes

Lecture SequenceLecture Sequence1.1. Basic tropical cloud typesBasic tropical cloud types

2.2. Severe convection & mesoscale systemsSevere convection & mesoscale systems

3.3. Tropical cloud populationTropical cloud population

4.4. Convective feedbacks to large-scales Convective feedbacks to large-scales

5.5. Monsoon convectionMonsoon convection

6.6. Diurnal variabilityDiurnal variability

7.7. Clouds in tropical cyclonesClouds in tropical cyclones

Diurnal cycle of tropical convection depends on: Diurnal cycle of tropical convection depends on:

• the the scalescale of the convective phenomenon—time scale of the convective phenomenon—time scale of large systems not separable from diurnal time of large systems not separable from diurnal time scalescale

• The The locationlocation of the convection—open ocean, near of the convection—open ocean, near coastline, over mountainscoastline, over mountains

This talk will illustrate that…

A Purely Oceanic Environment

Convective systems over the West Pacific

1 <80 km2 80-170 km3 170-300 km4 >300 km

Cloud shield <208 K

Chen & Houze 1997

Convective systems over the West Pacific

Convective systems over the West Pacific

Chen & Houze 1997

Cloud systems tracked in time in IR satellite data

Small Large

Convective systems over the West Pacific

Chen & Houze 1997

Time needed for large systems to reach maximum size

Convective systems over the West Pacific

Chen & Houze 1997

Relationship to surface air temperature

Cooler than Day 1

Convective systems over the West Pacific

Chen & Houze 1997

“Diurnal Dancing”MJO

MJO

Over Land Near Mountains

Diurnal cycles of different types of

extreme convective systems in

mountainous regions

Romatschkeet al. 2010

Romatschkeet al. 2010

Wide convective core occurrenceby time of day

during monsoon season

This illustrates the effect of nocturnal downslope flow on the diurnal cycle of wide convective core occurrence

Pre-monsoon Season in South Asia

Romatschke et al. 2010

Δ Small 600-8,500 km2

Medium 8,500-35,000 km2

CHF

MEP

Mountains can lead to small and medium sized systems having different diurnal cycles—mesoscale lifecycle effect

A Coastal Environment

Johnson & Houze 1987

December1978

January1979

WINTER MONEXWINTER MONEXDiurnal variation of high cloudiness near BorneoDiurnal variation of high cloudiness near Borneo

08 LST08 LST 20 LST20 LST

14 LST14 LST 02 LST02 LST

S. CHINA SEA

December1978

Mean fractional

area covered

high clouds in

IR images

Bintulu

BORNEO

Houze et al. 1981

Houze et al. 1981

WINTER MONEXWINTER MONEXDiurnal variation of precipitation near BorneoDiurnal variation of precipitation near Borneo

December1978

Mean fractional

area covered by radar

echo

BORNEO

Bintulu

.1.5

Houze et al. 1981

Radar Obs. of WINTER MONEX Borneo cloud systemRadar Obs. of WINTER MONEX Borneo cloud system

Stratiform Precipitation

BORNEOS. CHINA SEA

Bintulu

Mapes et al. 2003

Pacific

Pacific

South America

South America

Andes

Andes

Diurnal gravity wave generation of mesoscale convection over coastal South America

Propagational Diurnal Cycle

Pre-monsoon Season in South Asia

Romatschke & Houze 2010

Equator

NOAA ShipRonald H. Brown

Webster et al. 2002

60E 100E

JASMINE1999 Pre-monsoon

Bay of Bengal

300 mb wind & sfc pressure850 mb wind

Percent High Cloudiness in the Summer MonsoonPercent High Cloudiness in the Summer Monsoon

May-September 19991999

< 235 K < 210 K

Zuidema 2002

Diurnal cycle, mean percent high cloudiness, 1999

Cloud Top < 210 K

Zuidema 2002

Zuidema 2002

r < 85 km

r = 140-210 km

r = 85-140 km

r > 210 km

Location of cloud systems by horizontal dimension May-September 1999

CloudTop

< 210 K

Zuidema 2002

JASMINE Mesoscale Convective SystemsJASMINE Mesoscale Convective SystemsDefined & tracked by 218 K infrared thresholdDefined & tracked by 218 K infrared threshold

JASMINE 1999, Ship Track & Satellite DataJASMINE 1999, Ship Track & Satellite Data

85-90 E85-90 E

ShipTrack

Webster et al. 2002

Propagational diurnal cycle occurs over Bay Propagational diurnal cycle occurs over Bay of Bengalof Bengal

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

Ship radar

20:30 LST

2345 LST 22 May 99 0215 LST 23 May 99 0615 LST 23 May 99

JASMINE 1999JASMINE 1999

Ship Radar DataShip Radar Data

Doppler Radial Velocity

Reflectivity

Reflectivity

JASMINE 1999JASMINE 1999

Ship Radar DataShip Radar Data

22 May 19992300 LST

JASMINE Ship RadarData

TRMM PrecipitationRadar Swath

23 May 1999 0650 LST

23 May 1999 0650 LST

TRMM PR shows extensive stratiform structureTRMM PR shows extensive stratiform structure

~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

• 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 SequenceLecture Sequence1.1. Basic tropical cloud typesBasic tropical cloud types

2.2. Severe convection & mesoscale systemsSevere convection & mesoscale systems

3.3. Tropical cloud populationTropical cloud population

4.4. Convective feedbacks to large-scales Convective feedbacks to large-scales

5.5. Monsoon convectionMonsoon convection

6.6. Diurnal variabilityDiurnal variability

7.7. Clouds in tropical cyclonesClouds 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