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.Basic tropical cloud types 2.Severe convection & mesoscale systems 3.Tropical cloud population 4.Convective feedbacks to large-scales 5.Monsoon convection 6.Diurnal variability 7.Clouds in tropical cyclones

Diurnal cycle of tropical convection depends on: the scale of the convective phenomenon—time scale of large systems not separable from diurnal time scalethe 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 mountainsThe location of the convection—open ocean, near coastline, over mountains This talk will illustrate that…

A Purely Oceanic Environment

Convective systems over the West Pacific 1<80 km km km 4>300 km Cloud shield <208 K Chen & Houze 1997

Convective systems over the West Pacific Chen & Houze 1997 Cloud systems tracked in time in IR satellite data SmallLarge

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

Over Land Near Mountains

Diurnal cycles of different types of extreme convective systems in mountainous regions Romatschke et al. 2010

Romatschke et al 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

Pre-monsoon Season in South Asia Romatschke et al Δ Small 600-8,500 km 2  Medium 8,500-35,000 km 2 CHF MEP Mountains can lead to small and medium sized systems having different diurnal cycles— mesoscale lifecycle effect

A Coastal Environment

Johnson & Houze 1987 December 1978 January 1979

WINTER MONEX Diurnal variation of high cloudiness near Borneo 08 LST 20 LST 14 LST 02 LST S. CHINA SEA December 1978 Mean fractional area covered high clouds in IR images Bintulu BORNEO Houze et al. 1981

WINTER MONEX Diurnal variation of precipitation near Borneo December 1978 Mean fractional area covered by radar echo BORNEO Bintulu.1.5

Houze et al Radar Obs. of WINTER MONEX Borneo cloud system Stratiform Precipitation BORNEO S. CHINA SEA Bintulu

Mapes et al Pacific South America 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 Ship Ronald H. Brown Webster et al E 100E JASMINE 1999 Pre-monsoon Bay of Bengal

300 mb wind & sfc pressure850 mb wind Percent High Cloudiness in the Summer Monsoon 1999 May-September 1999 < 235 K< 210 K Zuidema 2002

Diurnal cycle, mean percent high cloudiness, 1999 Cloud Top < 210 K Zuidema 2002

r < 85 km r = km r = km r > 210 km Location of cloud systems by horizontal dimension May-September 1999 Cloud Top < 210 K

Zuidema 2002 JASMINE Mesoscale Convective Systems Defined & tracked by 218 K infrared threshold

JASMINE 1999, Ship Track & Satellite Data E Ship Track Webster et al Propagational diurnal cycle occurs over Bay of 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 LST 23 May LST 23 May 99 JASMINE 1999 Ship Radar Data

Doppler Radial Velocity Reflectivity JASMINE 1999 Ship Radar Data 22 May LST

JASMINE Ship Radar Data TRMM Precipitation Radar Swath 23 May LST

TRMM 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 Sequence 1.Basic tropical cloud types 2.Severe convection & mesoscale systems 3.Tropical cloud population 4.Convective feedbacks to large-scales 5.Monsoon convection 6.Diurnal variability 7.Clouds in tropical cyclones Next

This research was supported by NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G, NSF grants, ATM , ATM , DOE grant DE-SC / ER-6