Diurnal Variations of Tropical Convection

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Diurnal Variations of Tropical Convection

• Ohsawa, T., H. Ueda, T. Hayashi, A. Watanabe, and
  J. Matsumoto, 2001 Diurnal Variations of
  Convective Activity and Rainfall in Tropical
  Asia, J. Meteor. Soc. Japan, 79, 333-352.
• Kubota, H., A. Numaguti, and S. Emori, 2004
  Numerical Experiments Examining the Mechanism of
  Diurnal Variation of Tropical Convection, J.
  Meteor. Soc. Japan, 82, 1245-1260.

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Introduction

• The diurnal variations of convective activity and
  rainfall greatly influence the energy and water
  cycles on a daily basis.
• Over tropical oceans, the maximum convection
  appears between midnight and sunrise.

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Procedure of data

• The rainfall data from 101 stations in four
  countries (Bangladesh, Thailand, Vietnam,
  Malaysia) are used.
• The information on convective activity is derived
  from the data from GMS-5.
• The frequency of occurrence of
  is used as the index of convective activity.

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Convective activity
In the afternoon or early evening over both land
and sea.
In the late morning over land and in the late
evening over the sea.
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75
75
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same phase
more than 10 mm/day
57/101
about 7 mm/day
21/101
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Harmonic analysis
advance 3 hours
delay 2 hours
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Climatological winds
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Late night-early morning maxima
Bangladesh
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Thailand
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Vietnam
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Malaysia
relatively dry season
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Interim summary

• The late night-early morning maximum is found in
  the windward areas of mountains, in basins and
  valleys, and in coastal areas.
• These areas are the place where a low-level
  convergence is expected during nighttime due to
  local circulations such as mountain and land
  breezes or their interaction with a prevailing
  wind.

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• The westward shift seem to be caused by a
  westward advection of upper non-precipitating
  clouds such as an anvil by a strong easterly wind
  in the upper troposphere.

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Conclusions

• Over land, the maximum of convective activity
  tends to occur during the afternoon-early evening
  or during the late night-early morning.
• Over the sea, the maximum primarily appears
  during the afternoon.

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Conclusions

• These afternoon-early evening maxima are caused
  by the solar radiative heating on the surface
  during daytime.
• The late night-early morning maxima are caused by
  local circulations such as mountain and land
  breezes or their interactions with a prevailing
  wind.

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Diurnal Variations of Tropical Convection

• Ohsawa, T., H. Ueda, T. Hayashi, A. Watanabe, and
  J. Matsumoto, 2001 Diurnal Variations of
  Convective Activity and Rainfall in Tropical
  Asia, J. Meteor. Soc. Japan, 79, 333-352.
• Kubota, H., A. Numaguti, and S. Emori, 2004
  Numerical Experiments Examining the Mechanism of
  Diurnal Variation of Tropical Convection, J.
  Meteor. Soc. Japan, 82, 1245-1260.

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Purpose

• They want to clarify the mechanism that drives
  the diurnal variation of tropical convection over
  oceans by previous investigators as controlling
  the diurnal cycle.

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Model

• The 2-D Regional Atmospheric Modeling System
  (RAMS) is uesd.
• It was 400 km wide, with a 2-km horizontal
  resolution, and extended to 30 km vertically,
  with 41 levels.
• The model ran 15 days, and the data for the last
  10 days were analyzed.

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Initial conditions

• The IC consisted of a horizontally uniform
  monthly (Dec, 1992) average vertical
  distribution.
• The SST was fixed at the Dec 1992 average (302.5
  K).

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Standard experiment (STND)
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• The time series for the horizontal distribution
  of precipitation.

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UNICLD experiment

• Gray and Jacobson (1977) hypothesized that
  differences in radiative cooling between
  convective regions and surrounding clear regions
  might intensify circulation and convection more
  during the night than during the day.
• To test this, an experiment (UNICLD) included
  horizontally uniform cloud water when calculating
  the radiative scheme was conducted.

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UNICLD experiment
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NOCLDR experiment

• Randall et al. (1991) and Xu and Randall (1995)
  suggested that convection is activated at night
  due to destabilization caused by cloud radiative
  cooling in the upper troposphere over the ocean.
• The NOCLDR (no cloud radiative cooling)
  experiment including a reduced cloud radiative
  cooling effect.

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NOCLDR experiment
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NOPBLR experiment

• Randall et al. (1991) and Xu and Randall (1995)
  mentioned only the high-level cloud radiative
  cooling effect.
• The cloud radiative cooling at the top of the
  boundary layer was removed, and imposed uniform
  radiative heating and cooling (NOPBLR).

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NOPBLR experiment
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Water vapor budget in the boundary layer
E evaporation from the sea vertical
diffusion vertical advection P the
precipitation budget within the boundary layer Q
the large-scale advection of water vapor
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Conclusions

• The differences in radiative cooling between
  convective regions and surrounding clear regions
  are not the primary mechanism forcing the diurnal
  variation of precipitation.
• The cloud radiative cooling at both high levels
  and the top of the boundary layer is important in
  driving the diurnal variation of convection.

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Conclusions

• Boundary-layer cloud radiative cooling
  destabilizes the low-layer atmosphere, and
  provides moisture from the boundary layer to the
  free troposphere.
• This creates a favorable condition for shallow
  convection to develop to deep convection.

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