observational data

1
The chances of getting wet Louise Nuijens
UCLA - Dept. Atmospheric Oceanic
Sciences GCSS Toulouse - June 2, 2008
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Lots of rain
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Motivation
What regulates the large variability in shallow
precipitation within the undisturbed trades?
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Ideas

• random?
• deeper clouds, more clouds, more rain?
• Austin, 1948 - Stevens and Seifert, 2007
• Variability in meteorology?
• stability
• humidity (deep convection strong relation
  column water vapor and precipitation)
• Bretherton et al, 2004 - Holloway and
  Neelin, 2008
• Variability in the aerosol?
• more CCN, less rain? Colon-Robles
  et al, 2006 - Hudson and Mishra, 2006
• (more cloud?)

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Data and methods

• Data
• 3300 horizontal radar scans (SPolKa)
• 200 soundings from land (SPNT), ship (RVSJ),
  aircraft (C130)
• lidar data from C130 circles flown at 4.5 km
• Method
• composites (either 2, 3 or 6) of data
• for periods of little, moderate and widespread
• precipitation, based on six hour mean echo
• fractions of NE radar domain.
• excluding days with heavy precipitation
• using echo fraction as a proxy for rainfall

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Deeper and more clouds more rain
Trivej and Stevens The power law and echo size
distribution', poster Wednesday
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Stronger easterlies, deeper and moister layers,
more rain


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Relations between winds, precipitation and
humidity

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But higher winds is more aerosol?

CCN / MD data from Hudson and Mishra, 2007
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Vertical distribution of echo fraction for
flights with low vs high aerosol
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Stronger easterlies, higher humidity?
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Summary

• Precipitation varies most strongly with
  humidity and the strength of the easterlies
• Deeper clouds and greater inversion heights at
  times of more rain
• Between periods of little and moderate precip,
  moistening predominantly
• forced from bottom shifting to the top
  between periods of moderate and
• widespread precip
• and suggest that subtle variations in zonal
  wind speeds (and surface
• evaporation) and subsidence, even within this
  meteorological regime, can
• regulate humidity, hence precipitation
• Variability in the aerosol likely plays a minor
  role
• Nuijens, Stevens and Siebesma
• ' The environment of precipitating shallow
  convection' (poster Wednesday)