Phil Arkin, ESSIC

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Describing the Diurnal Cycle of Precipitation
Using Satellite Observations

• Phil Arkin, ESSIC
• University of Maryland
• With thanks to
• Pingping Xie, John Janowiak, and Bob Joyce
• Climate Prediction Center/NOAA

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• The diurnal cycle in precipitation remains a very
  difficult challenge for global and regional
  models of the atmosphere
• Observations from which the diurnal cycle of
  precipitation can be inferred have been limited
  until recently
• radar and some gauges over land
• inferences from geostationary imagery over oceans
• Newly available high resolution precipitation
  products (CMORPH, TRMM RT, PERSIANN, others) make
  more detailed description of many phenomena
  possible
• CMORPH is composite product using all available
  passive microwave-derived estimates with
  interpolation by advection inferred from
  geostationary IR (Joyce et al., 2004, J.
  Hydrometeorology)
• Basic dataset is 30 minute/8 km 3 hour totals
  for 0.25ºx 0.25º areas used for the most part

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Seasonal mean diurnal cycle IR (top), radar
(bottom)

• Seasonal mean diurnal cycles in deep convective
  cloud (lt215K) and radar rainfall are similar
• Interannual changes also similar
• Amounts, phases cant be compared effectively

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US/Central Amer./ Mexico JJA 2003
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215K
LAND ONLY
Climatology of diurnal cycle of cloudiness
(pentads for full year)
OCEAN ONLY
Mean diurnal cycle in fractional coverage (87-97)
at 215K averaged over the NAME region (upper
left), and over land (upper right) and ocean
(lower left) subregions. 0000 UTC at bottom
(4-6pm local), time increasing upward.
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1987
1988
1989
LAND 215K
1992
1990
1991
1993
1994
1995
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1987
1988
1989
OCEAN 215K
1992
1990
1991
1995
1994
1993
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May
June
July
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• Diurnal cycle prominent during May September
  when averaged over whole region
• Huge differences between land and water
• Begins in late June in both (northern/western
  subset of full domain)
• Over land, clouds begin to increase in
  mid-afternoon and peak around 7-8pm seasonal
  peak mid-July mid-August
• Over water, diurnal cycle much weaker but still
  clear peaks around 4-6am seasonal peak later
  August September
• Much more intraseasonal variability in 11-year
  average over water
• Interannual variations more in amplitude than
  phase over both land and ocean
• CMORPH allows us to visualize details of the
  influence of the terrain of the diurnal cycle of
  precipitation that may never have been seen
  before
• Precipitation dies away quickly to the west of
  the Sierra Madre Occidental
• Seasonal cycles in 2003 and 2004 are similar, but
  some differences are evident

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Difference between largest and smallest values in
mean diurnal cycle (left) and time of maximum
value (right). Index value of 1 corresponds to
0115 UTC, roughly 10pm local on average.
Mean precipitation through lifetime of Isabel
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• Isabel exhibited substantial mean diurnal cycle
  in precipitation during its lifetime
• Peak values found on northeast side of eye near
  local midnight
• Maximum values about 50 greater than minimum
• Ivan, Jeanne and Karl also show substantial
  diurnal (also some semidiurnal) variability
  averaged over their lifetime
• Not clear whether the (local) time of maximum is
  the same for each
• Mechanisms?
• Solar forcing?
• Inertial oscillation?
• Dynamic oscillation that aliases to 24 hour
  period?

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South America DJF 2002-03 2003-04
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Sea breeze induced convection near the coast on
Day 1 propagates westward reaching the western
Amazon Basin on Day 3.
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• Syntheses of available satellite data make it
  possible to describe diurnal cycle of
  precipitation (as well as other variations) in
  much greater detail than in the past
• CMORPH, PERSIANN, TRMM/RT, Osaka Pref. U.,
  NRL/Turk,
• Have to see if Regional Reanalysis capable of
  providing circulation details (global reanalyses
  so far cannot)
• Details still have to be validated
• CMORPH, radar and IR give similar results over
  U.S.
• Many other high resolution precipitation products
  have been developed (PERSIANN, TRMM RT, NESDIS
  Autoestimator, )
• All provide interesting and provocative detail
• Some validation is available (CPC for US, BoM for
  Australia)
• But a thorough evaluation and intercomparison is
  badly needed (more discussion tomorrow)