Seasonal Cycles of Precipitation and Precipitable Water

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Seasonal Cycles of Precipitation and Precipitable
Water and Their Use in Monsoon Onset and Retreat
Er Lu and Xubin Zeng Dept of Atmospheric
Sciences, University of Arizona
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Purpose of Study

• Understand the seasonal cycles of precipitation
  (P) and
• precipitable water (W) and their relations.
• Propose a globally unified monsoon index based
  on W, and
• use the PW relations to clarify the issues
  of the index.

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Background

• P W 2-D, observational, hydrological
  quantities
• P dW/dt (water balance equation)
• P W ?

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Synoptic P W correlation
Synoptic P W are positively correlated during
the year or any season.

• P W analysis (1988-97, daily)
• P NCEP/CPC
• W NASA/DAAC

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Seasonal Cycles of P W
Standard deviation of daily P
Out-of-phase
W
P
In-phase
Not correlated
SE (Southeastern U.S.)
SM (South Mexico)
WC (Western Coast of the U.S.)
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Part 1 Understand the Same Seasonal Pattern of W
Spatial coherence of seasonal W
W
So, W continentally increases from winter to
summer.
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Suppose RH takes values in 0,1 with an even
possibility, then the areas of the domains above
and below the contour of delta(Ts) represent,
respectively, the possibilities of having an
increase and decrease of W.
When delta(Ts) is large, as in seasonal changes
of mid-high latitudes, the possibility of having
an increase of W is high, and that of having a
decrease of W is low. So, W tends to increase
from winter to summer.
Contours are delta(Ts) obtained from delta(W)0
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Also, when delta(Ts) is large, in order to have a
decrease of W, the summer RHs2 needs to become
extremely small, which can only be satisfied for
an isolated atmospheric column. For the real
ocean-land-atmosphere system, the summer high
surface temperature can lead to enough increase
of W through evaporation and thermallyinduced
circulation that converges water vapor.
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Water balance equation
Assume in rainy time CE is offset by P, so only
dry time is included in the integration.
Divide the water vapor convergence into a dynamic
part and a thermally-induced part, which is
related to the change rate of surface
temperature
For synoptic change or seasonal change over
tropics, dynamic part is the dominant, so W can
increase or decrease. For seasonal change of
mid-high latitudes, in which delta(Ts) is large,
W always increases from winter to summer.
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Small delta(Ts) seasonal change over tropics or
synoptic change
Large delta(Ts) seasonal change of mid-high
latitudes
W and Ts have weak correlation
W and Ts have strong correlation
Dynamic
Thermodynamic
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Stephens (1990, J. Climate) Seasonal W over
oceans was prescribed from SST. Correlation
between seasonal W and SST is strong in some
regions, but weak in others.
Our inference Prescription of seasonal W from
surface temperature can be made over both oceans
and lands, but only for mid-high latitudes, not
tropics.
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Part 2 Understand Different P Seasonality Regimes
P seasonality regimes (Finkelstein Truppi
1991)
P
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Favorable circulation condition in WC (wettest
rainy seasons driest rainy seasons)
P, E (evaporation), and C (vapor convergence) in
rainy and dry seasons
Evaporation is not important to winter P in WC,
but may be to others.
North American Regional Reanalysis (NARR)
The formation of precipitation is affected by
complex dynamic and thermodynamic conditions
(e.g., evaporation and circulations), and, in
different seasons and locations, these conditions
can vary greatly. We attempt to understand the
different precipitation seasonality regimes from
the basic atmospheric fields.
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Seasonal tendency of P can be reflected from RH

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RH varies with water vapor and temperature, but
how to compare the changes of water vapor and
temperature?

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Change of temperature
Change of water vapor
Change of saturation extent (precipitation)
If
, then P does not change much.
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WC The change of temperature from winter to
summer is much greater than the change of water
vapor. So, relative to summer, the coldness of
the winter air is much more significant than the
dryness, which makes the winter have a large
saturation extent and thus precipitation. SM
The much more significant moistness of the summer
air than its warmness is important to the summer
monsoon precipitation. SE The changes of
water vapor and temperature are roughly
equivalent, so precipitation occurs throughout
the year.
Lu Zeng 2005, GRL, in press
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Different Relations between Seasonal P and W
Seasonal P W correlation
SM
In-phase
WC
Out-of-phase
SE
Not correlated
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• Part 3 A Globally Unified Monsoon Index by Using
  W
• Principle
• The annual cycle of W is used to represent the
  annual monsoon process including the summer and
  winter monsoons and the two transitional periods
  between them.
• The monsoon onset and retreat are regarded as
  phase-locked phenomena, with each corresponding
  to a relatively fixed stage of the annual cycle.

Zeng Lu 2004, J. Climate
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Onset Date
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Retreat Date
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• Is it reasonable to use W to determine monsoon
  onset/retreat (since conventionally P is used)?

• Since the in-phase relation between seasonal P
  and W, we can use W to determine the climate
  monsoon onset/retreat dates.
• The significant positive correlation between
  synoptic P and W during monsoon onset/retreat
  seasons implies that a larger (smaller) daily W
  statistically corresponds to a larger (smaller)
  daily P. So, an earlier (later) onset/retreat
  determined from P can also be determined from W.
  Therefore, the interannual variations of monsoon
  onset/retreat can also be determined from W.

• Why W cannot correctly determine the global
  monsoon regions?

W always increases from winter to summer in
mid-high latitudes. The large increase of W can
appear in both the monsoon and the nonmonsoon
regions. So, the global monsoon regions cannot be
obtained from W.
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Summary

• The W, as long as not in tropics (no matter in
  monsoon or nonmonsoon regions), always increases
  from winter to summer due to the large annual
  range of temperature.
• In regions not in tropics, though water vapor
  always increases from winter to summer, P may
  have different seasonality regimes, depending on
  the comparison of the change of water vapor with
  that of temperature. In monsoon regions, the
  change of water vapor is much greater than the
  change of temperature, so P also increases from
  winter to summer. In nonmonsoon regions, the
  change of temperature can be equivalent to or
  much greater than the change of water vapor, so P
  can occur throughout the year or mainly in
  winter.
• The positive correlations between P and W at
  seasonal and synoptic scales in monsoon regions
  suggest that it is suitable to use W to indicate
  both the means and the interannual variations of
  the monsoon onset and retreat. However, the
  global monsoon regions cannot be determined from
  the seasonal change of W, since W also increases
  from winter to summer in nonmonsoon regions.