Transcript Characterization of the Global Hydrologic Cycle from a Back

Characterization of the Global Hydrologic Cycle from a
Back-Trajectory Analysis of Atmospheric Water Vapor
Paul A. Dirmeyer
Kaye L. Brubaker
04 / 15 / 2008
Recycling Ratio
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Definition: the fraction of precipitation over a defined area that
originated as evapotranspiration from that same area, with no
intervening cycles of precipitation or surface evapotranspiration.
The recycling ratio is a diagnostic measure of the potential for
interactions between land surface hydrology and regional climate.
A change in regional evapotranspiration affects not only the
supply of water carried by the circulation of the atmosphere, but
can thermodynamically alter the atmosphere itself.
Bulk Methods
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The bulk approach assumes that locally evaporated and
externally advected moisture are well mixed in the air over the
region of interest.
One major drawback: contain an atmospheric moisture flux
term at the lateral boundaries defined as the product of two
time-mean quantities—wind and humidity
In actuality, perturbational expansion yields
nonlinear term can be quite significant and has much of its
signal on the time scale of synoptic waves.
Other Methods
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Another drawback: must be calculated over predefined volumes
using the wind and humidity information along the boundaries.
Fine for calculating a single value over a large area, but difficult
to produce a continuous map over a continent or the globe.
The most direct way of estimating recycling is to track the water
vapor in the air from source (evapotranspiration) to sink
(precipitation). (Isotopic Analysis)
Tracer modeling drawback: adds to the computational cost; any
changes require a complete reintegration of the general
circulation model; errors in the model climate contribute errors
in the estimates of the hydrologic cycle.
Back-Trajectory Analysis Methodology
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This approach uses a quasi-isentropic calculation of trajectories
of water vapor backward in time (QIBT) from observed
precipitation events, using atmospheric reanalysis to provide
meteorological data for estimating the altitude, advection, and
incremental contribution of evaporation to the water
participating in each precipitation event.
It relies on the use of high-time-resolution (daily or shorter)
precipitation and meteorological data to include the effects of
transients on the transport of water vapor.
 Chose an interval of 45 min to ensure
statistical stability of results at minimum
computational expense.
 Trajectories are calculated first backward
then forward and the average is taken to
minimize the impact of interpolation errors in
rapidly evolving or highly rotational flows.
 Assume that the diabatic processes
approximately balance out along the path
between the highly diabatic surface evaporation
and terminal precipitation events.
 Assume that the water evaporated from the
surface mixes uniformly through the
atmospheric column within the period of the
time step
Fig 1. Schematic of (a) the division of precipitation
over a pentad into increments of equal amount to be
assigned to advected parcels; (b) the launching of
parcels at random x–y locations and elevations of a
humidity-weighted vertical coordinate over a grid
box (humidity indicated by the curve labeled q);
(c) the apportionment of water vapor in a parcel
from a precipitation event to evaporation during
earlier time intervals along the isentropic backtrajectory path.
Fig. 3. The scaling regression curves from all test regions,
and (bold) the curve through the arithmetic mean of the
recycling ratios at each scale.
Fig. 2. Estimated recycling ratios as a function of area
from subregions over three of the test regions from Table
1, the average values for each scale (filled squares), and
the best-fit regression line through the average values.
 Areas of high terrain tend to stand out as having high recycling ratios. This may be an artifact
of the combination of low precipitable water and high warm-season reanalysis evaporation rates
over these regions.
 Relative minima in regions with strong advection from adjacent waters.
 Recycling appears to be relatively high over much of South America south of the Amazon River
all the way through the La Plata basin, etc.
Unshaded areas in seasonal maps occur
over deserts where no precipitation is
reported in the multiyear analysis.
Recycling ratios are higher during the local
warm or wet season, and lower in winter or
the dry season.
Robust recycling ratios at high northern
latitudes are a spring and summer
phenomenon.
The high-latitude regions of the
Northern Hemisphere, especially in the
Pacific region, show a very strong
annual cycle.
Areas of elevated terrain also show
large magnitudes of the annual cycle.
There are also isolated extrema in the
arid regions of northern Africa and
southwestern Asia, which is an artifact
of the rare sporadic rain events in the
region leading to statistically unstable
estimates.
At lower latitudes, high values in arid
regions and low in the deep Tropics.
Large mean and seasonal variability signals
at high northern latitudes are not evident
at interannual time scales.
Strong signals mainly in the dry regions in
the subtropics and midlatitudes that lie
outside the rainbelts
for a given season.
COV seems to be largest during the dry
season in regimes of strong seasonal
precipitation, consistent with an erratic
evaporation response dependent on the
availability of moisture from the previous
season’s rainfall.
A patchy distribution of weak
but significant positive trends
during boreal winter over
Canada and the northern United
States.
In boreal spring there is a broad
region of strong increases in
recycling over Canada, Alaska,
Fennoscandia,and the Arctic
coast of eastern Siberia, with
sporadic small regions of
positive and negative trends
elsewhere.
The high-latitude positive trends
are consistent with the warming
and extended growing season
trends in these areas.
For most of the globe, the
QIBT recycling exceeds the
bulk recycling.
Over most midlatitude regions,
the difference is less than 70%
of the bulk value; however, in
locations where the bulk
recycling is quite low, such as
the Saharain SON, the QIBT
estimate is more than double
the bulk estimate.
Notable exceptions, where the
QIBT estimate is lower than
the bulk estimate, are northern
South America (all seasons)
and equatorial Africa(DJF).
Conclusions
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Overall, the 25-yr global average recycling ratio for the 105 km2 spatial
extent is 4.5%. On both an annual and a seasonal basis, minima of
recycling are observed in regions with strong advection from adjacent
waters.
The overall patterns are similar, compared with those derived using a
bulk method, in terms of the locations of minima and maxima,
although there are differences in magnitude and detail.
Regions with strong interannual variability in recycling do not
correspond directly to regions with strong intra-annual variability.