A1262292527qbnkN

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Bias Correction of Global Gridded Precipitation
for Solid Precipitation Undercatch Jennifer C.
Adam and Dennis P. Lettenmaier Department of
Civil and Environmental Engineering, Box 352700,
University of Washington, Seattle, WA 98195
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Methodology Specific to Canada
Global Gridded Data Set Comparisons
ABSTRACT Systematic biases in gauge-based
measurement of precipitation can be particularly
severe in the case of solid precipitation. Of
these biases, wind-induced undercatch of solid
precipitation is by far the most significant. A
methodology for producing gridded monthly average
catch ratios for the correction of wind-induced
undercatch of snow is developed, suitable for
application to continental or global gridded
precipitation products. Catch ratios (defined as
the ratio of measured precipitation to true
precipitation) are developed for all countries
where snow accounts for a significant portion of
cold season precipitation and are gridded using a
½º spatial resolution. Catch ratio equations,
specific to gauge type and taken from the recent
World Meteorological Organization (WMO) Solid
Precipitation Measurement Intercomparison, were
applied to five years of daily data (1994 through
1998) from which mean monthly catch ratios were
estimated. Canadian catch ratios were determined
using somewhat more detailed information than for
the rest of the domain, and are therefore
expected to be more reliable. The gridded gauge
correction products are designed to be applicable
both to climatological estimates and to
individual years during the 1979-98 reference
period. As compared with other recent (but more
localized) studies that used a similar method to
account for wind-induced catch deficiencies our
estimates of mean precipitation tended to be
somewhat higher (from 4 to 12), mostly because
we did not attempt to correct for liquid
precipitation undercatch.
Two recent studies have attempted to create an
adjusted precipitation archive for Canada
(Groisman, 1998 and Mekis et al., 1999). Both
studies were more exhaustive than ours, and in
particular, evaluated metadata in more detail
than we did. Therefore, we attempted to make use
of the results of both of these studies.
Groisman (1998) completed a monthly analysis and
performed adjustments on 6,692 stations located
throughout Canada. Mekis et al. (1999), on the
other hand, had access to more detailed metadata
for a smaller set of 495 stations. Both data
sets make adjustments for wetting losses and
wind-induced undercatch for liquid precipitation
and Mekis et al. make an adjustment to account
for trace precipitation.

• The adjusted Willmott et al. data set
  approximates the widely used Legates et al.
  (1990) data during the winter months and
  especially in the Northern Hemisphere.
• The adjusted Willmott et al. data set is closer
  to the unadjusted data sets during the warmer
  months, because no corrections were made for
  liquid precipitation.

• The catch ratios were applied to twenty years of
  the Willmott et al. (2001) precipitation data.
  This adjusted data set was then compared to other
  existing global precipitation data sets.
• 5 global monthly precipitation data sets were
  used in our comparisons (3 time-series and 2
  climatologies).
• All data sets were interpolated to a common
  resolution before comparison.
• Climatologies were created from the data sets by
  averaging the time-series over the period 1979
  through 1998.

Dataset Details Measurement Biases Corrected For
Adjusted Willmott et al. (2001) Time-series 1979-1998 ½º Wind-induced solid precipitation undercatch in snow-dominated regions
Willmott et al. (2001) Time-series 1950-1999 ½º none
Legates et al. (1990) Climatology 1920-1980 ½º Wind-induced undercatch, wetting, and evaporation of liquid and solid precip.
GPCC - Rudolf et al. (1994) Climatology 1961-1990 1º none
CRU0.5 - New et al. (2000) Time-series 1901-1998 ½º none

• Between 20º and 35º all data sets are
  approximately equivalent
• Above 35º the adjusted Willmott et al. and
  Legates et al. data sets are considerably higher
  than the unadjusted data sets, owing to bias
  adjustment
• Between 38º and 45º in Eurasia the Legates et
  al. data set is much higher than the adjusted
  Willmott et al. data set

• For 485 stations mean monthly ratios of
  Groisman to Mekis et al. accumulated monthly
  precipitation estimates were derived
• The ratios were gridded to a ½ resolution and
  applied to the Groisman adjusted monthly station
  data to create an extensive network of station
  measurements that reflect the Mekis et al.
  adjustments.
• Mean monthly catch ratios were determined by
  dividing the original unadjusted mean monthly
  station data by the Mekis et al. adjusted
  Groisman mean monthly station data, both averaged
  over the same period.

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General Methodology
The purpose of this study was to develop a ½º
global gridded precipitation product suitable for
global modeling studies, that reflects as best we
could the known effects of measurement biases,
using the results from the WMO Solid
Precipitation Measurement Intercomparison
(Goodison et al., 1998). Our focus is not on the
exhaustive reduction or elimination of all
sources of error, but rather is intended to
reduce the largest component of the net annual
biases in estimation of climatological mean
precipitation. Therefore, catch ratios were
developed to take into account the wind-induced
undercatch of snowfall, which generally is the
greatest source of error in precipitation
measurements. Of particular concern is
precipitation estimates in those areas (which
constitute approximately ½ of the northern
hemisphere land area) where snow accounts for a
substantial fraction of the annual precipitation
available for runoff.
For our study, catch ratios were determined only
for the countries that experience at least half
of their coldest months precipitation as solid
precipitation in at least half of their land
areas. There were a total of 30 countries
selected.
Precipitation (mm/month)
Groisman / Mekis et al.
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Comparisons to Yang et al.
Comparisons to Canadian and Former USSR Adjusted
Data Sets
Globally Gridded Catch Ratios
Canada
Former USSR
Yang and others performed adjustments on the
precipitation measurements from several stations
in the USA (1996 and 1998), Greenland (1999a),
the Arctic Ocean (1999b), and Siberia (2000).
Yang made adjustments to rainfall and snowfall
measurements including the systematic biases
resulting from wind-induced undercatch, wetting,
and the treatment of trace precipitation as zero.
Yangs method to adjust for wind-induced
undercatch is similar to our method (e.g. WMO
intercomparison results used in both) with the
exception that Yang had access to specific
station information and therefore did not need to
make assumptions regarding gauge type, shielding,
gauge height, and wind-sensor height. We
determined the Yang catch ratios from their
published results by dividing the total
gauge-measured precipitation by the sum of the
total gauge-measured precipitation and the total
depth of wind-induced undercatch.

• The most appropriate regression (based on the WMO
  study) was assigned to each country.
• We assumed that a single prevalent type of gauge
  or shield is representative for a given country.
• We relied on information in Sevruk et al. (1989)
  to determine what type of gauge and what type of
  shielding, if any, was prevalent for a given
  country.
• Some countries did not participate in the WMO
  intercomparison and therefore there were no catch
  ratio equations derived for their national
  gauges. In these cases, regression equations for
  gauges that are similar in design, material, and
  shielding were applied.

Catch Ratio Percent (Measured / True
Precipitation)

• Groisman et al. corrected precipitation data for
  622 USSR stations on a monthly basis using
  Reference Book on the Climate of the USSR (1966
  1969). These were gridded to ½ resolution and
  averaged over the 1979 through 1990 period.
• The Groisman et al. estimates generally are much
  lower (approximately 20 to 30) than the adjusted
  Willmott et al. probably because of the out-dated
  methods that were used.

Monthly catch ratios were then gridded to a ½
resolution using the SYMAP algorithm of Shepard
(1984) as implemented by Widmann and Bretherton
(2000).
The catch ratios were derived using a time-series
of daily meteorological values for the period
during which all of the necessary variables (mean
precipitation, mean wind speed, maximum and
minimum temperature) were available (1994 through
1998). Station observations of these variables
were obtained from the NOAA Climate Prediction
Center Summary of the Day data archived at the
National Center for Atmospheric Research. (In
the figure, the Canadian stations are not shown
because other data were used for Canada.)

• The Groisman (1998) and Mekis et al. (1999) data
  were gridded to ½ resolution and averaged over
  the 1979 through 1990 period.
• The adjusted Willmott et al. data set captures
  the approximate values of the more detailed Mekis
  et al. Corrections.

• CONCLUDING REMARKS
• We believe that the adjusted precipitation data
  set described herein offers an improvement over
  current global products that are either
  unadjusted for solid precipitation catch
  deficiencies, or that use methods that predate
  the WMO intercomparison (Goodison et al., 1998).
• Development of high quality gridded global
  precipitation data sets suitable for large-scale
  modeling is an incremental process. We believe
  that the adjustment procedure, and accompanying
  adjusted (from Willmott et al, 2001) data set is
  a next step in a progression. Its major
  desirable features are that it is closely tied to
  the results of the most recent WMO precipitation
  measurement intercomparison (Goodison et al.,
  1998).
• Note See the author for a list of references.

The adjusted and unadjusted daily precipitation
values were summed to provide monthly totals, and
mean monthly catch ratios were determined for
each station by dividing the unadjusted sum by
the adjusted sum.
Using the monthly gridded catch ratios to adjust
an existing gridded precipitation data set
yielded an increase in the global mean annual
precipitation of 4.7 over the time-period 1979
through 1998, but the greatest increase of nearly
85 occurs at approximately the 80º latitude
during the winter (DJF).

• Our catch ratios are on average between 4.1 and
  7.4 higher than Yang et al.s
• The variation among catch ratios is generally
  inversely related to the mean.