Global Drought Monitoring Service through the GEOSS Architecture ...

Architectural Implementation Pilot, Phase 3 Version: 2.0
Global Drought Monitoring and European Drought
Observatory-Water SBA Engineering Report
Date: 11/Feb/2011
downscale the reanalysis, which is available at high temporal resolution, and at the same time
remove biases in the reanalysis. This work is described in detail in Sheffield et al. (2006).
2.5.1.2 Bridging the gap between reanalysis data and real time observing system
data
To bridge the data gap between the beginning of 2001 and near-real-time, these methods
were extended to blend reanalysis with available observations. Although reanalysis data are
available up to real-time, most observation-based datasets are generally only available some
months of even years behind real-time. Therefore for 2001-realtime we have used a number of
different datasets depending on their availability. For 2001-2006, we have used the recently
updated (to 2006) monthly gridded precipitation and temperature dataset of Willmott and
Matsura. This matches well the CRU dataset (used for 1950-2000) over their overlap period at
large scales. From the beginning of 2007, we have used the Global Precipitation Climatology
Project (GPCP) monthly dataset which is available a few months off real-time. Ongoing work is
looking at the differences between these various datasets during their overlap periods and
methods to ensure temporal consistency. For the last few months up to real-time, we are relying
on real-time precipitation products (PERSIANN 20 data from University California Irvine,
TRMMM data from NASA) and gauge telemetry (Global Telecommunication System (GTS)
gauge data from NOAA). These products are being downloaded on a daily basis and are
blended into a forcing dataset for VIC over Africa.
Having set the initial meteorological forcing into place, the VIC simulations have been
run, up until near-real-time, in order to establish operational running. Our immediate objectives
are to finalize the data streams for the real-time running of the VIC model. The rapid timing of
real-time operational monitoring creates problems, such as the need to assess whether input data
are available, as well as developing fall-back methods for when data are unavailable or fail
quality control checks. Furthermore, the real-time meteorological data are likely biased, creating
the need to periodically re-run the VIC model up to a few months off real-time when the longterm
gridded observation-based products (which are our best estimates of precipitation and
temperature) are updated, to avoid a drift in the land surface states.
The probability distributions of total column soil moisture and runoff for each grid cell
and each month constitute the climatology, against which current conditions can be compared.
The screening tools account for drought areal extent and duration using concepts adapted from
Andreadis et al (2005), which involve a form of spatial cluster analysis to identify drought
patterns from gridded model output. Based on the historic analysis, we will establish a set of
severity-area-duration thresholds that can be used to screen evolving droughts. Within the real
time monitoring framework, we will monitor where drought thresholds are crossed for either soil
moisture or runoff. Once the prescribed drought thresholds have been crossed, we will continue
to track drought evolution in time (i.e., in subsequent forecasts), until the nowcasts indicate that
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