Transcript pos_evansetal2002

A72B-0168
Simulation of the Climate of South-West Asia with a Regional Model
J.P. Evans ([email protected])
R. Smith ([email protected])
Yale University, P.O. Box 208109, New Haven, CT 06520-8109 United States
R. Oglesby ([email protected])
NASA/MSFC/NSSTC, 320 Sparkman Dr., Huntsville, AL 35805 United States
1. Introduction
South-West Asia (Figure 1) is interesting for several reasons. Over the
last 8 thousand years, the landscape has been massively altered by
developing human activity, including forest removal, rangeland
degradation by plowing, grazing and trampling, and water course
damming and diversion. Rapid population growth, political conflict and
water scarcity are common throughout the area, rendering it sensitive
to changes in climate. Indeed climate changes may have already had
significant impact on the history of the region. Our study region
includes the archeologically important Fertile Crescent, the birthplace
of agriculture and civilization.
The region is characterized by steep climatological gradients between
the central desert regions and the surrounding water bodies and
sizeable Tauros and Zagros mountain ranges.
The ability of the regional model RegCM2 to simulate the climate of South-West Asia is
examined. The climate of the region displays high spatial, seasonal and interannual
variability, providing a strong test of climate model capabilities. RegCM2 captures the spatial
variability of temperature despite cold biases being present in the model. RegCM2 does not
capture the annual cycle of precipitation on the Black and Caspian Sea coasts where very
steep topography exists, nor on the eastern Mediterranean coast where the coastal
mountains are not resolvable. RegCM2 does capture the seasonal cycle in the Fertile
Crescent and Zagros mountains, where it is strongly influenced by a plateau circulation
above the Iranian plateau. It is shown that accurate simulation of precipitation in these
regions, including the inter-annual variability, requires the correct simulation of both storm
tracks and topographic interactions. Through the use of regional climate models the
concentration and transport pathways of water vapor through the Middle East can be
explored. The results demonstrate that short lived 'events' have little impact on monthly mean
atmospheric fields yet provide a significant amount of the precipitated water which flows in
the Tigris river.
3.2 Influence of terrain
The terrain exerts a significant influence over the precipitation falling in
the Fertile Crescent and Zagros Mountains regions.
Figure 6 shows the Tigris-Euphrates valley being dominated by rising
motion during the winter months as the westerly winds are forced to
rise over the mountains. During summer the winds are turned down
the valley and a large zone of descending air is present.
Figure 7 shows this change in vertical motion is accompanied by the
appearance of a summer temperature anomaly over the Iranian
plateau.
2. Model, domain and experiment
RegCM2 Description
Dynamical component based on MM4
• hydrostatic
• compressible
• primitive equation
• terrain following s vertical
coordinate
modifications include
• detailed representation of radiative
transfer
• surface physics-soil hydrology
(BATS)
• planetary boundary layer
• convective precipitation scheme
RegCM2 model run
• Centered at 45N 35E
• ~8,000,000km2 total domain
• Topography and landuse are
interpolated from a global 10
min dataset
• Initial and boundary conditions
are extracted from the ECMWF
TOGA analysis
• Begins 1st january 1990 (1st
month of run is discarded)
• Covers 5 years
• grid spacing = 25 km
• time step = 90 s
Black Sea
Turkey
Figure 6: Omega and horizontal
wind @ 700hPa
Figure 7: Temperature and horizontal
wind @ 700hPa
Syria
Iraq
This summer temperature anomaly over the Iranian plateau is
associated with a break in the dominance of westerly winds. It induces
a kind of plateau circulation effect which can be seen in Figure 8. Over
the peaks of the mountain range easterly winds from the Iranian
plateau meet westerly winds from the Mediterranean. This
convergence of air masses above the 800 hPa level strongly restrains
air from below this level from moving up into the convergence zone.
Instead the low level air mass is more easily turned to flow along the
mountain range towards the south-east.
Iran
Saudi Arabia
Figure 1: RegCM2 domain
Figure 8: Vertical cross-section of temperature and
wind @ 34N
3. Results
3.3 Water Vapor transport
The fifteen largest precipitation events in the Fertile Crescent region
were studied to establish where the water vapor leading to each event
came from. Presented are results from a typical storm which occurred on
the 16th February 1990. Figure 9 shows snapshots of the storm, which
clearly shows the easterly movement through the domain.
3.1 Surface temperature and precipitation
While RegCM2 is able to capture the spatial distribution of
temperature quite well, Figure 2 clearly shows a significant
cold bias is present.
Figure 3 shows that the model is able to capture only some
aspects of the precipitation field well.
The annual cycle of precipitation is modeled well in the Fertile
Crescent and Zagros Mountains subregions however
problems exist in each of the other subregions.
Figure 3: Observed and modeled mean annual precipitation
a
b
c
Figure 12: Vertical
cross sections showing
the water vapor
transport into the box
defined by the white
square, 6 hours before
the precipitation
maximum. From the
west a broad region of
moist air approaches
while from the south a
more confined ‘river’ of
very moist air is evident.
Figure 9: Precipitation and winds. a) 24hrs
before event; b) 12hrs before event; c) the event
a
Figure 4: domain showing the focus
subregions. 1. South-East Black Sea
Figure 2: Observed mean annual temperature and model bias
coast, 2. South-West Caspian Sea coast, 3.
Eastern Mediterranean coast, 4. Fertile
Crescent (headwaters of Tigris river), 5.
Zagros Mountains and 6. Saudi desert.
Figure 5: Monthly average
precipitation for each subregion.
b
c
Figure 10: Water vapor mixing ratio at 800hPa. a)
24hrs before event; b) 12hrs before event; c) the event.
As well as the moisture coming from the west, an
intense ‘river’ of moisture also enters from the south.
Figure 11: Mass of water
entering the area of
interest. Clearly large
contributions come from
both the west and the
south.
While water vapor transport into the Fertile
Crescent region from the west is important, water
vapor approaching from the south was at least as
important in most of the precipitation events. Thus
the Persian Gulf/Arabian Sea are important water
sources for the Fertile Crescent.