WMOD00384

• Title: Weather Scenarios for Western River Basins Under Changed Climate Conditions
• Date: 1/13/1991
• Weather Modification - Doc Type: Report

2. METHOD OF APPROACH

Ideally, determination of the impacts of global climate change on a river basin might proceed
as follows. A series of models would be developed, including one or more GCMs,
orographic precipitation models, runoff models, and river-management models. Use of the
models in a chain would permit one to make predictions of the ultimate impacts of global
climate change upon water resources in the western United States. Work is proceeding on the
different models required, but decision makers can not wait until all the models are perfected
before starting to assess the potential impacts of climate change. For exanlple, river
management models are being used in sensitivity studies even as the runoff models are being
developed. Although sensitivity studies can be carried out with completely arbitrary
assumptions about potential changes in meteorological variables, GCCRP management has
requested some first-guess climate change scenarios, which are to be as realistic as the present
state of knowledge permits, of weather that may come to pass in several river basins in the
western states.

Numerous studies have established statistical relationships between the water supplies in
individual basins in the western United States and large-scale weather patterns over the North
Pacific and North America (e.g., Cayan and Peterson, 1989). Therefore, rather than develop
scenarios for individual basins, the scenario team will produce climate-change scenarios for
western North America as a whole, and derive from them the likely weather conditions for
the individual basins. As a result of this approach, the changed-climate scenarios developed
for the various basins should be consistent with one another. Consistency does not mean
uniformity; runoff from some major basins of the United States is independent of or
negatively correlated with runoff from other major basins (Meko and Stockton, 1984), and
this fact should be reflected in the scenarios to be developed.

Unless changes in cloudiness exert an unexpectedly strong negative feedback effect, it appears
that the net effect of enhanced concentrations of greenhouse gases will be to wann the surface
of the earth by a significant amount. There are different ways in which the world could get
wanner, but any scenario adopted should make sense from the point of view of a synoptic
meteorologist. One mode of wanning would be for the surface of the entire earth to wann up
by some amount L\T, and the position of jet streams, fronts, and other synoptic scale features
to stay as they are at present. Obviously, this picture is oversimplified. Climate fluctuations
of the recent past did not produce uniform wanning or cooling over the Northern Hemisphere
(Lough et al., 1983). Output from several GCMs has indicated that global wanning will be
more pronounced in the polar regions than in the tropics (Grotch, 1988), and there are
physical reasons for expecting such an outcome (Dickinson, 1989). Another scenario would
be to keep the polar and tropical ainnasses as they are at present, but move the mean position
of the polar front in each hemisphere toward the pole. This scenario does not look realistic
either, as it would provide maximum wanning in midlatitudes, and none at the poles or at the
equator. Furthermore, if the air mass characteristics do not change, why should the polar
fronts and the associated jet streams migrate poleward?

Printed January 18, 1991