Geoscience Reference
In-Depth Information
precipitation anomalies, with a time scale dependent on regional climate sta-
tistics including local seasonality and interannual variability.
Hydrological
drought
describes low levels in lakes, reservoirs, and the subsurface, and/or
low low rates in streams and rivers.
Agricultural drought
refers to low soil
moisture values, or deficits in surface water availability, and may be defined
relative to season and crop type. Socioeconomic drought is measured by its
effects on people.
Attempts to quantify drought have led to the development of various drought
indices
such as the
Palmer Drought Severity Index (PDSI)
, which uses both
precipitation and temperature information and is most useful for characteriz-
ing long-term drought. Another drought index, the
Standardized Precipitation
Index (SPI)
, is thought to be more useful for identifying regions of emerging
regional drought. It is based on calculations of the percentage of normal rainfall
for a time scale and region defined by the user for a given application.
11.4 EXERCISES
11.1. Calculate the amount of longwave radiation leaving an area of 1 m
2
at
the tropopause for three cases: cloudless atmosphere, low cloud, and
high cloud. In each case, model the atmosphere as a transparent slab
with a temperature structure similar to that observed for the troposphere.
Choose reasonable temperatures for the surface, a low cloud, and a high
(in the troposphere) cloud. Assume that the surface and clouds are perfect
blackbodies.
11.2. Calculate the longwave radiative forcing due to an increase in the height
of a midlatitude cloud from 4 to 4.5 km. Assume that the tropospheric
lapse rate doesn't change. Refer to
chapter 2
to choose reasonable values
for the tropospheric lapse rate.
11.3. Daisyworld
1
(a) The surface of Daisyworld I is covered by two species of daisies.
One species is pure white, the other is pure black. The white daisies
produce increasing numbers of seeds as the average surface air
temperature rises above 15°C. The black daisies produce increasing
numbers of seeds as the average surface air temperature falls below
15°C. For thousands of years the average surface air temperature of
Daisyworld I was 15°C, and the populations of the two species of
daisies were equal.
One day, a space traveler arrived to explore Daisyworld. She looked
around a bit but, being allergic to daisies, quickly departed. The
exhaust from her spaceship was highly concentrated CO
2
. The CO
2
was transported throughout the atmosphere, and the average surface
air temperature began increasing due to the direct radiative forcing.
What happened on Daisyworld I?
(b) The surface of Daisyworld II is covered by two species of daisies.
One species is pure white; the other is pure black. The white daisies
1
Daisyworld is the creation of James Lovelock, author of the Gaia hypothesis.
