AMIP II Diagnostic Subproject 18
Intercomparison of Surface Climate Extremes
Francis W. Zwiers and Viatcheslav V. Kharin
Canadian Centre for Climate Modelling and Analysis, Victoria,
(Also affiliated with the Department of Mathematics and Statistics,
University of Victoria, Victoria, B.C., Canada V8W 2Y2)
- Climatic extremes have acute effects on various aspects of human life.
For example, the viability of many crops is constrained by the number
of frost free days per year, the frequency and duration of high temperature
events that expose crops to damage from heat stress, and the availability
of moisture. Insurance schemes spread risk across large pools
of users and a number of years by assessing premiums actuarially
from historical claims data, historical extreme weather event data
and other related data. Changes in the frequency and/or intensity
of extreme events will affect these economic risk sharing arrangements.
Human health is affected by weather extremes directly through the
physiological effects of heat and cold and indirectly by floods,
pollution episodes, and the like.
- Evaluate and document the ability of current generation GCM's to simulate the extremes of surface temperature and precipitation.
- Relate the ability of GCMs to simulate the extremes to model's characteristics.
- 6- hourly data of total precipitation rate (pr) will be required to calculate 24-hour accumulated precipitation.
- Daily maximum and minimum surface (2m) air temperature (tasmax and tasmin).
- Corresponding station data and reanalysis data will be required to validate the simulated extremes. NCEP reanalysis data are already housed at CCCma. Yet to be identified collaborators are required to provide access to station data.
It is therefore of great interest to document the extremes of surface temperature and precipitation that are simulated by modern general circulation models and compare them to the observed extremes.
- Two approaches will be used to characterize the extremes of the
1) 10-, 20- and 50-year return values will be estimated at every grid point by inverting the estimated Generalized Extreme Value (GEV) distribution fitted to the sample of annual extremes (annual extremes of 24-hour accumulated precipitation, daily maximum surface temperature and daily minimum surface temperature). (Zwiers and Kharin, 1998a; Kharin and Zwiers, 1998b).
2) threshold crossing frequency and duration analysis.
The properties of the simulated extremes, and the extent to which
their behaviour can be represented by standard statistical models,
will also be examined. For example, Kharin and Zwiers (1998b)
find that the GEV distribution does not properly fit the annual extreme
minimum and maximum temperatures simluated by the CCC coupled model
at all locations. This happens when there is year to year variation
in the physical processes that govern the model's surface temperature.
For example, Kharin and Zwiers (1998b) display a point in central
Australia for which the median annual extreme minimum temperature
is about -10C. However, there are several years in the sample
for which the annual extreme minumum temperature is very nearly 0C.
This clustering is an artifact of the model's simple bucket type
land surface and the interpolating scheme that is used to diagnose
screen temperature. The temperature of the land surface remains
at 0C until all liquid soil moisture is frozen. Thus there
are instances when the atmosphere would like to cool the surface
to a temperature substantially below zero, but the one-layer land
surface prevents this because it has a large thermal mass of liquid
water that is not properly isolated from the atmosphere. We will
use goodness of fit tests and other diagnostic techniques to determine
whether other models exhibit similar types of pathological behaviour.
Our expectation is that models with modern, multilayer land surface
schemes (such as CCC's GCM3 which will be used in AMIP2) will exhibit
fewer such problems.
- Zwiers, F. W., and V. V. Kharin, 1998a: Changes in the Extremes
of the Climate Simulated by CCC GCM2 under CO2 Doubling.
Kharin, V. V., and F. W. Zwiers, 1998b: Changes in the Extremes in an Ensemble of Transient Climate Simulation with a Coupled Atmosphere-Ocean GCM. (http://ams.allenpress.com/amsonline/?request=get-abstract&issn=1520-0442&volume=011&issue=09&page=2200)
For further information, contact the AMIP Project Office (email@example.com).
Last update: 18 June 1999. This page is maintained by firstname.lastname@example.org