Trends in stratospheric humidity
and the sensitivity of ozone to these trends
Evans S.J. (1), Toumi R. (1), Harries J.E. (1), Chipperfield M.P. (2),
Russell J.M. (3)
(1) Blackett Laboratory, Imperial College of Science, Technology,
and Medicine, University of London
(2) Department of Chemistry, University of Cambridge, Cambridge, CB1 1EW
(3) Department of Physics, Hampton University, Hampton, VA,23668
ABSTRACT
Measurements of stratospheric water vapor and methane from the Halogen
Occultation Experiment (HALOE) mounted on the Upper Atmosphere Research
Satellite (UARS) are used to investigate changes in stratospheric water
vapor over the period 1992-1996 inclusive. An increase in water vapor mixing
ratio is found at levels between 30 km and 65 km across the globe which
fit, to first order, a linear trend varying with altitude from 40 parts
per billion by volume per year (ppbv yr(-1)) to a maximum of 90ppbv yr(-1)
at 45 km. These trends appear to be greater than that expected due to the
growth in tropospheric methane over the past several decades, and possible
mechanisms accounting for this are discussed. The trend of the combined
budget of 2 x CH4 + H2O is approximately constant with altitude with a
global mean value of 61 +/- 4 ppbv yr(-1). On the basis of these estimates,
sensitivity studies have been performed using a two-dimensional (2-D) radiative-chemical-dynamical
model to assess the impact on concentrations of stratospheric ozone of
this degree of change in stratospheric water vapor over timescales consistent
with doubling CO2 scenarios. We find that the impact of increased stratospheric
water vapor is to enhance the ozone increase in the midstratosphere by
similar to 1 - 2% compared to the response due to a doubling of CO2 itself
of similar to 5-10%. In the upper stratosphere the destruction of ozone
is enhanced and the changeover from production to loss is lowered to similar
to 50 km (from similar to 70 km). A chemical mechanism for these processes
involving enhanced OH and NO2 is identified.
Jour. Geo. Res-Atm. (1998)
Vol. 103, No. D8, pages 8175-8125
Back to HALOE
homepage.
Back to SPAT home
page.
Last modified 25/01/1999.
WWW page comments to Adam Hicks