How to include volcanic forcing in C20C runs
The Hadley Centre has used an updated version of the time series of Sato et al.
(1993), including the effects of Pinatubo (out to 1999, by applying a decay time
of 1 year, in line with observations.) After 1999, we use the lowest possible
value of volcanic forcing, which is ok if there are no major eruptions then. Note,
the Sato et al. data set is updated so this is not necessarily the latest version
BUT it is the one used in HadAM3 integrations. The data are derived largely from
optical depth measurements but also other data, obviously of varying quality &
coverage but generally getting better with time. We chose it for compatibility
with the GISS simulations which used it & for which it was derived: we think it
is also more detailed than most volcanic reconstructions and freely available
The Sato et al. time series is supplied as monthly optical depths at .55 micron,
in the middle of the visible. These are applied in HadAM3 as quarterspheric
monthly values. The conversion to average optical depths for each of the spectral
bands, and the single-scattering properties for each SW band, were calculated
using John Edwards' code assuming an effective radius of .25 microns, an empirically-based
droplet size distribution from a WMO publication (Deepak & Gerber) and a 3-1
mix of sulphuric acid and water by weight. The aerosol is spread over those
model layers above the diagnosed tropopause & below 1 kPa (the bottom of the
top layer in the standard 19 levels & about as high as volcanic aerosol is observed
to reach). For HadAM3 this is how the background stratospheric aerosol is defined,
so all that needs to be done is altering the optical depth from the standard
value (which is about the mean of the Sato & al data, so that the mean volcanic
forcing is zero).
To include the effects of stratospheric aerosols from volcanic eruptions, you
first need a climate model which has some radiation code that includes
stratospheric aerosols. If you do not have this, then unfortunately you will
have to omit
volcanic aerosols from the ensemble.
- This bit of code loads Sato et al (1993)
index into an array VOLCTS which has data for each 45°-latitude band,
month, and year between 1950 and 1999. The order of the latitude bands is
45N-90N, 0-45N, 45S-0, 90S-45S. If a row of the grid is "centered" on 45S,
0, or 45N then the average of the two adjacent quartersphere values is used.
Values prior to 1950 have been set to the minimum value of 1.
- This data must be included in the radiation code of your climate model.
If the radiation code in your climate model requires the data to be specified
as a mass mixing ratio or pathlength, these numbers must first be converted
from optical depth to mass per unit area. In HadAM3 it is included as a mass
mixing ratio. To calculate the volcanic mass per unit area, multiply VOLCTS
by the conversion factor MASCON defined in this
Note, this factor may not match the numbers assumed by your radiation code.
- The aerosol is distributed in each model level in the stratosphere. The
mixing ratio of the stratospheric aerosols must be calculated at each grid-point
and each model level BUT ONLY IN the stratosphere. This is done by dividing
the total aerosol (VOLCMASS) by the total air mass of all stratospheric levels
at that grid-point.
Then hopefully your radiation scheme can do the rest!
Johns, T. C., J. M. Gregory, W. J. Ingram, C. E. Johnson, A. Jones, J. F. B. Mitchell,
D. L. Roberts, D. M. H. Sexton, D. S. Stevenson, S. F. B. Tett, and
M. J. Woodage, 2001.
Anthropogenic climate change for 1860 to 2100 simulated with the HadCM3 model
under updated emissions scenarios. Hadley Centre Technical Note no. 22.
Sato M, J. E. Hansen, M. P. McCormick, and J. B. Pollack, 1993.
Stratospheric aerosol optical depths, 1850-1990. J. Geophys. Res. 98
Thanks to William Ingram for helping out with this HTML page.