1
Annual Progress Report
CLEO 2011
Project
1.4 Climate model development
Project leader
Michael Kahnert, SMHI, Folkborgsvägen 1, 60176 Norrköping; michael.kahnert@smhi.se
General Objective:
The main objective of this study is to develop a 3D aerosol-cloud model to investigate the indirect
effect of aerosols on a regional scale. During the past year, the work mainly focussed on the
development and validation of the model
Short description of activities 2011
During the period Aug 2010- Dec 2011, the research work focussed on the following two aspects:
1. Incorporation of cloud activation model into the MATCH-SALSA framework
2. Adaptations in RCA for offline ingestion of the cloud droplet number concentrations
(CDNC) from the cloud activation model.
Incorporation of cloud activation model into the MATCH-SALSA framework
The Multiple-scale Atmospheric Transport and Chemistry (MATCH) model is the chemistry
transport model that accounts for transport, chemical transformation and deposition of chemical
tracers in the atmosphere based on the emissions. MATCH is coupled to the aerosol dynamic
model, SALSA that considers the physical processes such as nucleation of particles from gas
phase, growth of particles by condensation and coagulation, thereby computes the size
distribution, number concentration and chemical composition of the aerosol species in the different
size classes. This information is coupled to a cloud activation model that computes the CDNC
based on the prognostic parameterization of Abdul-Razzak and Ghan (2002). By this scheme,
CDNCs are defined as the fraction of particles with radius larger than the critical radius of
activation at the time of maximum supersaturation. This means that, the efficiency of an aerosol
particle to be converted to cloud droplet depends on the size and chemical composition of the
particle as well as the updraft velocity and supersaturation of the air parcel. A full-fledged working
version of the MATCH-SALSA model coupled to the cloud model has been developed and
presented in the appendix are some results. Fig. 1 shows the seasonal means of CDNC in cm -3 in
the entire cloud over Europe. Fig.2 shows joint histograms of CDNC and height in hPa over the
Meditteranean region. It can be seen that the majority of the CDNC values are less than 500 cm -3,
but, values reaches as high as 1500 cm -3. It also shows an interesting seasonality of PDFs of
CDNC. For example, the maximum CDNC is observed in 875 and 700 hPa level. There are
however a number of features that are currently being investigated:
1. When compared to previous studies (Barahona et al., 2011; Bennartz, 2007), CDNCs, in
general, seems to be over-estimated by about 20-30%.
2. Cloud droplets are seen (fig.2) at very low pressure levels upto about 200 hPa. Although
supercooled liquid water has been observed well below freezing heights up to about -40C,
such high vertical location of supercooled liquid water is highly unrealistic.
3. Spatial distribution of CDNC during spring closely follows topography and the land sea
contrast seems to be stronger than expected.
Adaptations in RCA for offline ingestion of the CDNCs from the cloud activation model.
The CDNCs obtained are passed to the regional climate model, RCA4 that would give us
information on cloud microphysical properties such as cloud effective radii, cloud liquid water path
as well as the radiative fluxes. The total cloud particles in RCA4 were assigned fixed numbers
over the whole domain based on whether they are cold, oceanic or warm, continental that are
further used in the autoconversion process (conversion of cloud droplet to rain). Also, the
calculation of the effective radius of cloud droplets over land and ocean were based on these
constant CDNC numbers. These are now replaced by the 3D CDNC fields obtained from the cloud
activation model. The model is already set up. This would help us give an estimate of the indirect
aerosol forcing on a regional scale.
References:
Abdul-Razzak, H. and Ghan, S, J.: A parameterization of aerosol activation 3. Sectional representation, J. Geophys.
Res.,107, 4026, 2002.
Bennartz, R.: Global assessment of marine boundary layer cloud droplet number concentration from satellite, J. Geophys.
Res., 112, D02201, 2007.
Barahona, D., Sotiropoulou, R., Nenes, A.: Global distribution of cloud droplet number concentration, autoconversion rate,
and aerosol indirect effect under diabatic droplet activation, J. Geophys. Res., 116, D09203, 2011.
Deliverables 2011
D.1.4.1: Full-fledged version of MATCH-SALSA coupled to cloud activation model is done as
planned (Aug 2010-July 2011)
D.1.4.2: 1st order coupling between MATCH and RCA model (Aug 2011-present): The technical
model development is already done and tests will be carried out during the next couple of months.
Additional staff involved in project:
Manu Anna Thomas
Co-operation outside CLEO:
MACII project
Reports and publications:
n/a
Oral presentations:
n/a
Appendix:
DJF
JJA
MAM
SON
Fig.1: Seasonal means of CDNC (cm-3) in the entire cloud over Europe.
Fig. 2: Joint histogram of CDNC and height in hPa over the Meditteranean region.