Q. J. R. Meteorol. Soc. (2002), 128, pp. 973–989
Role of spatial and temporal variations in the computation of radiative forcing
due to sulphate aerosols: A regional study
By GUNNAR MYHRE1;3¤ , JAN E. JONSON2 , JERZY BARTNICKI2 , FRODE STORDAL3;1
and KEITH P. SHINE4
1
University of Oslo, Norway
2
Norwegian Meteorologica l Institute, Norway
3
Norwegian Institute for Air Research (NILU), Norway
4 University of Reading, UK
(Received 13 June 2001; revised 12 November 2001)
S UMMARY
A high-resolutio n regional model for sulphate aerosols is used to investigat e the effects of spatial and
temporal averaging of radiative forcing. Mie theory is used to calculate the aerosol optical properties . The
strong hygroscopi c growth with increasing relative humidity is taken into account. The results for the regional
area selected in our study (Europe and much of the North Atlantic) show that earlier global studies may have
underestimate d the magnitude of the radiative forcing due to sulphate aerosols by up to 30–40% due to coarse
spatial and/or temporal resolution, at least over certain regions. This underestimatio n in global models of the
water uptake is important for all strongly scattering hygroscopi c aerosols. Our results imply that representatio n
of relative humidity at even higher spatial resolution than used in this study may be of importance. This could be
incorporate d in models using subgrid-scal e parametrization s of the relative humidity.
K EYWORDS: Hygroscopi c aerosols Radiative transfer model Relative humidity
1.
I NTRODUCTION
For the well-mixed greenhouse gases that absorb and re-emit thermal infrared
radiation, the spatial and temporal resolution of meteorologica l data only modestly
affects their radiative forcing (Myhre and Stordal 1997; Freckleton et al. 1998). For
other radiative forcing mechanisms this insensitivit y to spatial and temporal resolution
may not hold. Haywood et al. (1997a) and Petch (2001) reported that due to strong
nonlinearitie s in the hygroscopic effect on the radiative forcing due to sulphate aerosols,
the magnitude of the forcing was substantially underestimate d for horizontal resolutions
normally used in general circulation model (GCM) studies.
The radius of hygroscopic aerosols grows by uptake of water in humid air. At the
deliquescence point this growth with higher humidities increases substantiall y, making
this process strongly nonlinear with relative humidity (Fitzgerald 1975; Tang 1996).
Observational campaigns (e.g. TARFOX, Hegg et al. 1997) show that water is the main
constituent of mass in aerosols over the eastern part of North America, a region with a
high industrial in uence on the large aerosol optical depth.
The range in the global estimates of the radiative forcing due to sulphate aerosols
is substantial, from about ¡0.2 to ¡0.8 W m¡2 (see Myhre et al. 1998; Haywood and
Boucher 2000). There are many reasons for the large spread, but two effects clearly
seem to be important: the forcing in cloudy regions, and the effect of relative humidity
on the aerosols. The difference in the hygroscopic effect may arise either from the
treatment of the hygroscopic effect of sulphate aerosols or from the global distributio n
of relative humidities and its spatial and temporal variation in the models calculating
the radiative forcing. Some previous global studies have used monthly mean data for
relative humidities; others have used high temporal resolution for relative humidities,
¤
Correspondin g author: Department of Geophysics, University of Oslo, PO Box 1022 Blindern, 0315 Oslo,
Norway. e-mail: gunnar.myhre@geofysikk.uio.n o
°
c Royal Meteorologica l Society, 2002.
973
974
G. MYHRE et al.
but monthly mean data for sulphate. The spatial resolution also differs in the previous
global studies.
This paper has three main focuses: (i) investigatio n of spatial and temporal resolution on the radiative forcing due to sulphate aerosols; (ii) consistency between distribution of clouds, relative humidity, and sulphate; and (iii) the radiative forcing due to
sulphate over Europe. A regional high-resolutio n chemistry transport model (CTM) and
a radiative transfer model are used in this study. In Haywood et al. (1997a) a limitedarea model with a high resolution (2 £ 2 km) was used and compared to the spatial
resolution used in one grid cell in GCMs. Petch (2001) considered a 1 km resolution
over 1-D (one-dimensional) domains of 256 and 500 km. This modelling study investigates spatial variations down to a somewhat larger scale (50 £ 50 km), but it covers a
larger regional area. In contrast to Haywood et al. (1997a) and Petch (2001) this study
includes the spatial variability of the sulphate aerosol itself, as well as relative humidity
and clouds.
2.
M ODELS
(a) CTM
The sulphate distributio n is calculated using the Co-operative Programme for
Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe
(EMEP) 3-D Eulerian Acid Deposition model. The EMEP model is an off-line regional
CTM centred over Europe, and its model domain includes most of the North Atlantic
Ocean and the Arctic region. The model uses 20 sigma layers of up to 100 hPa in
the vertical, of which ten are below about 3 km. In the horizontal the resolution is
50 £ 50 km, with a polar stereographic mapping true at 60± N. The model time step
is 10 minutes, but for the vertical advection the time step is reduced to 5 minutes. In the
present model version nine chemical species are included (SO2 , NO, NO2 , HNO3 , PAN,
NH3 , SO4 , ammonium sulphate, and ammonium nitrate).
Dry deposition is parametrized according to the resistance anology as described in
Jakobsen et al. (1997) with some modiŽ cations (Olendrzynski 2000). The model treats
in-cloud and below-cloud scavenging differently, based on the parametrization in Berge
(1993). In-cloud scavenging is based on a direct relation between cloud water, locally released precipitation and concentrations of pollutants. Below-cloud scavenging of gases
and particles is calculated based on information about the accumulated precipitation
from the layers above. The oxidation of SO 2 to sulphate is parametrized as a function
of cloud cover and water content as described in Jonson et al. (2000). The model is
described in more detail in Jonson et al. (1998) and Olendrzynski et al. (2000).
The meteorologica l input data are updated at 3 h intervals. The data are generated
from the regional numerical weather prediction (NWP) model HIRLAM (Källén 1996)
on the same grid as the acid deposition model. Meteorologica l data for 1998 are used in
this study. For sulphur the lateral boundary concentrations are based on seasonally averaged concentrations from a hemispheric model (Tarrason, personal communication). A
10-day period was used to spin-up the model before a 1-year run.
Emissions of SO2 , NOx and NH3 are from the EMEP database for 1998 with a
monthly variation (Bartnicki et al. 1998). A distinction is made between emissions
above and below 100 m. Emissions above 100 m are distributed into the three layers
above the lowest model layer.
Calculations of radiative forcing are performed for 4 months: January, April, July,
and October. Annual means in our results refer to averages over these 4 months. Sulphate
calculations are performed for anthropogeni c and natural emissions. In this study results
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
975
are shown for the difference between simulations including and excluding all sulphate
emissions.
The sulphate from the model is compared to surface observations in Bartnicki et al.
(1998), Olendrzynski et al. (2000) and Olendrzynski (2000) with reasonable agreement.
Figure 1 shows scatter plots for January and July 1998 from 57 EMEP stations for
monthly mean sulphate concentrations . The modelled sulphate concentrations are generally lower than those observed, especially in January. Correlations are 0.68 and 0.65
in January and July, respectively. Away from the surface there is inadequate data with
which to evaluate the model.
Figure 2 shows the distributio n of the modelled annual mean total (anthropogenic
and natural) sulphate burden. The highest values are over southern and central Europe.
The maximum in southern Italy is due to the large natural emission from volcanoes.
There is generally a higher burden in eastern Europe than in western Europe. Over the
ocean the sulphate burden is relatively high, as over the Mediterranean, the Black Sea,
the North Sea, and the Baltic Sea. The average sulphate burden over the model domain is
2.3 mg m¡2 . The maximum total sulphate burden of 22 mg m¡2 over Europe is similar
to the anthropogeni c sulphate burden over Europe found in previous global studies
(Langner and Rodhe 1991; Feichter et al. 1996; Restad et al. 1998). However, this
maximum is mainly from natural emission and the sulphate burden is up to 10 mg m¡2 in
regions with strong anthropogenic emissions. The large difference from previous global
studies can be explained by substantia l reductions in sulphur emissions between the
1980s used for emission data in the above global models, and 1998 used in this study.
Over Europe anthropogenic sulphur emissions were reduced by almost 60% from 1985
to 1998 (Vestreng and Støren 2000).
(b) Radiative transfer model
The solar radiative transfer model used here is the discrete-ordinate method
(Stamnes et al. 1988). In the calculations for this study the multistream model is used
with eight streams. Rayleigh scattering and clouds are included in the radiative transfer
model, and the exponential-su m Ž tting method (Wiscombe and Evans 1977) is used
to account for absorption by water vapour and ozone. Four spectral regions are used,
with the main emphasis on wavelengths below 1.5 ¹m. The spectral regions are 0.3–
0.5 ¹m, 0.5–0.85 ¹m, 0.85–1.5 ¹m, and 1.5–4.0 ¹m. The number of exponential-su m
Ž tting terms for each spectral region is two or three. The GENLN2 line-by-line model
(Edwards 1992) is used to calculate the transmission data for water vapour with spectroscopic data from the HITRAN-92 database (Rothman et al. 1992). The water vapour
continuum is included. Cross-sections for ozone in the ultraviolet and visible region are
from WMO (1985). Temperature and pressure dependencies are not included as these
effects in uence the radiative forcing due to the aerosols by less than 1%. As discussed
below, the effect of absorption of gases other than water vapour and ozone on the forcing
due to aerosols is also small.
Data for cloud fractions and cloud liquid content are from the NWP model and
therefore consistent with the CTM. The random cloud-overlap assumption is used, based
on radar observations (Hogan and Illingworth 2000). The optical properties of clouds are
calculated using the procedure described in Slingo (1989), with effective radii of 10 ¹m
for low clouds and 18 ¹m for high clouds (Stephens 1978; Stephens and Platt 1987).
The atmospheric water vapour content is also taken from the NWP model, which is
instantaneou s data sampled every 3 h. Surface albedo is calculated based on the surface
vegetation and monthly mean snow cover distributio n from the NWP model. For each
976
G. MYHRE et al.
0.02
0.04
0.06
0.08
0.1
0.2
0.4
0.6
0.8
1.0
EULERIAN
(a)
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
RU
0.1
0.1
0.08
0.08
0.06
0.06
IS
0.04
0.04
0.02
0.02
0.02
0.04
0.06
0.08
0.1
0.2
0.4
0.6
0.8
1.0
OBSERVED
0.04
0.06
0.08
0.1
0.2
0.4
0.6
0.8
1.0
(b)
1.0
1.0
0.8
0.8
0.6
0.6
LV
EULERIAN
0.4
0.4
0.2
0.2
IS
0.1
0.1
0.08
0.08
0.06
0.06
NO
0.04
0.04
0.06
0.08
0.1
0.2
0.04
0.4
0.6
0.8
1.0
OBSERVED
Figure 1. Scatter plot of observed and modelled sulphate for (a) January and (b) July 1998. The observation s are
from 57 EMEP stations (see text). Surface observation s and modelled values from the lowest level in the chemistry
transport model are used. Units in ¹g(S) m¡3 . The code associated with each observation refers to country and
station classiŽ cation. More information can be found at www.emep.int
977
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
mg/m2
Above
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Figure 2.
Annual mean sulphate burden (mg m¡2 ) calculate d with the chemistry transport model.
of the seven surface vegetation types in the model a surface albedo value is given. We
have taken into account that the solar zenith angle varies with season and latitude, and
over the diurnal cycle.
We use the term ‘radiative forcing’ not strictly as used in IPCC (2001) because we
include the natural sulphate in addition to the anthropopgenic sulphate. The radiative
forcing is calculated at the top of the atmosphere, as it was shown in Myhre et al.
(1998) that the difference between the radiative forcing due to aerosols at the top of
the atmosphere and at the tropopause was small.
(c) Optical properties
The optical properties (speciŽ c extinction coefŽ cient, single scattering albedo, and
asymmetry factor) used in this study for dry aerosols and for wet aerosols at relative
humidities above 30% are the same as in Myhre et al. (1998), where the growth of the
aerosols for relative humidities above 81% was based on Fitzgerald (1975).
(d) Validation of radiation model
The radiative transfer scheme is compared to a model again using the discreteordinate method (Stamnes et al. 1988) but calculating the water vapour absorption
directly from GENLN2. Therefore, the two radiative transfer schemes treat the gas
absorption differently. The two schemes are compared for a dry sulphate aerosol with
sulphate mass concentration of 10¡8 g m¡3 at the ground and decreasing concentrations
from 0 to 5 km, with a gradient of 2:0 £ 10¡9 g m¡3 km ¡1 .
Table 1 shows a comparison of the sulphate radiative forcing from the two radiative
transfer schemes. In both models Rayleigh scattering and absorption by ozone and water
vapour are taken into account. Calculations are performed for different surface albedos
978
G. MYHRE et al.
¡2
TABLE 1.
R ADIATIVE FORC ING DUE TO SULPHATE (W M )
FO R VARIOUS SURFAC E ALBEDOS AND SOLAR ZENITH ANGLES
CALCULATED WITH THE RA DIATIVE TRANSFER SCHEME WITH
GAS ABSORP TION FROM A LINE - BY- LINE MODEL (LBL) AND
USI NG AN EXPO NENTIAL SUM FITTING METHOD (ESF)
Surface Albedo
Solar Zenith
Angle (degrees)
0.0
0.0
0.2
0.2
0.2
0.5
0.5
0.8
0.8
0
75
0
30
75
0
75
0
75
LBL
ESF
¡1.31
¡2.17
¡0.51
¡0.71
¡1.65
0.27
¡0.95
0.45
¡0.35
¡1.31 (0.0)
¡2.10 (¡3.0)
¡0.52 (2.6)
¡0.72 (1.7)
¡1.61 (¡3.0)
0.24 (¡9.3)
¡0.93 (¡2.3)
0.41 (¡10.0)
¡0.36 (1.7)
Values in parentheses are percentag e deviation s between the ESF
and the LBL codes.
(a)
(b)
90.0 - 100.0
80.0 - 90.0
70.0 - 80.0
60.0 - 70.0
50.0 - 60.0
40.0 - 50.0
30.0 - 40.0
20.0 - 30.0
10.0 - 20.0
0.0 - 10.0
Figure 3.
Average relative humidities in (a) January and (b) July in the seven lowest levels in the model.
and solar zenith angles, and the radiative forcing due to the sulphate aerosols follows
the dependency on surface albedo and zenith angle as described in Haywood and Shine
(1997). It is worth noticing the positive forcing for high surface albedo and low solar
zenith angle which is explained in Haywood and Shine (1997).
The results agree for the two model versions within 5% for all cases except the ones
with high surface albedo and low zenith angle, where differences are up to 10%. In the
real world such a combination is rare. In the model treating the gas absorption with the
exponential-sum Ž tting method, absorption by other gases (most importantly oxygen
and carbon dioxide) is left out to save computer time. GENLN2 is used to investigate
the effect of this simpliŽ cation. A maximum difference of slightly above 1% is found.
3.
R ELATIVE HUMIDITY
In this section some aspects of the variation in relative humidity will be shown, as
there is a strong nonlinearity in the radiative forcing due to sulphate aerosols. Later it
will be shown that this effect induces large changes when spatial and temporal averaging
are carried out.
979
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
(a)
(b)
27.0 - 30.0
24.0 - 27.0
21.0 - 24.0
18.0 - 21.0
15.0 - 18.0
12.0 - 15.0
9.0 - 12.0
6.0 - 9.0
3.0 - 6.0
0.0 - 3.0
Figure 4.
Standard deviation (temporal variability) of the relative humidity during (a) January and (b) July.
(a)
(b)
27.0 - 30.0
24.0 - 27.0
21.0 - 24.0
18.0 - 21.0
15.0 - 18.0
12.0 - 15.0
9.0 - 12.0
6.0 - 9.0
3.0 - 6.0
0.0 - 3.0
Figure 5.
Vertical standard deviation of the monthly averaged relative humidity for (a) January and (b) July.
Figure 3 shows the averaged relative humidity for the seven lowest levels in the
model (up to about 1.5 km) for January and July. It shows that there are large spatial
variations in the relative humidity, with much higher values over ocean than land. Over
the ocean the relative humidity is above 80% whereas it is mostly below 60% over land.
It is somewhat larger in January than in July.
In Fig. 4 the temporal standard deviation of the relative humidity during January
and July is shown. The basis of the Ž gure is the averaged relative humidity for the seven
lowest levels in the model, and the temporal resolution of the data was 3 h. The standard
deviation is relatively homogeneous and mostly around 15%. It is generally higher over
ocean than land, with the largest standard deviations often in coastal regions. Figure 5
shows the standard deviations in the vertical variability over the seven lowest levels for
the monthly mean relative humidities for January and July. There is a large difference
between them: in January the largest standard deviation is over land, especially in
northern Europe with low and high pressure systems and inversions; in July the standard
deviation is much lower over land as the vertical mixing is much larger.
Figure 6 shows a comparison of the relative humidity for Ž ve meteorologica l
stations in one grid point of the model. The comparison is made for the lowest level
980
G. MYHRE et al.
(a)
(b)
Figure 6. Observed and modelled relative humidity for (a) January and (b) July. Surface observation s from an
area near Oslo, Norway, and modelled values from the lowest level in the model are used. Modelled relative
humidities are from 06, 12, 18, and 00 UTC whereas the observation s are from 06, 12, and 18 UTC. For 00 UTC
averaged values of 18 and 06 UTC are used. Obs 1: Rygge (59.23± N, 10.47 ± E, 157 m), Obs 2: Blindern (59.57± N,
10.43 ± E, 94 m), Obs 3: Fornebu (59.54± N, 10.37 ± E, 10 m), Obs 4: Tryvasshøgda (59.59 ± N, 10.41 ± E, 528 m),
Obs 5: Dønski (59.54 ± N, 10.30 ± E, 59 m).
in the model and surface observations . The observations are within 0.3± of each other,
and mostly within 0.15± . The altitude above sea level of the stations varies from 10
to about 500 m. Results are shown for January and July 1998. Figure 6 shows two
aspects. First, the relative humidity from the model is generally in accordance with the
observations , taking into account the variations between the observations . In the Ž rst
part of January the relative humidity from the model is lower than the observations ,
whereas in the latter part it is in the upper range of the observations. The model has
less variability than the observations . In July the diurnal variation in relative humidity is
much larger than in January. The model has generally a smaller diurnal variation than the
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
981
observations. A large number of observations covering the whole grid with a horizontal
resolution of 50 km are necessary to validate the model results further. The second
aspect shown in Fig. 6 is an important point—the difference in the relative humidity
between the observations . The meteorological observing stations are well within the
model horizontal resolution of about 50 km, indicating that relative-humidit y variations
within model grid boxes of 50 £ 50 km can be signiŽ cant. However, these observations
are from a coastal region, and we expect smaller variations in relative humidity over
ocean and non-coastal land regions.
4.
R ADIATIVE FORCING
In this section results of radiative forcing calculations will be presented. The main
focus is on spatial (horizontal and vertical) and temporal averaging with the main
emphasis on impacts of relative humidity and clouds. A special consideration regarding
the variation of the solar zenith angle and surface albedo has been necesssary in the
study of the horizontal averaging. Several factors in uence the radiative forcing due to
sulphate in a nonlinear way, e.g. surface albedo, solar zenith angle, relative humidity, and
clouds (see Haywood and Shine 1997 and Boucher et al. 1997). In order to investigate
the effect of spatial resolution on relative humidity and clouds, the surface albedos (a
value of 0.2) and solar zenith angles are kept Ž xed in the calculations. If this had not been
done, some of the variation in forcing would have been due to the change in solar zenith
angle and surface albedo as the resolution changed, and this would have confused the
analysis. We have imposed a Ž xed diurnal cycle for the solar zenith angle at all latitudes
and seasons. The solar zenith angle has been given values according to the time of day
(85± , 60± , 40± , 60± , and 85± at 06, 09, 12, 15, and 18 UTC, respectively). Note that this
procedure has only been followed in the study of horizontal averaging; in all other cases,
the solar zenith angle and surface albedo are varied as described in section 2(b).
A 3 h time step is used in the radiative transfer calculations. We have performed two
sets of calculations, one for clear sky and one including clouds (keeping the total cloud
cover Ž xed in the averaging procedure), in both cases averaging the relative humidity
and sulphate concentrations to various horizontal resolutions. Two averaging procedures
have been used for relative humidity: one is based on averages of speciŽ c humidity and
temperature, and one uses simple averaging of relative humidity. It will be shown that
there are only small differences between the two approaches. We have chosen to present
most results adopting the simple average in relative humidity.
(a) Spatial resolution
Calculations for eight different horizontal resolutions are performed ranging from
representing the investigated area in all the 151 £ 133 CTM grid boxes to groups of
2 £ 2 and 4 £ 4 gridboxes, and so on, until Ž nally grouping the entire grid into one grid
box. For lower horizontal resolutions than the original grid, averaged values are used
(except for solar zenith angle and surface albedo in the case of horizontal averaging, see
above).
Figure 7 shows the radiative forcing due to sulphate for the different horizontal
resolutions for clear sky and when clouds are included in the calculations. The forcing
is strongly dependent on the horizontal resolution, which is almost entirely due to the
hygroscopic effect of the sulphate aerosols, as the radiative forcing for dry sulphate
shows little spatial variability. Generally, the patterns for clear sky and the calculations
including clouds are similar. However, for the highest horizontal resolutions the rate
of change of forcing with resolution in the radiative forcing is slightly stronger in the
982
G. MYHRE et al.
Figure 7. Radiative forcing due to sulphate (W m¡2 ) as a function of horizontal resolution . Radiative transfer
calculations are performed for horizonta l resolution s (km) marked with C. ‘Clear’ indicates that clouds are
excluded in the calculations and ‘cloud’ indicates that clouds are included.
clear-sky case than in the case including clouds. This is mainly due to the fact that
the strongest nonlinear effect of the sulphate aerosols is for high relative humidities,
which are often in cloudy regions. For lower horizontal resolutions, averaging is over
larger spatial regions and the collocation of relative humidity and clouds is weaker,
consequently results for the clear-sky and cloudy cases are more similar.
The dependence of the radiative forcing on horizontal resolution is strongest for low
resolutions, but it is also signiŽ cant at higher resolutions indicating a need for further
investigation s with even greater horizontal resolution than used in this study. Typical
grid resolutions for models used in global estimates of the forcing due to sulphate are
about 2.5–10 degrees, indicating a 10–25% (15–30% for clear sky) lower forcing than
for the 50 km grid resolution used in the original version of our model.
In Table 2 results are shown for various horizontal resolutions and for the two
averaging procedures of relative humidity mentioned above. The difference between
the two procedures is small (within 1.5%) and is largest for the lowest horizontal
resolutions. For even lower horizontal resolutions (not shown) differences become
larger, but such resolutions are lower than have been used in previous 3-D studies of
radiative forcing due to sulphate aerosols.
Calculations with varying solar zenith angles and surface albedos are performed
yielding generally similar results. However, for the lowest resolution the results differ
slightly due to the nonlinear relationships of surface albedo and solar zenith angle to
the radiative forcing due to sulphate aerosols. In one experiment an averaged sulphate
concentration was used in the entire CTM grid. This leads to an even larger dependency
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
983
TABLE 2.
R ADIATIVE FO RCING DUE
¡2
TO SULPHATE (W M ) FOR DIFFER ENT HORIZONTAL RESOLUTIONS INCLUD ING CLOUDS AND FOR TWO AVERAGING
PROCEDURES FO R RELATIVE HUMIDITY:
ADOPTING SIMPLE AVERAGING OF RELA TIVE HUMIDITY (A PPROACH 1), AND US ING AVERAGES OF SPEC IFI C HUMIDITY
AND TEMPERATURE (A PPROACH 2)
Resolution
50 km
150 km
300 km
500 km
Approach 1
Approach 2
¡0.75
¡0.71
¡0.67
¡0.61
¡0.75
¡0.71 (0.2)
¡0.68 (0.7)
¡0.62 (1.5)
Values in parentheses are percentage differences between the two averaging approaches.
TABLE 3. R ADIATIVE FO RCING DUE TO SULPHATE
(W M ¡2 ) FOR DIFFERENT VERTICAL RESOLUTIONS
( FOR THE 0–2 KM ALTITUDE REGION ) UNDER CLEAR SKY CONDITIONS
Cases
Reference calculation s (8 layers)
4 layers
2 layers
1 layer
Radiative forcing
¡1.05
¡1.00 (¡5)
¡0.93 (¡11)
¡0.84 (¡20)
Values in parenthese s are percentag e deviation s from the
reference calculations .
on the horizontal resolution, as the forcing for the lowest horizontal resolution was
unchanged and for the highest horizontal resolution it was strengthened. This is because
the sulphate concentrations are higher over land regions with generally lower relative
humidities than over ocean.
In the above discussion horizontal resolution for radiative forcing due to sulphate is
investigated. In Table 3 calculations of the sensitivity to vertical resolution are shown.
The eight lowest layers are below about 2 km where the relative humidity and sulphate
are highest. We have performed three sensitivity experiments with these eight layers
reduced to four, two, and one layer(s), respectively, by simple averaging of relative
humidities and sulphate concentrations . Results in Table 3 are only shown for a clear
sky, and show an underestimatio n of the magnitude of the radiative forcing of about
10% for around 1 km vertical resolution , and 20% when representing the eight layers
by one layer. Including clouds in such calculations is problematic. However, we would
expect that this would, as in the other calculations shown in this paper, reduce the effect
of decreasing the spatial resolution.
(b)
Temporal resolution and consistency between sulphate and meteorological data
Table 4 shows results for daily averaged and monthly averaged inputs compared
to the reference calculations with 3 h resolution for sulphate, relative humidities and
clouds. The radiative forcing based on monthly averaged data is more than 25% weaker
than that based on the 3 h data, indicating that studies using monthly mean input data
have signiŽ cantly underestimate d the magnitude of the forcing. The underestimatio n
using monthly mean data for the relative humidity is similar to the underestimatio n due
984
G. MYHRE et al.
TABLE 4. R ADIATIVE FORC ING DUE TO SU LPHATE
(W M ¡2 ) FO R DIFFERENT TEMPORA L RESOLUTIONS
FOR CLOUDY CONDITIONS
Cases
Reference calculations
Daily average
Monthly average
Monthly average sulphate only
Radiative forcing
¡0.63
¡0.54 (¡14)
¡0.46 (¡26)
¡0.63 (¡0)
Values in parenthese s are percentag e deviation s from
reference calculations .
to low horizontal resolution used in global models compared to the 50 km resolution
used here. The forcing based on daily averaged data is also 14% lower than the 3 h
data, indicating that daily variations with even higher temporal resolution than used
here may be of importance. The difference between the two averaging procedures of
relative humidity has also been tested for temporal resolution. Also in this case it is
small, namely about 1%.
A set of calculations is performed which includes both spatial and temporal averaging to see to what extent these two effects are additive. Above we showed that for
a resolution of about 300 km the magnitude of the radiative forcing is underestimated
by 11% for cloudy condition and by 14% for clear sky, compared to that for 50 km
resolution. When monthly averaging of relative humidities is also included, the magnitude of the radiative forcing is underestimated by 24% and 33% for cloudy and clear-sky
conditions, respectively. These Ž gures are rather close to the case in which only monthly
averaging is included. Note that in the monthly average calculations realistic surface
albedo and solar zenith angles were used, whereas in the calculation with different horizontal resolutions Ž xed surface albedo and a Ž xed diurnal variation in the solar zenith
angle were used (as noted above). As shown in Table 4, use of monthly average data
resulted in a 25% underestimatio n compared to 3 h data. An additional calculation using
monthly mean data applying surface albedo and solar zenith angles values as in the case
of varying horizontal resolution led to a slightly smaller underestimatio n (21%). Finally,
an experiment was performed at 300 km horizontal resolution using monthly mean data,
in which vertical averaging was performed (the eight lowest layers were averaged to two
layers). This reduced the clear-sky radiative forcing by only 1%. From these calculations
we conclude that the underestimatio n of the magnitude of the radiative forcing due to
averaging over various spatial and temporal scales is clearly not additive.
Some global estimates of the radiative forcing due to sulphate have been performed
with relative humidity updated several times each day, but with monthly mean data
for sulphate from a CTM. Table 4 shows calculations where the results of such a
simpliŽ cation has been investigated. The forcing in the case with monthly averaged
sulphate data is very similar to that in the reference case. This Ž nding indicates, in
agreement with Koch et al. (1999), that consistency between the meteorological data (in
particular clouds and relative humidity) and sulphate for larger regions is not of large
importance, and that inconsistencie s at this point do not introduce major uncertaintie s
in the radiative forcing due to sulphate.
(c) Regional forcing
Figure 8 shows the geographical distributio n of the annual mean radiative forcing
due to the sulphate aerosols for clear sky and when clouds are included, adopting the
985
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
(a)
Wm-2
0.0
-1.0
-2.0
-3.0
-4.0
-5.0
-6.0
(b)
Wm-2
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
Figure 8. Annual mean radiative forcing due to sulphate (W m¡2 ) adopting the highest spatial and temporal
resolution : (a) clear sky, (b) includes clouds. Note the differenc e in the colour scales between panels.
986
G. MYHRE et al.
highest spatial and temporal resolution . The forcing is strong over the ocean in coastal
regions. This is due to the combination of high relative humidity and sulphate burden
and low surface albedo over the ocean. Over land areas over Europe the radiative forcing
due to sulphate aerosols is around ¡0.5 W m¡2 . In some previous global calculations
of the forcing due to sulphate (Haywood and Ramaswamy 1998; Penner et al. 1998;
Myhre et al. 1998) the forcing is stronger over the European continent than over ocean,
whereas in our study it is the reverse which was also found by Haywood et al. (1997b)
and Feichter et al. (1997). The main reason for the distributio n of forcing over ocean and
land is the Ž ner horizontal resolution in this model, which better captures the variation
in relative humidity and surface albedo. A somewhat weaker forcing in general in this
study is a result of the much lower SO2 emission in 1998 than in 1985, the year for
which most previous CTM studies calculated the sulphate concentration.
Clouds reduce the averaged radiative forcing over the region from ¡1.05 W m¡2
for clear sky to ¡0.63 W m¡2 . In particular the forcing is reduced by clouds over ocean.
A calculation for dry sulphate aerosols shows weaker radiative forcing due to sulphate.
The maximum forcing is then over land regions and re ects much more the distributio n
of the sulphate burden than the forcing shown in Fig. 8.
5.
D ISCUSSION AND CONCLUSIONS
In agreement with a few previous investigation s we have found that spatial and
temporal resolution is important for determining the radiative forcing due to sulphate.
Based on our calculations for a regional area at midlatitudes and high latitudes in the
northern hemisphere, we conclude that earlier global calculations of radiative forcing
may have underestimated the magnitude by over 30–40% due to coarse spatial resolution
(horizontal and vertical), and up to 25% due to temporal resolution. However, note
that simultaneous spatial and temporal averaging are not additive, as the high relative
humidities which play the most important role for the forcing are reduced in a similar
manner by spatial and temporal averaging. In other regions the magnitude of the
underestimatio n due to coarse spatial and temporal resolution may deviate from what
is found here. Important factors in uencing the magnitude of the underestimatio n of the
radiative forcing due to sulphate are the relative humidity and its spatial and temporal
variation.
For future global calculations the gain in accuracy will probably be larger from
improving the spatial resolution than increasing the temporal resolution. Most global
studies today are based on GCMs with relative humidities that are updated many
times each day. However, the spatial resolution used in global models is substantiall y
coarser than in the model presented in this study, which is shown here to signiŽ cantly
underestimate the magnitude of the radiative forcing due to sulphate. Therefore, based
on our results, present GCM calculations of the radiative forcing due to sulphate clearly
underestimate the magnitude of the forcing more due to the coarse spatial resolution than
temporal resolution. This study indicates that it may be important to represent variations
in relative humidity with even better resolution than used in this study. The observations
in Fig. 6 show that large variations exist in relative humidities even within the spatial
resolution used in this study. Subgrid-scale parametrizations in global models, and
further studies with high-resolutio n regional models, should be used to investigate the
effect of relative humidity on the radiative forcing due to sulphate aerosols. Earlier
global studies on the radiative impact of sulphate aerosols differ with respect to various
model assumptions. From this study it is found that different spatial and temporal
RADIATIVE FORCING DUE TO SULPHATE AEROSOLS
987
resolutions introduce much larger uncertainty than the degree of consistency between
the meteorological data and the sulphate aerosols.
The present study has focused on sulphate aerosols, but the spatial and temporal
resolution is important for all hygroscopic aerosols. Among the most important of such
aerosols other than sulphate are organic carbon, nitrates and sea salt. Black carbon,
which is a strongly absorbing aerosol, is to a small extent hygroscopic. However,
for strongly absorbing aerosols the effect of relative humidity is probably smaller, as
the extinction coefŽ cient and the single scattering albedo will increase affecting the
radiative forcing in opposite directions. The optical properties of mineral dust are found
to be weakly in uenced by the relative humidity (Li-Jones et al. 1998). Further, aerosols
are often a mixture of the above mentioned components. Internal mixing of absorbing
and scattering aerosols increases the effect of the absorbing aerosols compared to
external mixing (Haywood et al. 1997b; Myhre et al. 1998). Also in such cases the
spatial and temporal variations in relative humidity are important, as the water will
increase the single scattering albedo. In some regions this will even be important for
the sign of the direct aerosol radiative forcing.
A CKNOWLEDGEMENTS
The relative-humidit y data from meteorological stations were provided by the
Norwegian Meteorological Institute. This work has received support from the Research
Council of Norway under the ozone and climate program, including a grant through
RegClim. We are thankful for the technical support from Gunnar Wollan. We also thank
two reviewers for helpful comments.
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