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| Titel |
A Long-Term Study of the Aerosol Effects on Convective Clouds over Southern Sweden and Finland |
| VerfasserIn |
Moa Sporre, Erik Swietlicki, Paul Glantz, Markku Kulmala  |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053017
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| Zusammenfassung |
| MODIS (Moderate Resolution Imaging Spectrometer) cloud data have been combined with
ground based measurements of aerosol number size distributions from Vavihill (9 years) and
Hyytiälä (10 years) to investigate how convective clouds over Scandinavia are influenced by
the amount of aerosol particles present in the air.
Aerosol size distributions data from DMPS (Differential Mobility Particle Sizer)
instruments at Vavihill in Southern Sweden (56.01Ë N 13.9Ë E) and Hyytiälä in central
Finland (61.51Ë N 24.17Ë E) have been used to calculate the daily average particle number
concentrations over 80 nm (N80) and several other aerosol parameters. N80 has been utilized
as the main measure of the amount of CCN (Cloud Condensation Nuclei). Satellite images,
from the two MODIS instruments onboard the Terra and Aqua satellites, were investigated
for the days with available aerosol data from Vavihill and Hyytiälä. A smaller area
surrounding each station was examined and satellite scenes containing convective clouds
were selected. Level 1B data were used to calculate the cloud top temperature at a 1 by 1 km
pixel resolution and the cloud effective radius (re) data were obtained from the Level 2 cloud
product data. The approach developed by Rosenfeld & Lensky (1998) and Freud et al.
(2006) has been applied to find vertical re profiles of the clouds by plotting the re
against the cloud top temperature. Furthermore, Convective Available Potential
Energy (CAPE) data from ECMWF (European Centre for Medium-Range Forecasts)
has been used to examine how the instability of the atmosphere affects the clouds.
Finally, precipitation data from Hyytiälä and SMHI (Swedish Meteorological and
Hydrological Institute) for Vavihill have been investigated to find how the aerosols affect
precipitation.
The number of profiles included in the study is 569 for Vavihill and 474 for Hyytiälä. The
results show that the profiles of re are clearly affected by the aerosols since profiles with
higher aerosol number concentrations measured at the ground, have lower values of re. The
cloud base temperature (Tb) also has a strong effect on the re profiles, re decreases
with an increasing Tb. This is thought to occur since a higher Tb causes the ice
formation to occur higher up in the clouds resulting in smaller droplet sizes. The Tb and
N80 are positively correlated, (r equal to 0.37 and 0.68 for Vavihill and Hyytiälä,
respectively) so which one of the two that affect the profiles the most is hard to
determine.
The instability parameter CAPE does not affect the re profiles and we find no correlation
between any aerosol parameters and the amount of precipitation. There is however a weak,
but significant at a 99% confidence level, correlation between CAPE and the amount
of precipitation and also between the latter parameter and the re profiles vertical
extent.
A large study, including several hundred cases, of how aerosols affect convective clouds
at mid-latitudes has been performed using ground based aerosol measurements combined
with satellite data. The results show that aerosol number concentrations and cloud base
temperature are important for the cloud droplet sizes but that the atmospheric instability and
the vertical extent of the clouds control the amount of precipitation produced by the
clouds.
Freud, E., Strom, J., Rosenfeld, D., Tunved, P., & Swietlicki, E. (2006). Tellus B, Vol. 60,
No. 2, 286-297.
Rosenfeld, D. & Lensky, M. I. (1998). Bull. Amer. Meteo. Soc. 79(11), 2457-2476 |
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