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| Titel |
Aerosol impacts in the Met Office global NWP model |
| VerfasserIn |
Jane P. Mulcahy, Malcolm E. Brooks, Sean F. Milton |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039665
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| Zusammenfassung |
| An accurate representation of the direct and indirect effect of aerosols is of growing concern
for global numerical weather prediction (NWP). Increased scattering and absorption of
incoming shortwave (SW) and outgoing longwave radiation (OLR) fields due to the presence
of aerosol layers in the atmosphere modifies the atmospheric heating profile and can affect
large-scale circulation patterns. The current representation of aerosols in the global
NWP configuration of the Met Office Unified ModelTM (MetUM) is based on a
simple aerosol climatology (Cusack et al., 1998). Profiles of water soluble dust, soot,
oceanic and stratospheric sulphate aerosols are described separately for land and
ocean surfaces and are distributed over the boundary layer, free troposphere and
stratosphere (sulphates only). While this improved the reflected SW radiative bias at the
top-of-atmosphere (TOA), there is evidence that the climatology is too absorbing
leading to a temperature bias in the lower troposphere of approximately 0.5 K/day.
Furthermore, the omission of the scattering and absorption properties of mineral dust and
biomass burning aerosol particles in particular, is believed to be the principal cause of
significant model biases (in the region of 50-56 W m-2) in both the model OLR at the
TOA (Haywood et al., 2005) and the surface SW radiation fields (Milton et al.,
2008). One of the objectives of the Global Aerosols (G-AER) component of the
MACC (Monitoring Atmospheric Composition and Climate) project is to evaluate the
impact of an improved aerosol representation on the performance of global NWP
models.
In a stepwise approach of increasing the aerosol complexity in the MetUM, the Cusack
climatology is being replaced by the CLASSIC (Coupled Large-scale Aerosol Simulator for
Studies in Climate) aerosol scheme, developed for the HadGEM (Hadley Centre Global
Environmental Model) climate model. CLASSIC includes representations of external
mixtures of sulphate, black carbon, organic carbon, biogenic, sea salt, biomass-burning,
mineral dust, and nitrate aerosol particles (Bellouin et al., 2007). As a first step, monthly
mean climatologies of these species are implemented in the model. Stage two involves
running the CLASSIC scheme in a prognostic mode where the aerosols are fully interactive
with the model meteorological and radiation fields. Here we present an evaluation of both
stages of the aerosol implementation procedure. An objective verification of the model output
fields is carried out against forecast analyses and a wide range of satellite and in situ
data. The model aerosol optical depth (AOD) is evaluated against ground-based
AERONET observations and satellite aerosol retrievals available through the MACC
project (e.g., MODIS, SEVIRI). The impacts on model performance, in terms of the
general circulation patterns and in addressing the model radiation biases will also be
presented.
References:
Bellouin, N. et al. (2007), Improved representation of aerosols for HadGEM2,
Tech. Note 73, Hadley Centre, Met Office, Exeter, U. K.
Cusack, S. et al. (1998), The radiative impact of a simple aerosol climatology on
the Hadley Centre atmospheric GCM, Q. J. R. Meteorol. Soc., 124, 2517-2526.
Haywood, J. M. et al. (2005), Can desert dust explain the outgoing longwave
radiation anomaly over the Sahara during July 2003, J. Geophys. Res. 110,
D05105, doi:10.1029/2004JD005232.
Milton, S. F. et al. (2008), Modeled and observed atmospheric radiation
balance during the West African dry season: Role of mineral dust, biomass
burning aerosol and surface albedo, J. Geophys. Res., 113, D00C02,
doi:10.1029/2007JD009741. |
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