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Title: Evaluating mineral dust aerosol retrieval and its direct radiative effect with a view towards improving forecasts in the UK Met Office NWP model
Author: Ansell, Catherine
ISNI:       0000 0004 2741 9899
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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The UKMO has tested the effect of assimilation of aerosol optical depth (AOD) observations from the SEVIRI retrieval algorithm which has been developed at Imperial College and this has been shown to add skill to its short term forecasting ability. This thesis details the validation of the SEVIRI AOD retrieval algorithm prior to a longer assimilation trial and considers both internal and external factors which may affect its success using data from the GERBILS airborne field campaign during June 2007. It was found that of the factors tested, the retrieved AODs were most sensitive to the coarse mode in the size distribution assumed in the radiative transfer modelling used to derive the expected dust signal. Following on from this work, co-located SEVIRI retrieved AODs and GERB fluxes at the top of atmosphere (TOA) were used to provide, for the first time, an observationally based estimate of the net direct radiative effect (DRE) of mineral dust over North Africa from geostationary satellite observations, giving new insight into the influence of time of day on the SW, LW and net effects. As expected, mineral dust aerosol was found to reduce the outgoing longwave radiation at all times of day with the peak reduction clearly following the diurnal cycle of surface temperature. Instantaneous SW DRE was found to be large under certain conditions and showed strong dependencies on pristine sky albedo and solar zenith angle such that a given aerosol loading could induce a positive or negative SW DRE. Overall, the mean SW DRE over June 2007 was found to be negligible. The net DRE for June 2007 was hence found to be dominated by the LW component with mineral dust inducing a reduction in outgoing net flux of the order 10Wm-2. Besides being of interest in their own right, the methodology and results from this analysis should help to provide an important constraint on the success of the assimilation process in terms of assessing whether the assimilated aerosol has the 'correct' impact on the forecast radiative fields.
Supervisor: Toumi, Ralf ; Brindley, Helen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral