Proceedings - C-SRNWP Project

The Murk aerosol is based on the urban-industrial aerosol type (61% water soluble aerosols
e.g. sulphuric acid, sulphates, 22% soot (carbon) and 17% dust (quartz, silicaceous clays)
What problems are there with Murk?
• Large values of MURK (“dust storms”) at the beginning of the forecast
These are due to problems with the assimilation of visibility observations
(Figure 6)
• Unrealistically large values of MURK at later forecast ranges due to the “relaxation
term”
• Out of date source data again giving unrealistically large MURK values (Figures 11
and 12)
Figure 6: Unrealistically large analysis increment values to Murk aerosol
log10 vis is a very non-linear function of RH (Figure 7) and this causes data assimilation
problems. Low visibility obs in the U.K are mostly associated with large RH values (>90%).
However if the model background RH is ~60%-70%, the analysis will try and give a large
increment to MURK (as changing RH will have little impact on the height of the surface).
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