VISURB-UK

Jim Haywood

To improve forecasts of visibility in urban areas

Visibility is an NWP Index

The customer (e.g. MOD) is interested in three areas:-

• Fog• Cloud• Clear sky

It is this third area that we plan to investigate during VISURB

The principles of visibility prediction:

• In cloud-free areas the visibility is proportional to the aerosol extinction, Qext

• But Qext is non-linearly proportional to the relative humidity (aerosols are hygroscopic)

This shows the growth curve for secondary organic aerosols, but modelling/measurement work shows similar results for sulphate, nitrate etc.

VisibilityVisibility=-lnQtot

is the liminal contrast = 0.02 in the mesoscale model

Qtot=Qair+Qext(RH,Maerosol)

Qair= is negligible, and max(visibility) = 100km in the mesoscale model

This is the ‘aerosol’ mass concentration field from the mesoscale model. It is driven by climatological emissions of aerosol.

Industrial areas are clearly discernible.

This is the ‘visibility’ which is clearly not a 1:1 anti-correlation to the aerosol mass concentration as it also depends upon the relative humidity.

Some areas are reasonably well anti-correlated though e.g. the aerosol plume south of Norway.

VISURB objectives:

• Characterisation of aerosol size distributions.

• Characterisation of chemical composition (AMS).

• Characterisation of hygroscopic growth factors (wet-neph).

• Effect of aerosols upon the radiation budget.

VISURB flight planning:

• Head to oceanic areas with large concentrations of aerosol (predicted by the mesoscale model).

• Work in plumes, most likely in the North Sea.• Essential equipment:a) Nephelometer/wet-nephelometerb) PCASPc) AMSd) SWS/SHIMS/BBRs if clear skies existe) Core chemistry (particularly CO for plume

detection).

Transit

Operations at 500ft

Concentrated efforts once plume had been detected.

Where I’d imagine we’d be working …..