The uk Lidar-sunphotometer operational volcanic ash monitoring network

The Met Office completed the deployment of ten lidars (UV Raman and depolarization), each accompanied by a sunphotometer (polarized model), to provide quantitative monitoring of volcanic ash over UK for VAAC London. The lidars provide range corrected signal and volume depolarization ratio in near-real time. The sunphotometers deliver aerosol optical depth, Angstrom exponent and degree of linear polarization. Case study analyses of Saharan dust events (as a proxy for volcanic ash) are presented.

burning event at the end of May 2016; and a volcanic ash event early January 2017 interestingly also associated with buoyancy internal gravity waves.

Biomass burning pollution event
Smoke layers of Canadian forest fires were observed by Lidar systems over centre-Southern UK between 25 th and 31 st of May 2016 at Exeter, East Malling and Watnall and between 23 rd and 31 st of May at Camborne.Figure 2 shows RCS and VDR collected at Camborne on May 24 th , 2016.Note that VDR is shown in log10 scale to emphasize the depolarization layers.
The pollution event was observed in all four locations at high altitude (between ~5 km and ~10 km).On several occasions, the pollution layers coexisted along the Ci clouds.In the UV region the molecular component of the backscatter signal is important and thus, weak aerosol pollution layers are not always clearly observed in RCS (Fig. 2, upper plot).However, the signature of the non-spherical particles can be seen in VDR (Fig. 2, middle plot).High AOD were also observed (up to 0.45), especially between 15:00 and 18:00.In this period AE increases (from 0.5 to 0.9), which indicates the relevance of small particles.Consequently, it looks like the contribution of the thin pollution layers is insignificant.On May 26, the pollution layers can still be seen at 8-10km at all four locations (Figs.3-6).No sunphotometer data was available at East Malling and Watnall.For Camborne and Exeter, high AOD values (up to ~0.5) can still be seen but most probably they are due to PBL particles contribution.The full analysis of the particles optical properties will be shown during conference.

Buoyancy internal gravity waves
A weak pollution event took place over central UK towards the end of December 2016 and early January 2017.Preliminary examination suggests that the layer originates from western Alaska and it could have as source the eruption from Bogoslov volcano during December [6].Continuous measurements were taken at three locations including Glasgow Bishopton in Scotland.On January 3 rd , a pollution layer is seen descending towards the planetary boundary layer (PBL).On January 4 th and 5 th , a very thin depolarizing layer is noticeable at the top of PBL.
The assumption is that this depolarizing layer originates from the descendent pollution layer.Figures 7 and 8 show RCS and VDR zoomed over first two km above the ground for January 4 and January 5.The presence of LLC (see Figs 7-8) as  well as of Ci clouds (from ~15:00UTC on 04/01 until ~07:00UTC on 05/01) and middle clouds (after ~12:00UTC on 05/01) limited the retrieval of AOD.Sporadic AOD values show small values (below 0.1 for 340nm).The Met Office NRT for AOD and AE for these days are not in agreement with AERONET [4], for reasons currently unknown.This is under investigation and an update will be given during IRLC.
Figure 9 shows the MSG satellite imagery over Scotland, which confirms the presence of the gravity waves.

CONCLUSIONS
An operational network of ten lidars and sunphotometers was successfully set up over the UK by the Met Office in 2016 to monitor volcanic ash.NRT plots with RCS and VDR are used by the VAAC London, along with AOD and AE from sunphotometers.The algorithms to retrieve the particle optical properties are under development Detailed analysis of the events described here, including the aerosol particles optical properties and optical and microphysical properties as retrieved by AERONET, will be presented at ILRC.