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Severe storms around the UK: Project DIAMET

Logo of the DIAMET project

April 2012 - Heavy rain and strong winds are familiar parts of the weather scene in north-west Europe. Hence, they are also high priorities for improvements in the Unified Model.

Whilst there have been significant improvements in the forecasting of storms in recent years, there is a recognition that the most severe conditions can still be highly localised and more difficult to forecast than the main storm system itself.

Of particular interest are the ways in which localised heating and cooling processes that are termed "diabatic" can affect the mesoscale atmospheric dynamics. Here, "mesoscale" refers to phenomena that have horizontal scales typically in the range from a few to around a hundred kilometres. DIAMET was established to study DIAbatic influence on Mesoscale structures in Extra-Tropical storms.

DIAMET is a consortium funded partly by the Natural Environment Research Council (NERC) through groups from University of Manchester, University of Leeds, University of Reading and University of East Anglia and also including the Met Office as a project partner. Its main aim is to conduct studies in storm systems approaching the UK and as such, forms a contribution towards the World Meteorological Organization (WMO) THORPEX programme.

One example of the types of weather phenomena to be studied is the so-called "sting-jet". This is thought to be caused when ice cloud interacts with a descending stream of dry air to the rear of a mid-latitude cyclonic storm. Evaporation of the cloud causes additional cooling of the descending airstream, which in turn increases its momentum and generates stronger surface winds as the air reaches the ground. This is now thought to be the cause of the damaging winds over southern England in the Great Storm of October 1987.

The project is organised into three work areas. The first looks at real mesoscale structures in the atmosphere, using high-resolution in-situ and radar measurements to derive their morphology and dynamics. It makes use of the concept of potential vorticity (PV) which can be thought of as a product of the absolute rotation of the atmosphere and its thermal stratification (or static stability). It is clear that because diabatic processes alter the thermal stratification of the atmosphere, they also have an impact on PV.

A key task is to calculate the production of PV by different diabatic processes, especially those involving phase changes of water such as the evaporation of water from the ocean surface, condensation to form liquid cloud, freezing of cloud water to form ice and the evaporation of both liquid and ice precipitation in dry air. This work makes use of sophisticated diagnostics from high-resolution runs of the Unified Model (UM). These PV changes are compared with estimates derived from the measurements. Further studies of diagnostics from the UM will reveal the sensitivity of the forecasts to the correct representation of each of these processes and the dynamical consequences of diabatically-generated PV, both on the mesoscale and larger scales.

The MODIS visible image at 1226Z on 8 December 2011 overlaid with the flight track of the FAAM aircraft (Tim Baker, Leeds).

The second work area examines particular physical processes and the way these are represented in forecast models. Convection is only just beginning to be resolvable by high-resolution local-area forecast models and cannot be explicitly represented in current regional or global models. Hence, it needs to be parametrized, i.e. represented in terms of atmospheric properties that are resolved on the model grid scale. The schemes that are used are not optimised for mid-latitude storms, where convection often initiates at altitude rather than at the Earth's surface. A combination of novel diagnostics and new (or modified) schemes aimed at improving the representation of convection will be developed in this work area. Also addressed here will be the derivation of air-sea fluxes of heat and momentum from aircraft flights, and their use to derive a better parametrization for these fluxes in high-wind conditions. Lastly, microphysical measurements made within the clouds will be used to derive latent heating/cooling rates as a function of the microphysical environment and to improve microphysical parametrizations in the UM.

The final work area addresses the problem of predictability, using a combination of ensemble forecasting and data assimilation techniques. A unique archive of ensemble forecasts produced at the Met Office will be exploited to determine how well the forecasts actually generate realistic mesoscale features, using standard measures of skill. Model errors in representing convection, air-sea fluxes and microphysics will be investigated to determine their impact on the forecasts for different flow conditions. The relationships amongst different model variables on the mesoscale is poorly known at present and this will be investigated using ensembles and the results of the measurement programme. Finally, novel approaches to data assimilation will be investigated through a student project.

The 2011 observing campaign

A key observing platform for DIAMET is the Facility for Airborne Atmospheric Measurement (FAAM) BAe146 research aircraft. This has now completed two flight campaigns towards DIAMET, the first in September 2011 and the second in November and December 2011. The aircraft has a range of capabilities that can contribute to the project aims. It can launch dropsondes - small instrument packages that descend on a drogue measuring temperature, relative humidity, wind speed and wind direction. Closely-spaced dropsondes can identify mesoscale structures in the atmosphere and the data are relayed in real time via satellite networks to the Met Office, where they can be assimilated into the operational forecast runs of the UM. The aircraft can also make in-situ measurements of liquid- and ice-phase cloud water to identify similar mesoscale structures. Finally, it can also descend to altitudes of less than 250 feet (75 metres) over the ocean to measure the near-surface turbulent heat and water vapour fluxes using a combination of fast-response instruments.View from the aircraft cabin on 8th Dec 2011 as the aircraft crosses the frontal cloud band (Peter Knippertz, Leeds).

In a display of good fortune that does not always accompany observational field campaigns that are planned many months or years in advance, both flying periods so far have been able to encounter a range of events of interest to the overall project aims. In particular, the FAAM aircraft flew on 8th December, when a violent cyclonic storm caused widespread damage and disruption across large parts of Scotland. During the morning, it flew north from Exeter, west of Scotland towards Stornoway, launching around 20 dropsondes to document the storm structure in this region. After re-fuelling at Teesside airport, it conducted further measurements within the cloud bands of the storm as it passed over eastern Scotland.

Analysis of data from all the flights is now underway and further flights are planned to examine summer-season storms during 2012.

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