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DIAbatic influences on Mesoscale structures in ExTratropical Storms

The focus of DIAMET was on the understanding and prediction of mesoscale structures in synoptic-scale storms. Such structures include fronts, rain bands, secondary cyclones, sting jets etc, and are important because much of the extreme weather we experience (e.g. strong winds, heavy rain) comes from such regions. Weather forecasting models are able to capture some of this activity correctly, but there is much still to learn. By a combination of measurements and modelling, mainly using the Met Office Unified Model (UM), we worked to better understand how mesoscale processes in cyclones give rise to severe weather and how they can be better represented in models and better forecast.

An overview of the project and some preliminary results were published in a BAMS paper (Vaughan et al 2015), and a complete list of publications from the project is provided through the Publications link.

The project was organised into three broad work packages. The first of these looked at real mesoscale structures in the atmosphere, using high-resolution in situ and radar measurements to derive their morphology and dynamics. The key to the latter was to calculate the production of potential vorticity by diabatic processes - especially phase changes of water (vapour/liquid/ice) and air-sea fluxes of sensible and latent heat. The associated high-resolution modelling programme used the UM to simulate a representative number of events, diagnosing the PV tendency in the model and comparing with the measurements. Sensitivity studies and further diagnostics with the model revealed the sensitivity of the forecasts to the correct representation of these processes and the dynamical consequences of diabatically-generated PV, both on the mesoscale and larger scales. Two student projects investigated the role of boundary-layer processes in storm behaviour and conducted a statistical investigation of mesoscale precipitation features, based on archived radar and wind profiler data.

radar pic_3-11-09_hr

The second WP examined particular physical processes and the way these are represented in forecast models. Convection cannot be explicitly represented in current large-scale models (it is just beginning to be resolvable by high-resolution local-area models) so it needs to be parameterised. 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 were developed in this WP. Also addressed here was the derivation of air-sea fluxes of heat and momentum from aircraft flights, and their use (as part of a larger, ongoing international project) to derive a better parameterisation for these quantities in high wind conditions. Lastly, microphysical measurements made with the FAAM aircraft were used to derive latent heating/cooling rates as a function of the microphysical environment and used to improve the model simulations in the first WP and to improve microphysical parameterisations in the UM

The final WP addressed the problem of predictability, using a combination of ensemble and data assimilation techniques. A unique archive of forecast ensembles produced at the Met Office was exploited to determine how well the forecast ensemble actually generated realistic mesoscale features, and the skill with which this was done (using standard measures of skill). Model errors in representing convection, air-sea fluxes and microphysics were investigated to determine their impact on the forecasts for different flow conditions. The relationship between different model variables on the mesoscale is poorly known at present and this was investigated using ensembles and the results of the measurement programme. Finally, novel approaches to data assimilation were investigated through a student project.

DIAMET objectives

The main objectives of the project fell into three areas:

Characterising the generation of mesoscale potential vorticity anomalies in cyclonic storms, and their implication for the larger-scale development of the storm and for the effects (e.g. high winds, heavy rain).

  • To characterize the generation of diabatically-generated PV associated with mesoscale structures within cyclonic storms, and to identify the related modelling sensitivities that may lead to errors in forecasts of such events.
  • To use airborne, ground-based and remote-sensing measurements, along with numerical model analyses and forecasts, to describe mesoscale structures in frontal cyclones impacting the UK.
  • To determine the effect of Boundary-layer processes on PV anomalies.
  • To determine the statistical distribution of mesoscale precipitation anomalies over the UK based on long-term weather radar and wind profiler data.
  • To determine the influence of specific PV anomalies on mesoscale development in cyclonic storms.

Physical processes and improving model parameterisations:

  • To determine whether there are structural aspects of existing convective parameterisations which hamper our ability to produce accurate forecasts of mesoscale features in extratropical storms, and provide guidance for improved future parameterisations.
  • To quantify the contribution of surface turbulent fluxes to storm development and their role in determining mesoscale PV structures.
  • To measure microphysical properties and variability in mesoscale structures, use these to derive latent heating profiles and improve microphysical parameterisations.

Predictability

  • To quantify the predictability of mesoscale features and the relationship between predictability of detailed weather events (precipitation and high surface winds) and mesoscale features.
  • To investigate and simulate sources of model error that most affect the forecast skill at convective scale. Determine sensitivity of forecast error to sampling error and compare with observations.
  • To use ensembles of forecasts to assess the changing nature of multivariate relations at high-resolution and to understand their sensitivity to different sources of error (initial and boundary condition error, and model error). This will help to design a new formulation of forecast error statistics that will work with operational data assimilation (DA) systems, e.g. at the Met Office.
  • To investigate next generation data-assimilation methods for the convective scale.

Summary of IOPs

IOP Date Flight t/o land Comments
      Z Z  
OPa Sep 11-13       Ex-hurricane Katia crosses UK. Aircraft not ready yet but we have radar data
  Sep-15 B646 12:10 13:57 Test flight to the Bristol Channel
IOP1 Sep-16 B647 08:43 13:23 Convective band ahead of U/L PV max. Dropsonde leg across the PV max over Ireland followed by 4 legs through convection E-W just N of Anglesey
IOP2 Sep-20 B648 07:56 11:14 Waves running along trailing front running SW over Cornwall. 
      12:49 16:52 Microphysics legs across cloud band at five levels. GB radar showing triple banded structure to precipitation.
OPb Sep-21       Upper level PV feature. Two sondes from Arran
IOP3 Sep-23 B650 08:29 11:57 WCB over  secondary WF running up trailing front west of Ireland. Dropsonde flight with some microphysics
      13:22 17:25 In situ through WCB followed by flux runs, west of Scotland.
           
Test Nov-22 B651 12:31 17:17 test flight over North Sea
IOP4 Nov-26 B652 11:00 16:07 Flux flight SW of Hebrides. Winds at 150' ~ 20 m/s
IOP5a Nov-28 B654 11:15 16:00 Dropsonde flight across double front out to 20 W
IOP5b Nov-29 B655 08:07 13:06 Combined dropsonde/insitu flight across cold front between land's End and Ireland
      14:08 16:11 Follow-on flight along radial to Chilbolton
IOP6 Dec-01 B656 07:00 12:24 Flight across bent-back warm front near Shetland plus fluxes in high winds
      14:06 15:31 Refuel at Inverness for flight home
Opc Dec-03       Two sondes from Camborne to study very narrow frontal band
IOP7 Dec-05 B657 09:12 12:20 In situ measurements of organised convection west of Scotland. Hoped for a polar low but didn't spin up
      14:43 16:00  
IOP8 Dec-08 B658 10:48 16:07 Strong windstorm over Scotland: Windstorm Friedhelm
      17:29 21:10  
IOP9 Dec-12 B662 12:25 17:15 Warm front approaching from the west. Flight profiling front along 51°18' N
Opd Dec-15       The non-Armageddon storm
IOP10 Apr-30       Mesoscale convective low: Chilbolton observations
IOP11a May-09 B694 12:33 17:09 Warm front in SW approaches.
IOP11b May-10 B695 08:51 14:00 Warm front over Scotland
      15:30 16:45 Transit Prestwick-Cranfield
IOP12 Jul-10 B712 10:00 14:16 Rainband north of upper-level PV anomaly
IOP13 Jul-18 B715 07:46 12:22 Bent back front over Scotland
IOP14 Aug-15 B728 14:00 18:30 Bent back front over Ireland