Exploring the impact of ice phase microphysical processes on mesoscale structures in mid-latitude cyclones using the Weather Research and Forecasting (WRF) model

In this paper we use the WRF model to assess the impact of uncertainties surrounding ice phase microphysical processes in the context of mid-latitude weather systems affecting the UK. We have identified several case studies from the recent DIAMET (DIAbatic influences on Mesoscale structures in ExtraTropical storms) field campaign that will be simulated using the WRF model. Collectively the chosen case studies represent a suitable range of meteorological conditions at varying times of year, and so provide a robust testbed for exploring ice phase sensitivities in terms of the impact on mesoscale weather. The case studies are as follows:

1. IOP2/B648/20^th September 2011: Mesoscale waves running along trailing cold front, generating a rainband with distinct banded structure;

2. IOP3/B650/23^rd September 2011: Rainband developing in diabatic Rossby wave beneath a warm conveyor belt;

3. IOP5/B655/29^th November 2011: Cold front undergoing ana to kata transition as the front passes over the UK mainland;

4. IOP13/B715/18^th July 2012: Stationary bent-back warm conveyor belt over Scotland, bringing flooding;

5. IOP14/B728/15^th August 2012: Bent-back front of strong summer cyclone over Ireland

The methodology is based on the Factorial Method, whereby we explore the uncertainty surrounding n number of microphysical variables (factors), with two possible values assigned to each variable, thus requiring 2ⁿ simulations for each case study. In this paper, we consider three variables, yielding a 2³ Factorial design, thus requiring 8 simulations per case study. The chosen factors are:

1. Snow fallspeeds;

2. Size threshold for cloud-ice to snow autoconversion;

3. The role of secondary ice production via the Hallet-Mossop process

Use of the Factorial Method enables us to quantify the effect of each factor individually, but it also enables us to explore the sensitivity of a given variable in the context of changes in other variables, i.e. non-linear interactions between factors. We perform the WRF simulations using microphysics schemes of varying levels of complexity, from those utilizing simple spherical ice assumptions, to more sophisticated schemes that use an adaptive habit parameterization for growth of pristine crystals.

The ice phase sensitivities are assessed in terms of the impacts on the potential vorticity field, and the intensity and spatial/temporal distribution of surface precipitation relative to Met Office rainfall radar data. We also include additional 3-D diagnostic output of the diabatic heating/cooling rates associated with specific microphysical processes (e.g. deposition/sublimation; freezing/melting; condensation/evaporation) to help identify those processes that have the biggest impact on the evolution of the potential vorticity field, and therefore the biggest impact on the morphology of mesoscale structures. Here we show some preliminary results from our WRF simulations that demonstrate the importance of ice phase microphysical processes when simulating mesoscale structures associated with mid-latitude cyclones. Ultimately our findings will help to identify an appropriate level of complexity for representing ice phase processes in simulations of mid-latitude weather, and thus help to improve forecast skill at the mesoscale.