Banded precipitation in extratropical cyclones over the UK and Ireland

Tianhang Zhang and David M. Schultz Centre for Atmospheric Science, Department of Earth and Environmental Sciences, University of Manchester Lifetime of rainbands Introduction Climatology (Preliminary Results) Left figure shows the duration of single rainbands. The significance of the project Left figure shows the annual trend of 63.8% of rainbands lasted for • Extratropical cyclones produce much of the precipitation in the midlatitudes. Within the synoptic- less than four hours, scale precipitation in extratropical cyclones, the organization of heaviest precipitation often takes the rainband number the form of precipitation bands on the mesoscale. suggesting that bands are • Precipitation bands produce extreme weather events such as heavy rain or snow and strong winds, over the UK and generally short-lived. which can disrupt society, causing damage, injuries, and sometimes death. Ireland from 2017 to • It is difficult to predict precipitation bands as there are many mesoscale and convective-scale 2021. structures. • What conditions organize banded precipitation remain an active area of research. There are 177 Future Work The steps of the project rainbands in the study period. Lift The ingredients of deep, moist Rainbands in the convection are well known (lift, instability, and moisture). Most cold season (Oct to observed precipitation bands in Mar) are more extratropical cyclones are frequent than those convective, suggesting that an Instability Moisture ingredients-based approach in warm season. could be applicable for banding.

Distribution of rainbands Lifetime of rainbands An ingredients-based methodology for Climatology (Data and Methods) convection Case studies 1. develop a database of surface cyclones over the eastern North : to investigate the change in ingredients in the Atlantic Ocean and Europe from April 2017 to March 2021: the Aim environment of banded precipitation. Data Source D W D - B o d e n a n a l y s e s u r f a c e - m a p a n a l y s i s a r c h i v e (http://www1.wetter3.de/). Data source 2. establish a database of precipitation bands over the UK and European Center for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data. ERA5 hourly data on pressure levels are available from 1979 to Ireland from April 2017 to March 2021: the Metcheck website present. The vertical coverage is from 1000 hPa to 1hPa, including 37 pressure (https://www.metcheck.com/WEATHER/archived_radar.asp). levels. The horizontal coverage is globe, with 0.25°×0.25° horizontal resolution. The criteria of a cyclone: Above figure shows time of rainbands. Questions: The blue line represents the total time of rainbands from 2017 This project will address • at least one closed isobar with fronts to 2021 per month; the orange represents the average • The change in the low-level frontogenesis • the minimum central pressure <1000 hPa lifetime of a rainband in each month. • The change in temperature advection • the lifetime of a cyclone >24 hours • The importance of instability (symmetrical, conditional, and In August, the mean duration of rainband was the The criteria for a rainband inertial instability • a contiguously linear or quasi-linear feature, longest, nearly twice as long as that in May • The change in moisture • an intensity > 2 mm h-1 (shortest). Although there was longest time of • Explain why these evolutions above occurred. • a length > 200 km, rainband occurrence in January, the mean duration • an aspect ratio (the ratio of length to width) > of each rainband was relatively short. Sensitivity experiments Case studies will be modeled using the Weather Research and 3:1, Rainbands last for a longer time in summer (JJA) Forecasting (WRF) model. Setting up sensitivity experiments is a • a duration > 2 hours. Above figures show the distribution of than in winter (DJF), although the frequency good way to investigate what factors influence banded There were 121 cyclones that created rainbands rainbands (a) in MAM; (b) in JJA; (c) in SOM; showed an opposite picture. precipitation. For example, a majority of rainbands formed along over the UK and Ireland. The rainband in the (d) in DJF from 2017 to 2021. Black solid lines most rainbands formed near the coastal regions, the coastal regions in the UK and Ireland. This is a question that circle in right figure is an example of a rainband represent axes of rainbands when they first appeared over the UK and Ireland. whereas a few were present in inland areas can be going into sensitivity experiments to investigate coastal that meets above criteria. and orographic effects on rainbands.