Characterisation of convective regimes over the British Isles

Abstract

Convection-permitting modelling has led to a step change in forecasting convective events. However, convection occurs within different regimes which exhibit different forecast behaviour. A convective adjustment timescale can be used to distinguish between these regimes and examine their associated predictability. The convective adjustment timescale is calculated from radiosonde ascents and found to be consistent with that derived from convection-permitting model forecasts. The model-derived convective adjustment timescale is then examined for three summers in the British Isles to determine characteristics of the convective regimes for this maritime region. Convection in the British Isles is predominantly in convective quasi-equilibrium, with 85% of convection having a timescale less than or equal to 3 h. This percentage varies spatially with more non-equilibrium events occurring in the south and southwest. The convective adjustment timescale exhibits a diurnal cycle over land. The non-equilibrium regime occurs more frequently at mid-range wind speeds and with winds from southerly to westerly sectors. Most non-equilibrium convective events in the British Isles are initiated near large coastal orographic gradients or on the European continent. Thus, the convective adjustment timescale is greatest when the location being examined is immediately downstream of large orographic gradients and decreases with distance from the convective initiation region. The dominance of convective quasi-equilibrium conditions over the British Isles argues for the use of large-member ensembles in probabilistic forecasts for this region.

Synoptic‐Scale Precursors of Extreme U.K. Summer 3‐Hourly Rainfall

“…Santos et al () used weather types to investigate flash flooding across three river basins in Portugal, although the study was not limited to the summer, while Barbero et al () found that disturbances associated with the jet stream and cutoff upper‐level lows were related to the occurrence of 1‐hr annual maximum precipitation across much of the United States. Summer flash flood studies in the United Kingdom have generally focused either on individual case studies or convective‐scale processes (e.g., Flack et al, ; Golding et al, ; Warren et al, ). Although intense U.K. hourly rainfall has been shown to increase with temperature at a rate approximating CC scaling (Blenkinsop et al, ; Chan et al, ) with some evidence of the sensitivity of this scaling to the prevailing synoptic circulation (Blenkinsop et al, ), to date there has been no detailed examination of the synoptic‐scale drivers of intense U.K. subdaily rainfall.…”

Recommendations for Improving Integration in National End-to-End Flood Forecasting Systems: An Overview of the FFIR (Flooding From Intense Rainfall) Programme

“…On the other hand some events show less locational predictability with storms occurring in very different places between the different ensemble members [57]*. For 85% of the cases examined the meteorological conditions are close to convective quasi-equilibrium which means that there is more likely to be relatively large uncertainty in the position of the rainfall events (e.g., [57,58])*. This locational uncertainty has impacts for identifying areas most at risk as the potential area at risk could cover large urban areas, with one area being more susceptible than another area, or multiple catchments.…”

Improvements in Forecasting Intense Rainfall: Results from the FRANC (Forecasting Rainfall Exploiting New Data Assimilation Techniques and Novel Observations of Convection) Project

“…Convective quasi-equilibrium occurs when instability production in the atmosphere on synoptic scales is approximately balanced by convection [67]. FRANC research showed that this type of convection accounts for ∼85% of UK summer convection cases [68]*. Non-equilibrium convection occurs when atmospheric instability is inhibited from release, and so instability continues to build until the inhibiting factor (such as an inversion or "lid") can be overcome (see Emanuel [69], Chapter 11.2, for comparison between equilibrium and non-equilibrium and compare Figure 5a,b).…”