Sensitivities of a Squall Line over Central Europe in a Convective-Scale Ensemble

Abstract

Convective-scale ensemble simulations with perturbed initial and lateral boundary conditions are performed to investigate the mechanisms and sensitivities of a central European convection event from the Convective and Orographically Induced Precipitation Study (COPS). In this event, a ''primary'' squall line developed ahead of a decaying mesoscale convective system (MCS) upstream of the Vosges Mountains (France), weakened over the Rhine valley, then regenerated as a ''secondary'' squall line over the Black Forest Mountains (Germany). All ensemble members captured the squall-line evolution, but most suffered from a delay in the onset of convection and positional errors of 50-150 km over the COPS region. These errors in the secondary initiation were linked to errors in the primary initiation. Detailed analysis revealed a similar primary initiation mechanism in all members: in the ascending branch of a midlevel frontal circulation ahead of the MCS, convection initiated within a mesoscale moisture anomaly embedded within the prefrontal flow. The differences in the skill of the ensemble members were related to subtle differences in their initial upper-level representation of potential vorticity (PV). Members that verified well possessed a stronger PV gradient at the leading edge of an upper-level trough. This led to more rapid cyclogenesis over northern France and the United Kingdom and faster development and propagation of the midlevel front and the prefrontal moisture anomaly. As a consequence, the squall lines in these members developed earlier and closer to the COPS region. This case study provides an example of the subtle mechanisms by which errors on the larger scales may transfer to the convective scale and lead to errors in quantitative precipitation forecasts.

Ensemble Sensitivity Analysis Applied to a Southern Plains Convective Event

“…Sensitivity fields were investigated with regard to the driving synoptic flow aloft as well as the mesoscale features closer to the surface that might be important to the dynamics of the convective event. Like with Hanley et al (2013), the variability in the convection forecast was found to be related to positional shifts in synoptic-scale features both at upper and lower levels of the troposphere. A faster upper-level low would be consistent with faster progression of low-level features.…”

“…Each of these studies has shown the value ESA provides in revealing the atmospheric flow features important to the predictability of the specific phenomena they address. One study that has examined convective-scale ensemble forecast sensitivity is Hanley et al (2013), which used a precipitation-based response function within a 4-km grid to examine the role of the PV field aloft 12 h prior. It was found that ESA revealed an area at the edge of a PV anomaly aloft, as well as more interior PV magnitudes, which signaled the importance of the precise nature of the upper-level PV field to the predictability of squall-line convection later in time.…”

Extreme rainfall sensitivity in convective‐scale ensemble modelling over Singapore

“…Despite the potential benefits, little research has been done with convection‐permitting ensembles to understand and predict the occurrence of the squalls in tropical regions, compared to the midlatitude severe convective systems (Hanley et al ., ; Bednarczyk and Ancell, ).…”

Identification of key factors in future changes in precipitation extremes over Japan using ensemble simulations

“…For example, Hanley et al (2013) conducted convective-scale ensemble simulations to examine the mechanisms of a convective event in central Europe. They demonstrated that ensemble members that better represented upper-level potential vorticity were in closer agreement with observations in terms of the development of squall lines in the convective event.…”