Abstract. Cold fronts provide an environment particularly favourable for convective initiation in the mid-latitudes and can also be associated with convective hazards such as wind, rain and hail. We build a climatology of cold-frontal convective cells between 2007–2016 for April–September in a cell-front distance framework by combining a radar-based cell detection and tracking dataset and automatic front detection methods applied to reanalysis data. We find that on average around twice as many cells develop on cold-frontal cell days compared to non-cold-frontal cell days. Using the 700 hPa level as a reference point we show the maximum cell frequency is 350–400 km ahead of the 700 hPa front which is marginally ahead of the mean surface front location. The 700 hPa front location marks the minimum cell frequency and a clear shift in regime between cells with a weakened diurnal cycle on the warm-side of the 700 hPa cold front and strongly diurnally driven cells on the cold-side of the 700 hPa front. High cell frequencies are found several hundreds of kilometres ahead of the surface front and cells in this region are most likely to be associated with mesocyclones, intense convective cores and lightning. These results are an important step towards a better understanding of cold-frontal convection climatology and links between cold fronts and convective hazards.
<p>Squall lines are mesoscale lines consisting of numerous convective cells with an along-line extent of at least 100 km and a smaller across-line extent on the order of tenths of kilometers. Typically, squall lines form in a sheared environment by interactions of convective cells that are near to each other. Over time, the convective cells form a continuous cold pool that drives the motion of the squall line.</p> <p>Initiating single convective cells in a sheared environment leads to the production of vertical vorticity due to tilting. Hence, these cells are connected to positive and negative vertical vorticity patches. While a squall line forms a band of positive and negative vorticity patches along the line, single cells will form a vorticity dipole.</p> <p>In this work, we investigate the interaction between squall lines and convective cells initiated in the vicinity of the squall line. This interaction can be understood as an interaction between different vortices. We study different configurations using (1) idealized simulations with the non-hydrostatic, convection-permitting Cloud Model 1 (CM1) and (2) a theoretical point vortex model representative of the vorticity ensembles.</p> <p>Squall line simulations with CM1 are initially run for 3 hours until a mature squall line forms. After 3 hours, convective cells are initiated at different positions near the squall line. The impact of the convection on the squall line is studied by an analysis of the squall line intensity and motion. In addition, the setup is studied using a point vortex model of a comparable vortex ensemble. A point vortex model is an idealized, mathematical model that describes vortex dynamics of a two-dimensional, non-divergent, inviscid atmosphere. Despite these strong limitations, it can be used to understand vortex dynamics and interactions in a relatively simple, but descriptive way. Moreover, the model describes well the vortex dynamics on the larger, synoptic scale. For example, a vortex couple of zero total circulation with the positive vortex south of the negative one starts to move towards the west which counteracts the typical westerly flow of the midlatitudes and, hence, leads to a slowed-down motion of the couplet compared to the westerlies.</p> <p>The general aim of this work is to better understand the interaction between single cells and convective lines. The results will contribute towards improved squall line forecasting.</p>
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<p>Cold fronts provide an environment favourable for convective initiation in the mid-latitudes. Some studies also show the presence of a cold front can increase the chance of certain convective hazards, such as hail and heavy rain. Convection initiates in three locations in respect to cold fronts: <em>ahead</em> of the cold front in the warm sector of the cyclone, directly <em>at</em> the cold frontal boundary and also <em>behind</em> the cold front. Previous literature has typically focused on each initiation location independently, thus a comprehensive study investigating the link between cold fronts and convection is currently lacking from literature. This study seeks to better understand the climatology, scale interactions and forcing mechanisms of convection at each initiation location relative to the front (i.e., behind, at, ahead).</p><p>Automatic front detection methods are applied to reanalysis data and a convective cell-tracking dataset from the German Weather Service is used to build a climatology of cold fronts and convection between April&#8211;September. Convective cells are found to initiate most commonly 200&#8211;300km ahead of the cold front during late afternoon. Cells behind the front primarily initiate in north-western Germany and exhibit a strong diurnal cycle. On the contrary, cells at and ahead of the front initiate most frequently in southern Germany and exhibit a less prominent diurnal cycle, especially for cells at the frontal boundary. Lightning probability decreases with closing proximity to the cold front and the average number of cell initiations per day is significantly higher on days with cold fronts opposed to days without. The next stages of research will investigate the relative importance of various forcing mechanisms on the development of convective cells at different cell-front positions.</p>
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