Heavy rainfall in Mediterranean cyclones. Part I: contribution of deep convection and warm conveyor belt

Abstract

In this study, we provide an insight to the role of deep convection (DC) and the warm conveyor belt (WCB) as leading processes to Mediterranean cyclones' heavy rainfall. To this end, we use reanalysis data, lighting and satellite observations to quantify the relative contribution of DC and the WCB to cyclone rainfall, as well as to analyse the spatial and temporal variability of these processes with respect to the cyclone centre and life cycle. Results for the period 2005-2015 show that the relationship between cyclone rainfall and intensity has high variability and demonstrate that even intense cyclones may produce low rainfall amounts. However, when considering rainfall averages for cyclone intensity bins, a linear relationship was found. We focus on the 500 most intense tracked cyclones (responsible for about 40-50% of the total 11-year Mediterranean rainfall) and distinguish between the ones producing high and low rainfall amounts. DC and the WCB are found to be the main cause of rainfall for the former (producing up to 70% of cyclone rainfall), while, for the latter, DC and the WCB play a secondary role (producing up to 50% of rainfall). Further analysis showed that rainfall due to DC tends to occur close to the cyclones' centre and to their eastern sides, while the WCBs tend to produce rainfall towards the northeast. In fact, about 30% of rainfall produced by DC overlaps with rainfall produced by WCBs but this represents only about 8% of rainfall produced by WCBs. This suggests that a considerable percentage of DC is associated with embedded convection in WCBs. Finally, DC was found to be able to produce higher rain rates than WCBs, exceeding 50 mm in 3-hourly accumulated rainfall compared to a maximum of the order of 40 mm for WCBs. Our results demonstrate in a climatological framework the relationship between cyclone intensity and processes that lead to heavy rainfall, one of the most prominent environmental risks in the Mediterranean. Therefore, we set perspectives for a deeper analysis of the favourable atmospheric conditions that yield high impact weather.

Vertical cloud structure of warm conveyor belts – a comparison and evaluation of ERA5 reanalysis, CloudSat and CALIPSO data

“…The distinction between mesoscale, slantwise ascent of the WCB and upright embedded convection had already been made in 1993 in a study on the mesoscale frontal structure of an explosively intensifying cyclone measured during the ERICA field experiment (Neiman et al, 1993). Embedded deep convection was also documented in a number of WCBs observed in the Mediterranean region (Flaounas et al, 2016;Flaounas et al, 2018) and during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) field campaign in the North Atlantic (Oertel et al, 2019;Blanchard et al, 2020). While slantwise WCB ascent leads to large-scale stratiform precipitation and the formation of widespread regions with low-PV air at upper levels, convective WCB ascent goes along with peaks of particularly strong surface precipitation and the formation of mesoscale upper-level PV dipoles, including regions with negative PV (Oertel et al, 2020).…”

Convective activity in an extratropical cyclone and its warm conveyor belt – a case‐study combining observations and a convection‐permitting model simulation

“…The WCB typically ascends poleward from the boundary layer to the upper troposphere ahead of the cyclone's cold front (Harrold, ; Browning, ; ) and plays a key role for the formation of clouds and precipitation in midlatitudes (e.g. Browning, ; Eckhardt et al ; Madonna et al ; Flaounas et al ).…”

“…Browning, ). However, recent remote‐sensing observations (Binder, ; Crespo and Posselt, ; Flaounas et al ; ) and high‐resolution simulations (Rasp et al ) have indicated that convective activity is commonly embedded in the WCB. This concept was already proposed by Neiman et al () as the escalator–elevator mechanism, which describes the alteration between gentle slantwise frontal ascent (the escalator ) and rapid vertical motion induced by convective activity (the elevator ).…”

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

“…Recent studies also corroborated the occurrence of convection embedded in the warm conveyor belt (WCB) airstream (Binder, 2016;Crespo and Posselt, 2016;Rasp et al, 2016;Flaounas et al, 2016Flaounas et al, , 2018Oertel et al, 2019Oertel et al, , 2020Blanchard et al, 2020) as originally proposed by Neiman et al (1993) in their so-called 'escalator-elevator' concept. In the classical perspective, the WCB (e.g.…”

“…Although recent case studies have corroborated the presence of embedded convection in WCBs (Rasp et al, 2016;Crespo and Posselt, 2016;Flaounas et al, 2016Flaounas et al, , 2018Oertel et al, 2019), several major issues are still uncertain: (i) Where and how often is convection generally embedded in the WCB ascent region? (ii) How variable is the strength of embedded convection in WCBs?…”