How an uncertain short-wave perturbation on the North Atlantic wave guide affects the forecast of an intense Mediterranean cyclone (Medicane Zorbas)

Abstract

Abstract. Mediterranean cyclogenesis is known to be frequently linked to ridge building over the North Atlantic and subsequent anticyclonic Rossby wave breaking over Europe. But understanding of how this linkage affects the medium-range forecast uncertainty of Mediterranean cyclones is limited, as previous predictability studies have mainly focused on the relatively rare cases of Mediterranean cyclogenesis preceded by upstream extratropical transition of tropical cyclones. This study exploits a European Centre for Medium-Range Weather Forecasts (ECMWF) operational ensemble forecast with an uncertain potential vorticity (PV) streamer position over the Mediterranean that, 3 d after initialization, resulted in an uncertain development of the Mediterranean tropical-like cyclone (Medicane) Zorbas in September 2018. Later initializations showed substantially lower forecast uncertainties over the Mediterranean. An ad hoc clustering of the ensemble members according to the PV streamer position in the Mediterranean is used to study the upstream evolution of the synoptic to mesoscale forecast uncertainties. Cluster differences show that forecast uncertainties were amplified on the stratospheric side of a jet streak over the North Atlantic during the first day of the ensemble prediction. Subsequently, they propagated downstream and were further amplified within a short-wave perturbation along the wave guide, superimposed onto the large-scale Rossby wave pattern. After 3 d, the uncertainties reached the Mediterranean, where they resulted in a large spread in the position of the PV streamer. These uncertainties further translated into uncertainties in the position and thermal structure of the Mediterranean cyclone. In particular, the eastward displacement of the PV streamer in more than a third of the ensemble members resulted in a very different cyclone scenario. In this scenario, cyclogenesis occurred earlier than in the other members in connection to a pre-existing surface trough over the Levantine Sea. These cyclones did not develop the deep warm core typical of medicanes. It is proposed that the eastward-shifted cyclogenesis resulted in reduced values of low-level equivalent potential temperature in the cyclogenesis area. As a result, latent heating was not intense and deep enough to erode the upper-level PV anomaly and allow the formation of a deep warm core. The westward displacement led to surface cyclones that were too weak, and a medicane formed in only half of the members. The central, i.e. correct, PV streamer position resulted in the most accurate forecasts with a strong medicane in most members. This study is the first that explicitly investigates the impact of PV streamer position uncertainty for medicane development. Overall, results extend current knowledge of the role of upstream uncertainties in the medium-range predictability and unsteady forecast behavior of Mediterranean cyclones including medicanes.

A global climatological perspective on the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events

“…Such PV streamers and cutoffs can favour EPE formation through inducing enhanced moisture transport and forcing for ascent via reduced static stability beneath the positive upper tropospheric PV anomaly and dynamical lifting ahead (Funatsu and Waugh, 2008;Schlemmer et al, 2010). Accordingly, many case studies have linked Rossby wave breaking to EPEs in several regions, including, but not limited to, southwestern North America, northwestern Africa, the Alpine region, the Mediterranean, southern Africa, the Middle East, Pakistan and the Himalayas, and East Asia (Massacand et al, 1998;Knippertz andMartin, 2005, 2007;Argence et al, 2006;Hart et al, 2010;Martius et al, 2013;De Vries et al, 2016;Vellore et al, 2016;Tsuji and Takayabu, 2019;Portmann et al, 2020b). Systematic climatological analyses, however, are limited to the Alpine region (Martius et al, 2006), the Middle East (De Vries et al, 2018), South Africa (Favre et al, 2013), and North America (Abatzoglou, 2016;Barbero et al, 2019;Moore et al, 2019).…”

Role of moist and dry air advection in the development of Mediterranean tropical‐like cyclones (medicanes)

“…Portmann et al . (2020) showed that the uncertain position of a PV streamer was critical for the proper simulations of the September 2018 medicane, extending current knowledge on the role of upstream uncertainties in medium‐range predictability.…”

“…Although the complex topography of the region traversed by the October 1996 vortex makes the interpretation of the results for this case less straightforward, the simulations in both cases agree that the reduced humidity inhibits the development of the two cyclones. The presence of dry intrusions in many (if not all) medicanes described in literature (e.g., Cioni et al ., 2016; Carrió et al ., 2017; Miglietta et al ., 2017; Portmann et al ., 2020) and the reduction in convective activity and rainfall observed after the upper‐level dry air has reached the cyclone core (e.g., Claud et al ., 2010; Miglietta et al ., 2013; Dafis et al ., 2018; Marra et al ., 2019) suggest the generality of these results. However, further cases need to be examined before reaching more definitive conclusions.…”

“…Following Portmann et al . (2020), the air–sea interaction may be already important before cyclogenesis: for this reason, sea‐surface fluxes were switched off during the initial 30–36 h of each simulation (“No‐fluxes” runs). For the 1996 case, an additional test was undertaken, in which the fluxes were activated in the first and last part of the run, but were not allowed to increase during the phase (24 h, from 1800 UTC, 5 October to 1800 UTC, 6 October) of maximum cyclone intensification (“No‐fluxes‐peak” run); hence, the latter experiment permits us to investigate the importance of the fluxes in the phase when they strongly intensified (from about 500 to about 800 W m −2 ; see below).…”

“…In particular, we analyse the role of dry air near the cyclones' center, which is associated with a dry intrusion, that is, a slantwise‐descending airstream of cold and dry lower stratospheric/upper tropospheric air advected equatorward into the mid/lower troposphere, characterised by a high PV anomaly (the so‐called PV streamers). This feature is typical of the initial stages of a medicane's lifetime (e.g., Cioni et al ., 2016; Miglietta et al ., 2017) and needs to be properly represented for adequate simulation of the entire lifetime of these cyclones (Portmann et al ., 2020).…”