Local and remote moisture sources for extreme precipitation: a study of the two catastrophic 1982 western Mediterranean episodes

Damián Insua-Costa,María Carmen Llasat,Gonzalo Miguez-Macho

doi:10.5194/hess-23-3885-2019

Abstract

Abstract. Floods and flash floods are frequent in the south of Europe resulting from heavy rainfall events that often produce more than 200 mm in less than 24 h. Even though the meteorological conditions favourable for these situations have been widely studied, there is a lingering question that still arises: what humidity sources could explain so much precipitation? To answer this question, the regional atmospheric Weather Research and Forecasting (WRF) model with a recently implemented moisture tagging capability has been used to analyse the main moisture sources for two catastrophic flood events that occurred during the autumn of 1982 (October and November) in the western Mediterranean area, which is regularly affected by these types of adverse weather episodes. The procedure consists in selecting a priori potential moisture source regions for the extreme event under consideration, and then performing simulations using the tagging technique to quantify the relative contribution of each selected source to total precipitation. For these events we study the influence of four possible potential sources: (1) evaporation in the western Mediterranean; (2) evaporation in the central Mediterranean; (3) evaporation in the North Atlantic; and (4) advection from the tropical and subtropical Atlantic and Africa. Results show that these four moisture sources explain most of the accumulated precipitation, with the tropical and subtropical input being the most relevant in both cases. In the October event, evaporation in the western and central Mediterranean and in the North Atlantic also had an important contribution. However, in the November episode tropical and subtropical moisture accounted for more than half of the total accumulated rainfall, while evaporation in the western Mediterranean and North Atlantic played a secondary role and the contribution of the central Mediterranean was almost negligible. Therefore, remote sources were crucial: in the October event they played a similar role to local sources, whereas in the November case they were clearly dominant. In both episodes, long-distance moisture transport from the tropics and subtropics mostly occurred in mid-tropospheric layers, via well-defined moisture plumes with maximum mixing ratios at medium levels.

Highlights

The western Mediterranean region (WMR) is characterised by a high frequency of the occurrence of torrential rainfall episodes and floods that cause severe damage, with a very high social and economic impact (Llasat et al, 2010)
The main mechanism generating these heavy precipitation events (HPEs) is the strong instability induced by the warm, moist air at low levels which sits over the mild Mediterranean waters for most of the year, and the ensuing vigorous convection is usually triggered by the surrounding mountains or convergence lines (e.g. Buzzi et al, 1998; Rotunno and Ferretti, 2003; Llasat, 2009)
Most cases occur in autumn, when the combination of a warm sea surface temperature and a southward displacement of the jet stream, which usually favours the appearance of Atlantic lows or cut-off lows (COLs; e.g. Nieto et al, 2005) affecting the WMR, make this season the most favourable for the development of these extreme events

Summary

Introduction

The western Mediterranean region (WMR) is characterised by a high frequency of the occurrence of torrential rainfall episodes and floods that cause severe damage, with a very high social and economic impact (Llasat et al, 2010). Jansa et al (2014) and Reale and Lionello (2013) showed that heavy precipitation in the Mediterranean is usually directly or in-. D. Insua-Costa et al.: Moisture sources for the 1982 extreme western Mediterranean flood events directly related to intense, weak or moderate cyclones. Insua-Costa et al.: Moisture sources for the 1982 extreme western Mediterranean flood events directly related to intense, weak or moderate cyclones They found that in more than 80 % of heavy rain cases produced in the western Mediterranean, a cyclone was situated nearby, and was in a suitable location to organise a warm, moist inflow into the affected area (Jansa et al, 2001; Campins et al, 2011). For a detailed review of the most frequent atmospheric conditions resulting in Mediterranean HPEs, please refer to Dayan et al (2015)

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