Cut‐off low systems over Iraq: Contribution to annual precipitation and synoptic analysis of extreme events

Abstract

We combine daily in situ precipitation data with meteorological reanalysis data in order to explore the contribution of cut‐off low systems to the seasonal and inter‐annual rainfall variations over Baghdad from 2005 to 2016. During these 12 years (average rainfall of 131 ± 67 mm/year), 38 rainy cut‐off lows brought 43% of the total precipitation, with extreme inter‐annual variations. Indeed, precipitation associated with autumn cut‐off lows was the principal factor that turned an arid into a wet year: during the three most arid years cut‐off lows contributed about 25% of the average rainfall (10 out of 40 mm/year) while during the three wettest years they contributed near 67% (171 out of 254 mm/year). The extreme‐rain cut‐off low systems displayed analogous synoptic characteristics: upper‐atmosphere divergence, upwards vertical motions in the middle atmosphere, and lower‐atmosphere winds into central Iraq at times when the surface Red Sea and Persian Gulf waters were warmer than the surface air. During those days previous to an extreme event, the surface waters cooled substantially and the amount of precipitable water increased largely, suggesting high latent heat transfer. In order to characterize those conditions that favour rainfall, we focus on the November 18–20, 2013 cut‐off low system, which led to the largest flooding and wettest year in Baghdad between 2005 and 2016. The distribution of properties in the middle (500 hPa) and upper (250 hPa) troposphere shows that the region was affected by intense horizontal divergence and upwards motions, coinciding with a surface low over the Arabian Peninsula that caused intense northwards winds over the Persian Gulf and brought substantial moisture to central Iraq. The analysis of several stability indexes indicates that convective instability played a secondary role during the episode.

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

“…Another meteorological process that has been associated with EPEs is Rossby wave breaking. This process refers to the non-linear phase of planetary wave amplification, which can result in irreversible mixing of air masses from stratospheric high-latitude origins into tropospheric low-latitude regions, and vice versa, as observed in early studies (McIntyre and Palmer, 1983;Appenzeller and Davies, 1992). In a potential vorticity (PV) framework, Rossby wave breaking can be diagnosed from the formation of elongated PV filaments, referred to as PV streamers, that eventually can split off from the main stratospheric (tropospheric) air reservoirs over the poles (tropics), known as cutoffs (Hoskins et al, 1985;Wernli and Sprenger, 2007;Portmann et al, 2020a).…”

The three-dimensional life cycles of potential vorticity cutoffs: a global and selected regional climatologies in ERA-Interim (1979–2018)

“…In the subtropics and midlatitudes, these upper-level closed cyclones often form when air from the poleward side of the jet stream is transported far equatorward, forming an elongated cold-air tongue. Subsequently, the tongue breaks up into one or more upper-level cyclonic vortices separated from the main polar reservoir (Appenzeller and Davies, 1992), which are usually located equatorward of the jet stream and isolated from the main westerly flow. This process is known as Rossby wave breaking (RWB, Berggren et al, 1949), and the resulting upper-level cyclonic systems, which are often termed cutoff lows (COLs), were first characterized comprehensively by Palmén and Newton (1969).…”

Mapping Flash Floods in Iraq by Using GIS

“…Most of the rainfall from December through March. Although the largest number of low pressures passes over Iraq during the month of February, it is noticed that the largest amount of rain occurs during January and March, while most of the heavy rains occur in the month of November [3].…”