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Universitat Politècnica de Catalunya (2018)

Dynamics of cyclones and precipitation over the Middle East

Al-Nassar, Ali Raheem Tuaimah

Titre : Dynamics of cyclones and precipitation over the Middle East

Auteur : Al-Nassar, Ali Raheem Tuaimah

Université de soutenance : Universitat Politècnica de Catalunya

Grade : Doctoral Tesis 2018

Precipitation is one of the most important, and also difficult to predict, elements of climate. This difficulty is associated with the transport of moisture through weather fronts that change their pathway, shape and intensity at different spatial and temporal scales. Despite this difficulty, a proper rainfall prediction is necessary both to quantify the resources and infrastructures necessary to bring water to farm fields and cities and to anticipate the occurrence of extreme precipitation episodes that may cause human death and huge economic losses. The main objective of this dissertation is to investigate the influence of weather systems on the patterns and amount of precipitation over Iraq, from its variability at different temporal scales (daily to inter-decadal) to the contribution of severe precipitation events to total rainfall and the mechanisms underlying these extreme episodes. With this objective, we use monthly (1938-2016) and daily (2005-2016) precipitation records from a meteorological station in the city of Baghdad, ran by the Iraqi Meteorological and Seismology Organization. Additionally, we use the temperature, humidity, geopotential height, horizontal and vertical velocity, horizontal divergence, relative vorticity and potential vorticity fields from the ERA-Interim global atmospheric reanalysis, available four times a day on a 0.75º × 0.75º latitude-longitude grid from the European Centre for Medium-Range Weather Forecasts (ECMWF). These data, downloaded for a domain extending from 0°E to 60°E and from 10°N to 60°N, are then used to calculate the equivalent potential temperature, precipitable water, moisture flux, moisture flux convergence, Q-vector divergence, relative vorticity and potential vorticity advection, as well as several instability indices (K, CAPE, SWEAT and LI). In Chapter 2 we focus on the analysis of the monthly rainfall data for the 1938-2016 period. The mean annual precipitation is 135.8 ± 61.9 mm/yr and the mean monthly precipitation is 11.3 ± 18.7 mm. Despite the linear trend for the entire period is small, there are decades (1950s and 1960s) of relatively high mean-annual precipitation and other ones (1990s and 2000s) with substantially lower values. The monthly rainfall time series also shows the existence of substantial inter-annual variations, with annual precipitation values ranging between 29.3 and 307.7 mm (2012 and 1974, respectively). The seasonal cycle changes substantially between arid and wet years, with December-February receiving most during the arid years, and November-April collecting most precipitation during the wet years. The inter-annual changes in precipitation show no correlation with a global index for El Niño-Southern Oscillation. In Chapter 3 we analyse a 12-year time series of daily data (2005-2016) from the Baghdad meteorological station. We identify the contribution of cut-off lows to precipitation : 38 events contributed to 43.4% of the total precipitation over Baghdad. Cut-off lows occur all year long but those happening between October and December account for one-third of the total annual precipitation. In Chapter 4 we have considered the dynamics behind the 18-20 November 2013 extreme rainfall episode, the largest one since 1980. The middle and upper atmospheric conditions resulted from the development of an omega block into a Rex block. The eastern part of the cyclonic structure of the Rex block was experiencing horizontal divergence and the associated upward motions reached all the way from 1000 to 250 hPa. Simultaneously, the lower atmosphere (1000 to 700 hPa) exhibited intense southerly cool winds that brought large amounts of moisture into central Iraq. Finally, in Chapter 5 we again combine the daily Baghdad rainfall data with the ERA-Interim analysis to explore the weather patterns associated with the 20 extreme precipitation events for the 2005-2016 period. These events brought 805.2 mm (51.3%) of the total precipitation to the Baghdad station. Présentation

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