University of Sussex
Alamirew, Netsanet.pdf (7.11 MB)

Comprehensive analysis of thermodynamics,dynamics and associated variability

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posted on 2023-06-09, 14:44 authored by Netsanet Alamirew
During summertime Saharan heat low, a region of low pressure system, is formed as a result of large solar insolation superimposed with the convergence of west African South westerly monsoon flow and dry north easterly Harmattan flow along the intertropical discontinuity. This region plays significant role in the initiation and development of the West African Monsoon. The Saharan heat low is co-located with region of maximum load of dust aerosol which is known to have impact on the climate. Further the Saharan heat low plays key role in the global circulations including its role in formation of African Easterly Jets and African Easterly Waves. Despite its role in influencing the dynamic and thermodynamics of the region, the Saharan Heat low is not extensively studied partly due to lack of comprehensive data due to the harsh weather conditions of the region. Climate system of the Saharan heat low is a result of different complicated atmospheric and land surface processes most dominantly immense solar input at the surface, large convergence of sensible heat flux from the ground into the atmosphere, and low level cooling by horizontal advection of moisture from the surrounding area. These dynamical and thermodynamical processes take part in transport and redistribution of heat and transport of the moisture in the region. This thesis aims at providing a detailed analysis of the physical processes responsible for the development, maintenance, and decadal variability of the Saharan heat low region. I investigate three specific aspects of the Saharan heat low region. 1. Heat and Moisture Budget: Heat and moisture are drivers of dynamics and thermodynamics of a region. Previous studies presented heat and moisture budget of the Saharan heat Low without attributing to the detailed mechanisms by which heat and moisture is transported from the surrounding area to the Sahara heat low and vice versa. This thesis presents components of heat and moisture budget resulting from mean and transient flows that are responsible for heating/cooling and moistening/drying of the Sahara heat low region. Heat and moisture budget are derived using commonly used reanalyses simulations (ERA-I, NCEP, and MERRA) and comparison of the results between the three reanalyses are made. I investigate the mechanisms responsible for the decadal variability of intensity of the Sahara heat low and provide implications. This work has not been done previously to the best of knowledge. 2. Role of Dust and Water vapor in controlling the radiative flux: Recent studies show that water vapour greenhouse forcing is responsible for intensification of the Saharan heat low and as a consequence recovery of Sahel rainfall. Dust aerosol is known to have impact on the climate through its interaction with radiation. The large dust load in the Sahara heat low makes it important in controlling the variability in the radiative budget of the region. Previous studies have quantified the role of dust and water vapour in the region in controlling day to day variability in the radiative flux in the heat low. There is still uncertainty in the radiative forcing and associated variability partly due to lack of observational data. Furthermore separating the radiative effect of dust from that of water vapour is challenging due to the co-variability of dust and water vapour. This thesis quantifies separate and combined effect of dust and water vapour in controlling the radiative flux of the Saharan heat low using the recently made FENNEC observations of meteorological variables and dust loading. Theoretical experiments are made to study sensitivity of radiative flux to variations in dust and water vapour. 3. Characteristics of convective density currents: Convective down drafting density currents (cold pools) are ubiquitous features of the Saharan Heat low region which are shown to play important role in the transport of moisture and emission of dust in the region. Despite this, the characteristics of these atmospheric processes are not well studied in the Sahara Heat Low. Improving our knowledge of properties of convective density currents is imperative to better understand atmospheric processes within boundary layer of the Saharan heat low and thus improve model simulation performance. Here I provide magnitude, spatial distribution, and seasonal variability of cold pools using data from the Automatic weather Station (AWS) spread over the Sahara desert. I implement a unique identification method which is further verified by satellite observations of cold pool signatures. Once cold pools are identified at all stations, statistical description of the occurrence frequency and distribution are presented. Finally I asses reanalyses model simulation of convection triggered cold pool outflows through comparison with measurements.


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  • doctoral

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  • eng


University of Sussex

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