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Title: Cloud characteristics of the Andes/Amazon transition zone
Author: Halladay , Kate
ISNI:       0000 0003 9964 9362
Awarding Body: Oxford University
Current Institution: University of Oxford
Date of Award: 2011
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This research is motivated by the need to conserve tropical montane cloud forests (TMCFs) as a biodiversity hotspot threatened by environmental change. A defining feature ofTMCFs is frequent cloud immersion, which also provides moisture to the ecosystem in the dry season. Therefore, TMCFs may be sensitive to any changes in the height or frequency of cloud. The thesis consists of four papers using a combination of field and satellite-based estimations of cloud heights and frequencies with reanalysis data to investigate cloud climatology, trends and variability for a TMCF in SE Peru. The first paper uses satellite-based cloud observations from the International Satellite Cloud Climatology Project (ISCCP) (1983 to 2008), MODIS (Moderate Resolution Imaging Spectroradiometer) (2000 to 2009) and ground-based o~i'rvations to characterise cloud frequency at local and regional scales. The study area is divided into highlands (or puna), eastern slope and lowland zones to investigate spatial variability in the seasonal/diurnal cycles. Greater amplitude annual cycles and an early dry season minimum were found for the puna. For the lowlands, the minimum occurred in the late dry season except in the early morning. The second paper examines differences in interannual variability of cloud frequency between the three zones and its relationship with SSTs. Cloud frequencies are correlated with SSTs areas in the Pacific, Indian Ocean and tropical Atlantic, revealing differences between zones in the SST region of highest correlation, which also varies with season. Composites of cloud frequency, circulation and moisture flux for four combinations of high and low SSTs in the tropical North Atlantic and tropical Pacific suggest that high(low) SST anomalies in these regions are associated with reduced(increased) cloud frequency in all zones in both MAM and SON. Significant decreasing trends are found for the lowlands in January, March and September and in March on the eastern slope. The third uses radiosonde observations from the field site with station data from four elevations and cloud top heights from MODIS to investigate seasonal, diurnal and intra-seasonal variability of atmospheric structure and its relationship with cloud height and frequency. On a seasonal basis the cloud base height varies by approximately 400 m and top height by approximately 4000 m resulting in a minimum relative humidity in September at lower elevations but in June at higher elevations on the slope. Diurnal variability is dominated by daytime upslope and night-time downslope flows. The direction of mid to upper level winds are a dominant feature of intra-seasonal variability and influence inversion frequency, cloud height and frequency in all zones. The final paper compares direct observational methods (photographs, infrared thermometer and disappearance time of the radiosonde), with profile-based methods using relative humidity or temperature thresholds or gradients to estimate the positions of cloud boundaries. Methods to estimate cloud base height using the MODIS Cloud Product were also tested but potential applications were found to be limited. A method is proposed for cloud base height estimation involving consensus estimates from three methods that could be applied on long time scales. 11
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available