Wavelet analysis of downslope flows in Materhorn 2012 experiment

In the last years, increasing attention has been devoted to the study of complex terrain flows, with the specific goal in achieving a better understanding of air circulation which usually develops over mountainous regions. Progress in this field of research is fundamental for improvements of numerical weather prediction models for irregular topography areas, which are commonplace in human settlements across the world. Mountainous regions are often characterized by the diurnal mountain wind system, a thermally driven circulation involving winds following the shape of the underlying surface, such as, for example, downslope flows which originate during nighttime and travel down the mountain slopes. Nocturnal downslope flows are investigated in this work, using a subset of experimental data collected at Granite Mountain, in Utah, during MATERHORN campaign. The dataset consists of data measured by sonic anemometers as well as slow sensors mounted up to seven levels on 4 towers deployed along a main lower slope (angle about 2-3°) of Granite Mountain. The study is performed through the Wavelet Transform method, a mathematical tool which improves the classical Fourier Transform, as it provides the location in time and/or space in addition to frequencies of physical phenomena. The final purpose of the present thesis is to evaluate the ability of Wavelet Analysis of detecting relevant flow features in the flow dynamics and to characterize their behavior. Wavelet Transform resulted to be an appropriate method to perform this type of study. In particular, it was proved capable of detecting transitions between flow regimes, as well as the presence of flow oscillations at different frequencies. Moreover, it has been efficient in highlighting specific events such as collisions between wind fronts and abrupt variations in measured signals

Observational evidence of intensified nocturnal urban heat island during heatwaves in European cities

A heatwave (HW) is a large-scale meteorological event characterised by persistent and extremely high-temperature condition. At the local scale, the urban heat island (UHI) is another thermal-related phenomenon defined as an urban area warmer than its surrounding regions due to different surfaces’ capabilities to absorb and store heat. However, the assessment about the effect produced on UHI by HW events is not homogeneous. Indeed, regarding the capability of HWs to influence the urban-rural temperature difference, several studies report different conclusions describing both an exacerbation and a reduction of UHI during HW events. In this context, the present study analyses in situ long records of temperature measurements (20 years) to provide observational shreds of evidence of UHI modification under HW conditions. We examine data from the European Climate Assessment & Dataset and World Meteorological Organization computing the UHI index (UHII) to quantify the UHI effect intensity in 37 European cities during the last 20 summers. The results show an UHII intensification for 28 of the 32 cities affected by positive UHI during extremely high temperatures at night, while substantial variations are not observed during the daytime. The time evolution of UHI during a HW highlights that a more significant and persistent urban-rural temperature gradient explains the UHI intensification. Finally, the relationship between the large and local-scale temperature phenomena reveals that continental high-temperature periods are often associated with prominent temperature differences between small-scale urban and rural environments, assessing the impact of large-scale features on thermal stress at the local scale

Multi-Scale Analysis of Agricultural Drought Propagation on the Iberian Peninsula Using Non-Parametric Indices

Understanding how drought propagates from meteorological to agricultural drought requires further research into the combined effects of soil moisture, evapotranspiration, and precipitation, especially through the analysis of long-term data. To this end, the present study examined a multi-year reanalysis dataset (ERA5-Land) that included numerous drought events across the Iberian Peninsula, with a specific emphasis on the 2005 episode. Through this analysis, the mechanisms underlying the transition from meteorological to agricultural drought and its features for the selected region were investigated. To identify drought episodes, various non-parametric standardized drought indices were utilized. For meteorological droughts, the Standardized Precipitation-Evapotranspiration Index (SPEI) was employed, while the Standardized Soil Moisture Index (SSI), Multivariate Standardized Drought Index (MSDI), and Standard Precipitation, Evapotranspiration and Soil Moisture Index (SPESMI) were utilized for agricultural droughts, while their ability to identify relative vegetation stress in areas affected by severe droughts was investigated using the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) Anomaly provided by the Copernicus European Drought Observatory (EDO). A statistical approach based on run theory was employed to analyze several characteristics of drought propagation, such as response time scale, propagation probability, and lag time at monthly, seasonal, and six-month time scales. The retrieved response time scale was fast, about 1–2 months, and the probability of occurrence increased with the severity of the originating meteorological drought. The duration of agricultural drought was shorter than that of meteorological drought, with a delayed onset but the same term. The results obtained by multi-variate indices showed a more rapid propagation process and a tendency to identify more severe events than uni-variate indices. In general terms, agricultural indices were found to be effective in assessing vegetation stress in the Iberian Peninsula. A newly developed combined agricultural drought index was found to balance the characteristics of the other adopted indices and may be useful for future studies

Analysis of heat-related phenomena and their interactions at different spatio-temporal scales

This thesis analyzes the impact of heat extremes in urban and rural environments, considering processes related to severely high temperatures and unusual dryness. The first part deals with the influence of large-scale heatwave events on the local-scale urban heat island (UHI) effect. The temperatures recorded over a 20-year summer period by meteorological stations in 37 European cities are examined to evaluate the variations of UHI during heatwaves with respect to non-heatwave days. A statistical analysis reveals a negligible impact of large-scale extreme temperatures on the local daytime urban climate, while a notable exacerbation of UHI effect at night. A comparison with the UrbClim model outputs confirms the UHI strengthening during heatwave episodes, with an intensity independent of the climate zone. The investigation of the relationship between large-scale temperature anomalies and UHI highlights a smooth and continuous dependence, but with a strong variability. The lack of a threshold behavior in this relationship suggests that large-scale temperature variability can affect the local-scale UHI even in different conditions than during extreme events. The second part examines the transition from meteorological to agricultural drought, being the first stage of the drought propagation process. A multi-year reanalysis dataset involving numerous drought events over the Iberian Peninsula is considered. The behavior of different non-parametric standardized drought indices in drought detection is evaluated. A statistical approach based on run theory is employed, analyzing the main characteristics of drought propagation. The propagation from meteorological to agricultural drought events is found to develop in about 1-2 months. The duration of agricultural drought appears shorter than that of meteorological drought, but the onset is delayed. The propagation probability increases with the severity of the originating meteorological drought. A new combined agricultural drought index is developed to be a useful tool for balancing the characteristics of other adopted indices

Evidence of intensified Urban Heat Island during Heatwaves in European cities through observational data

Heatwaves (HWs) are meteorological extreme events on the scale of a few thousand kilometres characterized by persistent extremely hot temperature conditions. Due to their duration and intensity, HWs have been proven to affect thermal risk and mortality in different kinds of environment, but they result particularly dangerous in urban areas with high population density. Urban areas can be also exposed to the local scale phenomenon of Urban Heat Island (UHI), which occurs when a city experiences much warmer temperatures than nearby rural areas essentially due to the different surfaces’ heat absorption and retention capacities. The dynamical interaction of HW and UHI is not entirely understood. Many studies report partially incoherent outcomes, assessing both the existence and the absence of a synergistic behaviour exacerbating the urban-rural temperature difference during HWs. In this study, in situ air temperature measurements are analyzed to estimate the different UHI metrics during HWs events. Data provided by the European Climate Assessment and Dataset and the World Meteorological Organization relative to 41 European cities are examined for the 2000-2019 summer periods. The intensity of the UHI effect is estimated through the Composite UHI Index (UHII), defined as the difference between averaged urban and non-urban air temperatures. Regarding the nighttime, 28 cities present positive UHII increase ranging from 0.2 to 2.0 ºC, which means an average UHI intensification of about 0.7ºC during the HW events. Also, an increment in the percentage of days with positive UHII is observed. The analysis of the temporal evolution of UHII during HW periods highlights that the main responsible for this exacerbation is the rural temperature which tends to amplify less than rural in HW conditions. Furthermore, a composite analysis using the UHII and the temperature anomaly during HW events shows UHI amplification directly dependent on the HW intensity. This study is placed in the wide context of how urban areas respond to severe hot periods, and it could be useful to improve adaptation strategies aimed at reducing HWs’ impacts