Abstract-The aim of this study was to identify typical and specific features of land surface temperature (LST) distribution in the city of Krakow and its surroundings with the use of Landsat/ ETM? data. The paper contains a detailed description of the study area and technical properties of the Landsat program and data, as well as a complete methodology of LST retrieval. Retrieved LST records have been standardized in order to ensure comparability between satellite images acquired during different seasons. The method also enables identification of characteristic thermal regions, i.e. areas always colder and always warmer than a zonal mean LST value for Krakow. The research includes spatial analysis of the standardized LST with regard to different land cover types. Basic zonal statistics such as mean standardized LST and percentage share of hot and cold regions within 10 land cover types were calculated. GIS was used for automated data processing and mapping. The results confirmed the most obvious dependence of the LST on different land cover types. Some more factors influencing the LST were recognized on the basis of detailed investigation of the LST pattern in the urban agglomeration of Krakow. The factors are: emission of anthropogenic heat, insolation of the surfaces depending first of all on land relief and shape of buildings, seasonal changes of vegetation and weather conditions at the time of satellite image acquisition.

In cities located in concave landforms, urban heat island (UHI) is an element of a complicated thermal structure and occurs due to the common impact of urban built-up areas and orography-induced processes like katabatic flows or air temperature inversions. Kraków, Poland (760,000 inhabitants) is located in a large valley of the river Vistula. In the years 2009-2013, air temperature was measured with the 5-min sampling resolution at 21 urban and rural points, located in various landforms. Cluster analysis was used to process data for the night-time. Sodar and synoptic data analysis provided results included in the definition of the four types of nighttime thermal structure representing the highest and the lowest spatial air temperature variability and two transitional types. In all the types, there are three permanent elements which show the formation of the inversion layer, the cold air reservoir and the UHI peak zone. As the impact of land use and relief on air temperature cannot be separated, a concept of relief-modified UHI (RMUHI) was proposed as an alternative to the traditional UHI approach. It consists of two steps: (1) recognition of the areal thermal structure taking into consideration the city centre as a reference point and (2) calculation of RMUHI intensity separately for each vertical zone.

This paper assesses the merits and drawbacks of several data sets of solar downwelling radiation received at the horizontal surface of the tropical Atlantic Ocean where the magnitude of this radiation and its spatial and temporal variability are not well known. The data sets are compared to quality-controlled measurements of hourly means of irradiance made at five buoys of the PIRATA network for the period 2012-2013. The data sets comprise the re-analyses MERRA-2 and ERA5, and three satellite-derived data sets: HelioClim-3v5, SARAH-2 and CAMS Radiation Service v2. It was found that the re-analyses MERRA-2 and ERA5 often report cloud-free conditions while the actual conditions are cloudy, yielding an overestimation of the irradiance in such cases; and reciprocally, they report actual cloud-free conditions as cloudy, yielding an underestimation. The re-analyses exhibit more bias in irradiance in cases of medium and highlevel clouds than for low-level clouds. They correlate well with the hourly means of irradiance (as a whole, correlation coefficients greater than 0.85 for MERRA-2 and 0.89 for ERA5); they correlate very poorly with daily means of irradiance (coefficients of less than 0.48 and 0.59 for MERRA-2 and ERA5, respectively) and with the hourly and daily clearness indices (coefficients of less than 0.53 and 0.46 for MERRA-2 and less than 0.63 and 0.59 for ERA5). The irradiance pattern at both hourly and daily timescales is spatially distorted by re-analyses, especially for MERRA-2. The three satellite-derived data sets exhibit similar performances between them. The correlation coefficients are greater than 0.95 and 0.78 for irradiance and clearness index, respectively, in most cases for hourly values and 0.90 and 0.88, respectively, for daily values. The relative standard deviation of errors is of the order of 15 % for hourly values and 8 % for daily values. It is concluded that these data sets reproduce well the dynamics of the irradiance and clearness index at both hourly and daily timescales. They exhibit overestimation, with the lowest biases reached by the CAMS Radiation Service v2 and ranging between 11 and 37 W m −2 depending on the buoy. It is suggested that HelioClim-3v5 and CAMS Radiation Service v2 are suited for reproducing the spatial gradients of the irradiance and reflecting the spatial variability in the irradiance.

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