Modeling the urban climate of Budapest using the SURFEX land surface model driven by the ALADIN-Climate regional climate model results

Urbanized areas modify the local climate due to the physical properties and morphology of surface objects. The urban impacts on local scale interact with the regional climate resulting in an amplification of certain climate aspects in the cities (e.g., higher maximum air temperature), which may be further enhanced with climate change. Regional climate models provide adequate tool for assessing the regional characteristics of global changes, however, they are incapable for describing the local impacts, e.g., in cities, due to their relatively low resolution (usually 10–25 km horizontally) and due to the lack of detailed description of relevant physical processes. To investigate the future climate change in cities, surface models provide a scientifically sound and cutting-edge solution for the previous problem. In this study, the behavior of SURFEX externalized land surface model including the TEB urban canopy scheme and coupled to the ALADIN-Climate regional climate model in offline mode is investigated. A 10-year-long simulation for 2001–2010 was achieved on 1 km resolution for Budapest. The main goals of our investigation are i) to assess how the biases of the regional climate model inherited and modified by SURFEX, ii) what is the added value of SURFEX to ALADIN-Climate, and iii) what are the capabilities of SURFEX in terms of describing urban and suburban seasonal temperature cycle and daily urban heat island (UHI) evolution in Budapest. Quantified validation is conducted using the measurements of two stations located in the city center and in the suburban area. It was found that SURFEX overestimates the 2 m temperature in both locations throughout the year, in spite of the too cold ALADIN forcings. The strength of nocturnal UHI is overestimated from autumn till spring, while it is slightly too weak in summer. Moreover, the evolution and collapse of daily UHI is imperfectly simulated, namely some delay and slower daily dynamics occur, which might be caused by the method applied for deriving the atmospheric forcings

Assessment of the Urban Impact on Surface and Screen-Level Temperature in the ALADIN-Climate Driven SURFEX Land Surface Model for Budapest

Land surface models with detailed urban parameterization schemes provide adequate tools to estimate the impact of climate change in cities, because they rely on the results of the regional climate model, while operating on km scale at low cost. In this paper, the SURFEX land surface model driven by the evaluation and control runs of ALADIN-Climate regional climate model is validated over Budapest from the aspect of urban impact on temperature. First, surface temperature of SURFEX with forcings from ERA-Interim driven ALADIN-Climate was compared against the MODIS land surface temperature for a 3-year period. Second, the impact of the ARPEGE global climate model driven ALADIN-Climate was assessed on the 2 m temperature of SURFEX and was validated against measurements of a suburban station for 30 years. The spatial extent of surface urban heat island (SUHI) is exaggerated in SURFEX from spring to autumn, because the urbanized gridcells are generally warmer than their rural vicinity, while the observed SUHI extent is more variable. The model reasonably simulates the seasonal means and diurnal cycle of the 2 m temperature in the suburban gridpoint, except summer when strong positive bias occurs. However, comparing the two experiments from the aspect of nocturnal UHI, only minor differences arose. The thorough validation underpins the applicability of SURFEX driven by ALADIN-Climate for future urban climate projections.Peer reviewe

Future temperature and urban heat island changes in Budapest: a comparative study based on the HMS-ALADIN and SURFEX models

Cities, due to their warmer and dryer local climate in addition to their dense population, are subjected to large future climate change risks. Land surface models, with detailed urban parameterization schemes, serve as an adequate tool to refine the rough regional climate projections over the cities. In this study, the future temperature conditions in Budapest are studied with the SURFEX land surface model (LSM), driven by the HMS-ALADIN5.2 regional climate model (RCM) and considering the high-emission RCP8.5 scenario. Special attention is dedicated to explore the differences between the RCM and LSM in terms of the results, their interpretation, and further use in impact models. According to the investigated model combination, the winter season may warm the most, with 1.9 °C in 2021–2050 and 4.3 °C in 2071–2100, although the magnitude of this change is smaller in SURFEX than in ALADIN. Besides the mean changes, four climate indices, based on high and low temperature thresholds, were studied, and it was found that the low temperature indices (frost days and very cold days) may relatively decrease more in SURFEX compared to ALADIN over Budapest, and in the city center compared to the suburbs and rural areas. In addition, the urban heat island (UHI) intensity is projected to decrease in SURFEX mainly in spring and summer (by 2071–2100 with 0.35 °C and 0.32 °C, respectively). Finally, a simple method is provided to correct the SURFEX temperature fields, using the ALADIN model, with eliminated systematic biases and the simulated UHI field

ALADIN-Climate at the Hungarian Meteorological Service: from the beginnings to the present day’s results

This study is focusing on the past and, in particular, the present of the ALADIN-Climate model used at the Hungarian Meteorological Service. The currently applied model version is 5.2 (HMS-ALADIN52). In the recent experiments, the CNRM-CM5 global model outputs were downscaled in two steps to 10 km horizontal resolution over Central and Southeast Europe using RCP4.5 and RCP8.5 scenarios. Temperature and precipitation projections are analyzed for 2021-2050 and 2071–2100 with respect to the reference period of 1971–2000 with focus on Hungary. The results are evaluated in comparison to 26 simulations selected from the 12 km horizontal resolution Euro-CORDEX projection ensemble (including two additional versions of ALADIN-Climate: CNRM-ALADIN53 and CNRM-ALADIN63) to get more information about the projection uncertainties over Hungary and to assess the representativeness of HMS-ALADIN52. The HMS-ALADIN52 simulations project a clear warming trend in Central and Southeast Europe, which is more remarkable in case of greater radiative forcing change (RCP8.5). From the 2040s, the Euro-CORDEX simulations start to diverge using different scenarios. The total range of the annual change over Hungary is 1.3–3.3 °C with RCP4.5 and 3.2–5.7 °C with RCP8.5 by the end of the 21st century. HMS-ALADIN52 results are approximately near to the median: 2.9 °C with RCP4.5 and 4 °C with RCP8.5. CNRM-ALADIN53 shows generally similar results to HMS-ALADIN52, but simulations with CNRM-ALADIN63 indicate higher changes compared to both. In terms of seasonal mean precipitation change, the HMS-ALADIN52 simulations assume an increase between 9% and 33% (less in spring, more in autumn) over Hungary in both periods and with both scenarios. Most of the selected Euro-CORDEX simulations show a precipitation increase, apart from summer, when growth and reduction can be equally expected in 2021–2050, and the drying tendency continues towards the end of the century. Increase projected by HMS-ALADIN52 is mostly confirmed by CNRM-ALADIN53, while CNRM-ALADIN63 predicts precipitation decrease in summer. Precipitation results do not show a significantly striking difference between the scenarios, likely due to the fact that internal variability and model uncertainty are more relevant sources of uncertainty in precipitation projections over our region