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T. Halenka et al.
Under increasing resolution of numerical models, which is common trend both
in regional climate modelling and numerical weather prediction, the role of proper
parameterization of urban processes is rising as well. Model grid-boxes at scales of
10 km and less can be fully filled with urban land use and for bigger cities even
different parts of the city with different parameters can be resolved. The inclusion
of the specific urban processes affecting energy balance and transport, i.e. heat,
humidity, momentum fluxes, is vital to get the urban environment effects on urban
canopy layer correctly, which is necessary for further proper planning of adaptation
and/or mitigation options by city authorities, as well as to inform citizens on the real
conditions in the city, especially under some specific events like heat waves, smog
conditions etc.
The artificial urban surfaces clearly differ from natural surfaces and processes
involved represent additional sinks and sources of momentum, heat and moisture
affecting the properties of local atmosphere and having specific impact on the meteorological conditions, which is a well-known phenomenon since the 1980’s [5, 12,
13]. One of the most comprehensively studied aspects of urban surfaces effect is the
Urban Heat Island (UHI) phenomenon. UHI forms as a result of modified energy
budget due to the canyon-like geometry of the canopy layer and the specific thermal
parameters of the artificial surfaces [12]. Due to their decreased albedo, urban surfaces store more heat compared to rural areas and after sunset this heat is released
with a reduced intensity because of the decreased sky-view factor [6], which makes
UHI typical especially for late afternoon and nighttime. Moreover, there is missing
cooling due to lack of evaporation.
There are further impacts of urban surfaces on other meteorological parameters.
Richards and Oke [17] studied the changes of surface humidity, while e.g. Roth
[18] focused on the impact on roughness and turbulence. Many studies dealt with
the structure of the urban boundary layer including the impact on the height of the
planetary boundary layer [15] In addition, urban-meteorology [10] and wind speeds
[7]. Urbanization-triggered changes in precipitation and hydrological processes got
as well into the attention of research [19, 20]. In addition, urban-meteorology interaction may significantly influence air-quality ([8], with further references). These
influences are connected within the phenomenon of UHI having a direct impact on
the human health [16] and, in general, on the comfort of living in the cities, especially
in context of vulnerable minorities or groups of population.
To provide a reliable, numerical modeling based perspective of the UHI phenomenon, and of other related impacts (e.g. on wind speed/direction, precipitation,
mixing layer height, etc.), the complex nature of the mechanical, thermo-dynamical
and radiative processes has to be represented in models. The inclusion of urbancanopy-models (UCM) is necessary in this regard [1, 2, 9, 14] which are specially
designed to parameterize the processes specific to the urban environment that are not
resolvable at the model’s scale. Most of the modelling studies focused on a particular
city with minor interest in the impact on regional scale further from the urban area
itself and mostly just for case studies of several days. For more references see Huszar
et al. [9], where we attempted long term view on the urban effects in regional climate simulation for central Europe. Building upon this study we further analyse and
T. Halenka et al.
Under increasing resolution of numerical models, which is common trend both
in regional climate modelling and numerical weather prediction, the role of proper
parameterization of urban processes is rising as well. Model grid-boxes at scales of
10 km and less can be fully filled with urban land use and for bigger cities even
different parts of the city with different parameters can be resolved. The inclusion
of the specific urban processes affecting energy balance and transport, i.e. heat,
humidity, momentum fluxes, is vital to get the urban environment effects on urban
canopy layer correctly, which is necessary for further proper planning of adaptation
and/or mitigation options by city authorities, as well as to inform citizens on the real
conditions in the city, especially under some specific events like heat waves, smog
conditions etc.
The artificial urban surfaces clearly differ from natural surfaces and processes
involved represent additional sinks and sources of momentum, heat and moisture
affecting the properties of local atmosphere and having specific impact on the meteorological conditions, which is a well-known phenomenon since the 1980’s [5, 12,
13]. One of the most comprehensively studied aspects of urban surfaces effect is the
Urban Heat Island (UHI) phenomenon. UHI forms as a result of modified energy
budget due to the canyon-like geometry of the canopy layer and the specific thermal
parameters of the artificial surfaces [12]. Due to their decreased albedo, urban surfaces store more heat compared to rural areas and after sunset this heat is released
with a reduced intensity because of the decreased sky-view factor [6], which makes
UHI typical especially for late afternoon and nighttime. Moreover, there is missing
cooling due to lack of evaporation.
There are further impacts of urban surfaces on other meteorological parameters.
Richards and Oke [17] studied the changes of surface humidity, while e.g. Roth
[18] focused on the impact on roughness and turbulence. Many studies dealt with
the structure of the urban boundary layer including the impact on the height of the
planetary boundary layer [15] In addition, urban-meteorology [10] and wind speeds
[7]. Urbanization-triggered changes in precipitation and hydrological processes got
as well into the attention of research [19, 20]. In addition, urban-meteorology interaction may significantly influence air-quality ([8], with further references). These
influences are connected within the phenomenon of UHI having a direct impact on
the human health [16] and, in general, on the comfort of living in the cities, especially
in context of vulnerable minorities or groups of population.
To provide a reliable, numerical modeling based perspective of the UHI phenomenon, and of other related impacts (e.g. on wind speed/direction, precipitation,
mixing layer height, etc.), the complex nature of the mechanical, thermo-dynamical
and radiative processes has to be represented in models. The inclusion of urbancanopy-models (UCM) is necessary in this regard [1, 2, 9, 14] which are specially
designed to parameterize the processes specific to the urban environment that are not
resolvable at the model’s scale. Most of the modelling studies focused on a particular
city with minor interest in the impact on regional scale further from the urban area
itself and mostly just for case studies of several days. For more references see Huszar
et al. [9], where we attempted long term view on the urban effects in regional climate simulation for central Europe. Building upon this study we further analyse and