Characterization of potential temperature hotspots during urban heat island episodes examined by large eddy simulation and land use regression for the city of Augsburg

This study investigates the spatio-temporal distribution of potential temperature (theta) hot- and coldspots in an
urban environment for one day during a summerly heat wave, as reflected by a large eddy simulation (LES) model as well
as reproduced by a multiple linear regression (MLR) model based on an observation network. The spatial variation of static
surface characteristics only partly explains the observed patterns for both approaches. The question of which additional
factors, mainly those related to atmospheric circulation, are essential for the development of theta hot- and coldspots is
addressed. For this purpose, real case simulations with the LES model PALM-4U were conducted for the city of Augsburg,
Southern Germany. Hot- and coldspots were detected in the modelled theta fields with the G$_i$* statistic. The theta and G$_i$*
patterns were compared to the results of the MLR model, using only static surface characteristics for the referring daytime,
season and weather type as predictors. For some times of the day, the patterns from the two approaches show good agree-
ment, but there are considerable differences for other situations, although the weather type does not change over the studied
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period. In a next step, the detected hotspots are classified into expected and unexpected hotspots according to their surface
characteristics, and differences in the meteorological variables for both groups are investigated. A similar procedure is
applied to areas apart from hotspots, which could be expected to be hotspots, and those which are not expected to be one.
The results indicate that even in summerly anticyclonic conditions with low wind speeds, horizontal circulation and vertical
mixing play a significant role in manifesting urban theta patterns. It is concluded that more than a single simulation may be
required to represent typical urban temperature patterns during heat waves since they cannot reflect the critical influence of
varying circulation dynamics at different synoptic conditions. This should be considered in urban planning.