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Intra-urban nocturnal cooling rates: development and evaluation of the NOCRA model

Journal article
Authors Shiho Onomura
Björn Holmer
Fredrik Lindberg
Sofia Thorsson
Published in Meteorological Applications
Volume 23
Issue 3
Pages 339-352
ISSN 1469-8080
Publication year 2016
Published at Department of Earth Sciences
Pages 339-352
Language en
Keywords nocturnal cooling rate, two-phase cooling, intra-urban heat island, empirical modelling, urban planning applications
Subject categories Meteorology and Atmospheric Sciences, Physical Geography


A nocturnal cooling rate model (NOCRAM) to simulate nocturnal air temperature at urban sites is presented. The model is designed for urban planners, practitioners and researchers who demand meteorological information for urban planning and research applications. The model is based on the concept of nocturnal cooling, progressing in two distinct phases, i.e. site-dependent cooling around sunset and site-independent cooling from about 1 or 2 h after sunset until sunrise. Cooling rates are usually determined predominantly by prevailing weather conditions (i.e. clearness of the sky and wind speed), followed by maximum daily air temperature and by sky view factors. Second phase cooling is chiefly determined by sky clearness and wind speed. The findings and statistical results from the analysis of observational data during warm months (May–September) from Gothenburg, Sweden, as well as from past studies, were used. The model requires standard meteorological variables (i.e. wind speed, incoming short-wave radiation, air temperature, relative air humidity, air pressure) at a reference station as well as geometrical information (i.e. the sky view factor of the site and the geographical co-ordinates of the reference meteorological station). The model simulates the characteristic development of cooling rates in the two phases at open sites and built-up sites with different sky view factors under a wide range of weather conditions in warm months. Using the modelled cooling rates, nocturnal air temperature is estimated easily with the accuracy of root mean square error (RMSE) ≤1.54 °C and R2 ≥0.78.

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