Detection of the urban heat-Island effect from a surface mobile platform

A.

Description

Resumen
Se ha medido la temperatura superficial de las calles estrechas, largas y profundas de la ciudad de Sao Paulo mediante el uso de termómetros infrarrojos de precisión (IRTs). Estos instrumentos van montados en una plataforma móvil, que se mueve por las calles. Los diagramas térmicos se determinaron a lo largo de transceptos sobre diferentes tipos de suelo y de ocupación del mismo, en las primeras horas del típico periodo nocturno de la estación seca. La temperatura del aire también se midió, junto con el flujo convectivo QH, entre la atmósfera y los edificios. La presencia de una isla de calor atmosférica alrededor del centro urbano se identificó bien, con un valor de 2ºC. Sobre la vertical parece que este fenómeno no es tan claro, aunque se detectó una oscilación térmica máxima de 6ºC entre las superficies más frías y la atmósfera.
Abstract
The surface temperature of the urban canyons in the city of São Paulo is remotely estimated through the use of precision infrared thermometers (IRTs). These instruments are set up on a mobile platform, which moves through the bottom of the canyons. The thermal patterns are verified, along a traverse, through the different kinds of soil coverage and occupation, during the early hours of a typical nocturnal period during the dry season. Air temperature measurements are also taken as well as estimates of the convective QH flux, between the atmosphere and urban buildings. The presence of an atmospheric urban heat island is well identified around the urban center, with a magnitude of 2°C. Over the vertical surface it appears that this phenomenon is not pronounced, although maximum thermal amplitude of around 6°C between the colder analyzed surfaces and the warmer atmosphere is identified.

Sujets

• Cartografía
• Moderate-Resolution Imaging Spectroradiometer
• Incendio
• Teledetección
• Temperature
• Urban
• Sensible heat

Informations

• Publié par: erevistas
• Publié le: 01 janvier 2007
• Nombre de lectures: 12
• Langue: Español

Extrait

Revista de Teledetección. 2007. 27: 59-70
Detection of the urban heat-Island effect from
a surface mobile platform
A. Jaschke Machado and T. Rezende de Azevedo
[e-mail] and [e-mail]
Laboratory of Climatology and Biogeography. University of São Paulo
Prof. Lineu Prestes Avenue, n 338, Cidade Universitária - Zip Code: 05508-000, São Paulo - Brazil
RESUMEN ABSTRACT
Se ha medido la temperatura superficial de las The surface temperature of the urban canyons in the
calles estrechas, largas y profundas de la ciudad de city of São Paulo is remotely estimated through the use
Sao Paulo mediante el uso de termómetros infrarrojos of precision infrared thermometers (IRTs). These instru-
de precisión (IRTs). Estos instrumentos van montados ments are set up on a mobile platform, which moves
en una plataforma móvil, que se mueve por las calles. through the bottom of the canyons. The thermal patterns
Los diagramas térmicos se determinaron a lo largo de are verified, along a traverse, through the different kinds
transceptos sobre diferentes tipos de suelo y de ocu- of soil coverage and occupation, during the early hours
pación del mismo, en las primeras horas del típico of a typical nocturnal period during the dry season. Air
periodo nocturno de la estación seca. La temperatura temperature measurements are also taken as well as esti-
del aire también se midió, junto con el flujo convecti- mates of the convective Q flux, between the atmosphe-
H
vo Q , entre la atmósfera y los edificios. La presencia re and urban buildings. The presence of an atmospheric
H
de una isla de calor atmosférica alrededor del centro urban heat island is well identified around the urban cen-
urbano se identificó bien, con un valor de 2ºC. Sobre ter, with a magnitude of 2°C. Over the vertical surface it
la vertical parece que este fenómeno no es tan claro, appears that this phenomenon is not pronounced, alt-
aunque se detectó una oscilación térmica máxima de hough maximum thermal amplitude of around 6°C bet-
6ºC entre las superficies más frías y la atmósfera. ween the colder analyzed surfaces and the warmer
atmosphere is identified.
KEY WORDS: mobile transect, surface temperature,PALABRAS CLAVE: Cartografía, áreas quemadas,
MODIS, incendios, teledetección. urban, sensible heat.
mal band (between 8 and 14 m). The results canINTRODUCTION
show an interesting panorama of the simultaneous
thermal processes that occur in time and space, andGeographical investigations in Brazil are
should be considered a priori.currently undergoing a wide-ranging renewal pro-
The temporal and spatial variability, of the urbancess (MORAES, 2002). In terms of urban climate,
surface and the neighboring atmosphere temperatu-tele-detective methods do not consist entirely of
res, is a result of the complex exchange of energynew frameworks; since the 80s orbital platforms
fluxes through a volume containing this surface. Inhave been being used to estimate, for example, pat-
general, these fluxes are represented by a relations-terns of surface heat islands. The use of a temporal
hip of balance (OFFERLE et al. 2006) among theseries of meteorological variables to represent sur-
energy sources and sinks,face climate is also nothing new.
A new framework in field experiments consists of
Q* + Q = Q + Q + Q + Q + S (1)the utilization of mobile platforms over a surface F H E S A
and the indirect analysis of the patterns observed.
Q and Q*, respectively, represent the anthropo-These platforms can be mounted, in most cases, F
genic and natural energy sources. The natural sour-with tele-detection sensors, thermometers and
ces are derived from the net radiative processesradiometers that are sensitive to the infrared ther-
N.º 27 - Junio 2007 59
6+6A. Jaschke Machado and T. Rezende de Azevedo
which come from solar energy and atmospheric A more comprehensive spatial distribution of the
emission. However, this determination is complex, temperatures in urban areas can be reached using
especially in the case of the long wave radiative flu- mobile transects over the surface (VOOGT and
xes, where what are anthropogenic and what are OKE, 1997; VOOGT and OKE, 1998; MACHADO
natural observed parcels is not evident. In the and AZEVEDO, 2005). In these transects the air
canopy layer the term Q* can turn up a sink or a temperature and the wall temperatures of the urban
source of energy, and a direct relationship with the canyons are usually observed. These investigations
period of the day is never possible. can be teledetective, in a like manner to the orbital
Q and Q represent the turbulent exchanges of platforms. Then, it is possible to use infrared ther-H E
energy, by convection, between the surface and the mometers (IRTs) over vehicles in displacement on
air. They are defined, respectively, by a sensible and the streets. The apparent surface temperatures that
a latent component. are observed must be adjusted (WITTICH, 1997) to
By defining a volume that concerns the interface the emissions effects.
between the surface and the atmosphere (OKE, Oke (1987) indicates that these procedures of
1988), the term Q indicates the variability of sto- measurement can lead to a maximum error of bet-S
rage energy in this volume and is mainly associated ween 1°C and 2°C, when no adjustments are made.
to the sink of energy by conduction. On the other Because the observed target is generally placed
hand, Q represents the variability of energy near the vehicle, the atmospheric effect is reducedA
advected among physically distinct regions. considerably. The main sources of error are due to
As the balance energy expressions are only an surface emissivity and sensor precision. In the case
idealized representation of the inherent complexity of micro-scale experiments, the use of IRTs is more
of the environmental energy exchanges, the term S satisfactory if the vision field is reduced more in
indicates the fluxes that are still not parameterized, respect to the target (MASSON et al., 2002).
for example: the removal of the surface energy by In the analysis presented here of the metropolitan
varying types of water runoff, the accumulated and region of São Paulo, a period of two hours follo-
dissipated photosynthetic energy of the vegetative wing sunset, with a night of clear skies, during the
processes, and also the various manners in which dry season, and under calm conditions was chosen.
energetic materials are stored. This episode denotes the transition between the
The observed thermal patterns in the city of São daily and the nocturnal period of an atypical Friday,
Paulo are complex, not only because of the com- because there was an extended weekend due to a
plexity of the urban surface, but also due to the sea- four-day vacation. However, it is possible to verify
sonal climatic variability. The tropical climate in the dynamic of the thermal environment and its spa-
the region of São Paulo has an annual variability tial distribution.
which is well defined between the dry and wet sea- The atmospheric urban heat-island effect is veri-
sons. Yet, it can show daily thermal amplitudes hig- fied, and now it must be related to the city’s popu-
her than the annual average thermal amplitude. This lation. In order to compare the results of this paper
characteristic makes it so that the estimates of terms with those obtained for other cities, it is worthwhi-
in the energy balance (MORAES et al., 1977; le to mention that the official population of the city
MACHADO and AZEVEDO, 2007), have to be of São Paulo is 10,434,252 inhabitants (PMSP,
even done more precisely. A more detailed descrip- 2000). If the Metropolitan Area of São Paulo is con-
tion of the spatial distribution of temperatures must sidered , the population is 17,878,703 inhabitants
include the types and the geographical orientations (PMSP, 2000).
of the surfaces.
Temperature fields observed by remote sensors
installed in orbital platforms are not always adjus- INSTRUMENTS AND METHODS
ted for effects caused by the relative orientation to
the surfaces targeted (SOUX et al., 2004). Many Calculate of the energy balance terms
times, it is necessary to validate them in respect to
surface observations. In other cases, the moisture The thermal state of a volume between the surfa-
quantities on the surface should be reported ce and the atmosphere is defined by the balance
(SPRONKEN-SMITH et al., 2000), especially among the various energy fluxes (Eq. 1) that pass
when urban parks are investigated. through this volume.
60 N.º 27 - Junio 2007
66Detection of the urban heat-island effect from a surface mobile platform
In the inside of this volume there is a boundary On the other hand, the radiation fluxes can also
that represents the interface between the surface and be represented using this same notation basis. By
the atmosphere. Because of its irregular shape, it is considering the Stefan-Boltzmann constant ( ) and
very difficult to determine this boundary, which is the radiation proprieties of the various types of sur-
defined by one or a combination of the following: faces (including the atmosphere), such as albedo
public routes, urban vegetation, the grass or imper- ( ) and emissivity ( ), it is possible to obtain the
meable can