A Low-cost Back-calculation Method for Massive On-site Diurnal Microclimate Observation
Keywords: Microscale meteorological observation, Air velocity, Short-wave radiation, Mean radiant temperature, Globe thermometer
Microclimate, different from macrometeorology or mesometeorology, is the direct meteorological influence on outdoor human thermal healthy and labour safety. Massive field measurement is important for the study of urban microclimate. But the scientific meteorological measuring instruments, either a portable weather station or a combination of several different functional meteorological instruments, are usually expensive and cumbersome. A real massive field experiment is costly and complex. It is impracticable for most research institutions to carry out massive on-site observations, especially for those in developing countries and rural regions.
To fill this gap, this paper presents a low-cost back-calculate method for the massive on-site diurnal data observation. The target meteorological parameters are the mean radiant temperature (tmrt), short-wave radiation, local air velocity and relative humidity. These four parameters govern outdoor human thermal balance and are the most representative elements that can describe a specific outdoor microclimate [1-4]. Inspired by the globe thermometer  and the pan-radiometer , the proposed easy setup measures temperatures of three same sized but different coloured globe thermometers at the same time, then back calculates target parameters by solving real-time heat balance equation sets. The calculation results are compared with the field measurement by standard meteorology instruments.
Local micrometeorological measurements were conducted on the flat roof of the Main Building at the University of Hong Kong during a semi-sunny day in July 2014, from 8:00 to 18:00. Three hollow copper spheres, 40mm in diameter and 0.5mm in shell thickness, are constructed. Those three spheres are identically except that one is painted black, one is painted white and the remaining one is polished. Each sphere is equipped with a thermocouple at the middle of its cavity. Air temperature and relative humidity are measured by an iButton, which is also a component of the designed setup.
Other standard meteorological measuring instruments are used to provide reference data. Wind speed is recorded by an ultrasonic anemometer. 3-demensional radiation flux intensities are measured by three net radiometers. Reference tmrt is calculated according to the integral radiation measurements and view factors method .
The equation set (1) derives from the heat balance that heat stored in the sphere system is equal to the radiative heat gain and convective heat loss. The temperature data is collected every 10sec, thus the 5-minute mean value of Rs, Te and u can be approximated by conducting Riemann integral of equation set (1). Spectral analysis of the collected temperature signals is used to generate a better estimation of the convective influence of air velocity.
The convective heat transfer coefficient h is formulated according to the empirical correlation of forced convection data from sphere in air proposed by Hey ,
A new equation for the estimation of mean radiant temperature tmrt (°C) is proposed as equation (3):
Fig 1 shows the velocity calculation result. The estimation accuracy is not high enough to compare with that of professional anemometers, but it is capable to illustrate the influence of convection on an outdoor working site.
Fig 1. Calculated air velocity (5-min mean) versus measured air velocity
The natural air flow of the experimental site is blocked by surrounding buildings. The calculation accuracy of a wider range of wind velocity needs to be further investigated in the future.
2. Estimation of mean short-wave radiation (Rs)
According to equation (1), the calculated short-wave radiation is the mean value from all directions. It can be used as a presentation of solar radiation. The estimated Rs is more fluctuant. It is always lower than the measurement but they follow the same trend (Fig 2. a), which verifies the rationality of calculation. This underestimation may be explained by the uncertainty of surface optical parameters of each sphere in calculation. The calculation can be systematically calibrated. 10min mean value can decrease the scattering (Figure 2. b).
Fig 2. (a) the 5-min mean calculation result of mean short-wave radiation; (b) the systematically corrected 10-min mean calculation result of mean short-wave radiation.
Compared with the tmrt calculated by interval radiation method, sphere groups give a relatively fluctuant estimation; however the trend is coincident (Fig 3 a). The pattern seems very similar to the estimation of short wave radiation. The proposed method underestimates the mean radiant temperature in most case, but it can be systematically corrected just as the result of short-wave radiation (Fig 3 b).
Fig 3. (a) the 5-min mean calculation result of mean radiant temperature; (b) the systematically corrected 10-min mean calculation result of mean radiant temperature.
This calculation-depended low-cost method is capable of the on-site observation of real-time local air velocity, short-wave radiation, mean radiant temperature and relative humidity, though it is still limited to a certain precision. Its simple construction and cheap price set the stage for massive observation.