Air temperature of high altitude is considered as an important factor in both weather forecast and climate change research. Due to the increasing amount of attention that has been focused on these areas, it is desired that the accuracy of upper air temperature measurement can be reduced to 0.1 °C. During the day, direct solar radiation produces a considerable larger error on the temperature measurement, which has become the bottleneck when improving the accuracy of upper air radiosonde temperature sensors.
At present, experiments and theoretical calculations are the methods used to study the correction of solar radiation heating error. Unfortunately, it is difficult to setup a low-pressure radiation wind tunnel, which is crucial for these experiments. The Reynolds number is a key physical parameter for calculating solar radiation errors. This parameter is influenced by ascend velocity, air density, geometry of the bead and leads, thermal conductivities of the materials, and a number of factors. Nevertheless, this number has long been obtained by empirical estimation.
This paper proposes a new method of numerical simulation to improve the measurement accuracy of the temperature sensor and make up for the shortages of the traditional solutions. A computational fluid dynamics (CFD) method may solve computational problems of complex flow and heat transfer in various fields, and it is employed to simulate sounding environment from sea level to 32 km altitude. Physical parameters of a sensor, such as lead diameter, lead length, lead angles, lead materials, solar azimuths, solar elevation angles, the diameter size of the resistor body and the reflectivity of the surface coating, are analyzed. The modeling results of this method have a potential to scale the solar radiation errors down to roughly 0.1 °C magnitude.
CFD modeling of sounding temperature sensor
Bead thermistors are widely used in the arena of meteorological observation, since it has the advantages of small size, high sensitivity and the ability of heat. The bead thermistor climbs from sea level to an altitude of approximately 32 km. The atmospheric pressure decreases at least three orders of magnitude in the process, which affects the convective-conductive heat transfer of the temperature sensor significantly. The relationship between the air pressure and the altitude is given by the U.S. standard atmosphere published in 1976.
The bead thermistor has an irregular geometric shape. Hence it is necessary to use strong adaptive unstructured grid. In order to accurately capture the physical phenomena in the boundary layer, a boundary layer grid technology is implemented, which helps to improve the calculation simulation accuracy. The entire mesh of the bead thermistor model and the peripheral air domain is shown in Fig.1.
Numerical simulation results and analysis
a. Simulation of solar radiation as a function of solar elevation angles and azimuths
While time and hanging orientation of the radiosonde vary, the radiation angle changes as well. As shown in Fig.2.
Fig.3 displays the relationships between solar elevation angle, azimuth and solar radiation error from sea level to 32 km altitude. The solar radiation error and the altitude is approximately an exponential function, and the solar radiation error monotonically increases with altitude.
At sea level or near sea level, the discrepancy of the solar radiation error resulted from different sun elevation angles and azimuths are approximately 0.1 °C. As the altitude increases, this discrepancy increases rapidly. When the altitude is up to 32 km, the discrepancy of the radiation errors is up to 0.8 °C between the two radiation errors induced by two solar radiation orientations, which are X-axis direction and Z-axis direction, respectively. When the solar elevation angle is 90 °, the solar radiation error of the bead thermistor model is minimal.
b. Simulation of solar radiation as a function of lead materials
In order to discover how various lead materials influence solar radiation heating, five lead materials are studied. The solar radiation error as a function of various lead materials and altitude is illustrated in Fig.4. It can be reflected from the modeling results that relative low thermal conductivity of a lead material causes a relatively large solar radiation error. Pt lead material produces the largest solar radiation error, while Ag lead material generates the smallest error.
Figure1. The grids of the bead thermistor model and peripheral air domain
Figure2. Schematic of sun elevation angle and azimuth
Figure3. The discrepancy of the solar radiation errors of the different solar elevation angles, azimuths