Tropopause Height Inferred from Twilight Photometry

Abstract

Abstract The tropopause height has been measured by sounding balloons for more than 70 years. More recently, lidar has found the tropopause height by detecting aerosols or subvisible cirrus that accumulate at the tropopause. Here we present a twilight photometer technique to detect the tropopause height with results close to the traditional World Meteorological Organization (WMO) method from sounding balloon measurements. The twilight intensity (I), although radiometrically uncalibrated, can be normalized by the derivative of log(I) with respect to time (t), or I’(t)/I(t), to infer stratospheric aerosol layer properties using Earth’s shadow to orderly shield the sunlight below different altitudes. A conventional twilight photometer detects the twilight glow from atmospheric aerosol scattering with a photodiode detector equipped with a near-infrared optical filter. We have replaced the photodiode and optical filter with a Light-Emitting Diode (LED) that functions as a wavelength-dependent photodiode with a narrow-band response near 1050 nm. We found that the layer height from twilight photometer observations is highly correlated with the tropopause height. 76 twilight tropopause heights during the first six months of 2024 at Geronimo Creek Atmospheric Monitoring Station (GCAMS) in Central Texas agree well with those reported by sounding balloons launched from Corpus Christi, Texas, 207 km from GCAMS (standard deviation of 265 m). The twilight method is inexpensive, easily implemented, and has potential for use at remote sites with solar power and developing countries that lack resources for sounding balloons. It can also provide a learning tool for students of meteorology and the atmospheric sciences.