The OH<sup>*</sup>(3–1) layer emission altitude cannot be determined unambiguously from temperature comparison with lidars

摘要： I investigate the nightly mean emission height and width of the OH?(3–1) layer by comparing nightly mean temperatures measured by the ground–based spectrometer GRIPS9 and the Nalidar at ALOMAR. The dataset contains 42 coincident measurements between November 2010 and February 2014, when GRIPS9 was in operation at the ALOMAR observatory (69.3?N, 16.0?E) in northern Norway. To closely resemble the mean temperature measured by GRIPS9, I weighted each nightly mean temperature profile measured by the lidar using Gaussian distributions with 40 different centre altitudes and 40 different full widths at half maximum. In principle, one can thus determine the altitude and width of the OH?(3–1) layer by finding the minimum temperature difference between the two instruments. On most nights, several combinations of centre altitude and width yield a temperature difference of ±2 K. The generally assumed altitude of 87 km and width of 8 km is never an unambiguous, good solution for any of the measurements. Even for a fixed width of ～8.4 km, one can sometimes find several centre altitudes that yield equally good temperature agreement. Weighted temperatures measured by lidar are not suitable to determine unambiguously the emission height and width of an OH? layer. If the OH?(3–1) rotational temperature is used as a proxy for the temperature at an altitude of 87 km with a width of 8.4 km, this proxy is representative to within ±16 K.