Mean diurnal variations of noctilucent clouds during 7 years of lidar observations at ALOMAR

J. Fiedler,G. von Cossart,G. Baumgarten

doi:10.5194/angeo-23-1175-2005

J. Fiedler, G. von Cossart + Show 1 more

Open Access

https://doi.org/10.5194/angeo-23-1175-2005

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Abstract

Abstract. From 1997 to 2003, noctilucent clouds (NLC) were observed by lidar above the ALOMAR observatory in Northern Norway (69° N) during a total of 1880 measurement hours. This data set contains NLC signatures for 640h, covering all local times, even during the highest solar background conditions. After data limitation imposing a threshold value of 4x10-10m-1sr-1 for the volume backscatter coefficient of the NLC particles, a measure for the cloud brightness, local time dependencies of the NLC occurrence frequency, altitude, and brightness were determined. On average, over the 7 years NLC occurred during the whole day and preferably in the early morning hours, with a maximum occurrence frequency of ~40% between 4 and 7 LT. Splitting the data into weak and strong clouds yields almost identical amplitudes of diurnal and semidiurnal variations for the occurrence of weak clouds, whereas the strong clouds are dominated by the diurnal variation. NLC occurrence, altitude, as well as brightness, show a remarkable persistence concerning diurnal and semidiurnal variations from 1997 to 2003, suggesting that NLC above ALOMAR are significantly controlled by atmospheric tides. The observed mean anti-phase behavior between cloud altitude and brightness is attributed to a phase shift between the semidiurnal components by ~6h. Investigation of data for each individual year regarding the prevailing oscillation periods of the NLC parameters showed different phase relationships, leading to a complex variability in the cloud parameters.

Highlights

Introduction90 km and are bound to the existence of water vapor in combination with very low temperatures
Noctilucent clouds (NLC) are the highest clouds of the Earth’s atmosphere and a visible sign of the extreme conditions at the polar mesopause region
We find a distinct modulation of NLC occurrence, altitude, and brightness with respect to local time

Summary

Introduction

90 km and are bound to the existence of water vapor in combination with very low temperatures. During the 1960s this method was renewed, leading to long data sets of 33 and 40 years for observations from northwest Europe (Gadsden, 1998) and Moscow (Romejko et al, 2003), respectively These data series include information about the seasonal and interannual variation of NLC occurrence frequency and brightness. NLC observations on a regular basis by lidar require a location well inside the polar cap of their usual occurrence, the border of which is around 60◦ for the Northern Hemisphere (Gadsden, 1998) Measurements at these high latitudes during summer require daylight capability of the lidar systems, which are only featured in a few stations (Muller et al, 1997; Chu et al, 2003; Thayer et al, 2003; Fiedler et al, 2003; Hoffner et al, 2003). We report on the diurnal variability of NLC occurrence frequency and layer parameters, as seen above ALOMAR between 1997 and 2003

Instrumentation and data analysis

Observations

Discussion

Conclusion