Multi‐instrument zenith observations of noctilucent clouds over Greenland on July 30/31, 1995

Abstract

Abstract. Results are presented for zenith observations of a noctilucent cloud (NLC) display over the Sondrestrom atmospheric research facility near Kangerlussuaq, Greenland, on July 30/31, 1995. The observations were made with a Rayleigh lidar, which measured the NLC particle volume backscatter coefficient, and with a UV spectrograph, which measured the intensity and degree of linear polarization of solar light scattered from the NLC. The intensity and polarization measurements were made at solar depression angles of -1.8 ø to -4.6 ø . These data allowed the first simultaneous observation from the ground of the altitude and thickness of the NLC and of the radius of the NLC particles. The NLC was found to be between 86 and 84 km in altitude with a thickness of 1 to 2 km and the NLC particles had a radius at or below 0.07/zm. We also report the first observation of an NLC sublimating due to the passage of an AGW through the 85-km altitude region. These observations are generally in agreement with models of noctilucent clouds. IntroductionWhile noctilucent clouds (NLCs) have been known for over 100 years, it has been difficult to make quantitative groundbased measurements of many of the NLC characteristics. the NLC particles more accurately than the backscatter ratio. We therefore will present the volume backscatter coefficient in describing details of the NLC and its evolution. As the volume backscatter coefficient is equal to the product of the particle number density and the particle backscatter cross section, some assumption about the particle size, composition, and shape is required to determine the density of NLC particles using lidar data. Even multiwavelength lidar observations of an NLC may have difficulty due to the unknown shape of the particles.Recently, Hecht et al. [1994] have reported on a newly developed prism-based UV spectrograph, the high-resolution ozone imager (HIROIG), which is capable of simultaneously measuring the sky spectrum from 270 to 370 nm at any particular polarization angle. By rotating a half-wave plate, the polarization angle can be changed, so that depending on the sky brightness, three of the four Stokes parameters can be obtained in approximately 30 s or less. This instrument can therefore be used to measure both the wavelength dependence of light scattered by NLC particles and the degree of linear polarization of light scattered by those particles. According to Gadsden and SchrOder [1989] both methods can be used to put limits on NLC particle size.The UV spectrograph was deployed at the Sondrestrom research facility from July 30 to August 10, 1995, and was run in conjunction with the Rayleigh lidar to make the first zenith measurements of an NLC using the two complementary techniques. By combining the data sets, additional features of the NLC can be derived such as particle number density and estimates of the NLC sublimation rate.

Gravity wave observations in the summertime polar mesosphere from the Cloud Imaging and Particle Size (CIPS) experiment on the AIM spacecraft

“…Experimental evidence that short-period GW activity is inversely proportional to PMC backscattering was provided by Gerrard et al (1998Gerrard et al ( , 2004; see also Thayer et al, 2003) using lidar backscatter measurements at Sö ndrestrom, Greenland. Other observations from Sö ndrestrom have also shown evidence of GWs causing PMC sublimation (Hecht et al, 1997). Thayer et al (2003) reproduced the measured weaker PMC backscatter and thinner clouds using a model that included short-period GWs.…”

Responses of polar mesospheric cloud brightness to stratospheric gravity waves at the South Pole and Rothera, Antarctica

“…It has been suggested that GWs cause PMC brightness to vary through wave-induced undulation of cloud layers and wave-induced convergence and divergence of cloud particle concentration (Jensen and Thomas, 1994), through wave breaking (Fritts et al, 1993), or through wave-induced temperature perturbations in the PMC vicinity (Turco et al, 1982;Jensen and Thomas, 1994;Rapp et al, 2002). Observations from Sondrestrom, Greenland (67.01N, 50.91W) showed evidence of the passage of acoustic GWs in the mesopause region causing PMC sublimation (Hecht et al, 1997). Gerrard et al (1998Gerrard et al ( , 2004a analyzed the Rayleigh lidar data from Sondrestrom and found that the strength of stratospheric GWs (30-45 km) was negatively correlated with the PMC brightness.…”

“…Therefore, PMC brightness is more suitable for long-term trend monitoring. However, PMC brightness is strongly influenced by tides (von Zahn et al, 1998;Chu et al, 2001bChu et al, , 2003Chu et al, , 2006Fiedler et al, 2005), planetary waves (Merkel et al, 2003), and gravity waves (GWs) (Hecht et al, 1997;Gerrard et al, 1998Gerrard et al, , 2004a. To derive reliable long-term trends and to understand the meaning of these trends, it is important to understand and quantify PMC brightness response to various dynamic factors in addition to the basic water vapor, temperature, and nucleation parameters.…”

Concerning the upper stratospheric gravity wave and mesospheric cloud relationship over Sondrestrom, Greenland

“…On the other hand, over shorter time scales (i.e., less than ∼day) these waves are believed to be responsible for the MC's overall "banded" structure and the potential destruction of such clouds (e.g., Fritts et al, 1993;Jensen and Thomas, 1994;Hecht et al, 1997;Rapp et al, 2002). Particularly, it has been shown in microphysical MC models that gravity waves with short periods (less than ∼6 h) and high amplitudes reduce the cloud strength over time scales of several wave periods (e.g., Jensen and Thomas, 1994;Rapp et al, 2002).…”