Year-round stratospheric aerosol backscatter ratios calculated from lidar measurements above northern Norway

Abstract

We present a new method for calculating backscatter ratios of the stratospheric sulfate aerosol (SSA) layer from daytime and nighttime lidar measurements. Using this new method we show a first year-round dataset of stratospheric aerosol backscatter ratios at high latitudes. The SSA layer is located at altitudes between the tropopause and about 30 km. It is of fundamental importance for the radiative balance of the atmosphere. We use a state-of-the-art Rayleigh-Mie-Raman lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) station located in northern Norway (69 • N, 16 • E; 380 m a.s.l.). For nighttime measurements the aerosol backscatter ratios are derived using elastic and inelastic backscatter of the emitted laser wavelengths 355, 532 and 1064 nm. The setup of the lidar allows measurements with a resolution of about 5 min in time and 150 m in altitude to be performed in high quality, which enables the identification of multiple sub-layers in the stratospheric aerosol layer of less than 1 km vertical thickness.We introduce a method to extend the dataset throughout the summer when measurements need to be performed under permanent daytime conditions. For that purpose we approximate the backscatter ratios from color ratios of elastic scattering and apply a correction function. We calculate the correction function using the average backscatter ratio profile at 355 nm from about 1700 h of nighttime measurements from the years 2000 to 2018. Using the new method we finally present a year-round dataset based on about 4100 h of measurements during the years 2014 to 2017.

Small-scale structure of thermodynamic phase in Arctic mixed-phase clouds observed by airborne remote sensing during a cold air outbreak and a warm air advection event

“…Most frequently, a liquid-water-dominated layer is observed from which ice crystals precipitate (Shupe et al, 2006;McFarquhar et al, 2007;Ehrlich et al, 2009;Mioche et al, 2015). Spatial differences in the cloud phase vertical distribution can, in turn, occur on horizontal scales down to tens of meters (Korolev and Isaac, 2006;Lawson et al, 2010). Therefore, understanding the radiative properties and temporal evolution of Arctic mixed-phase clouds requires a three-dimensional (3D) characterization of the thermodynamic phase partitioning, which relates the vertical distribution of liquid droplets and ice crystals to the small-scale structures observed close to the cloud top.…”

Issues related to the retrieval of stratospheric-aerosol particle size information based on optical measurements

“…Aerosol particle size information can in principle be obtained based on measurements of (a) the spectral dependence of the aerosol extinction or scattering coefficients (e.g., Yue and Deepak, 1983;Bingen et al, 2003), (b) the spectral dependence of radiance, e.g., in limb geometry (e.g., Rieger et al, 2014;Malinina et al, 2018), (c) the scattering phase function (e.g., Gumbel et al, 2001;Renard et al, 2008) or (d) the polarization of the radiation scattered by aerosols (e.g., McLinden et al, 1999). In the present study the spectral method (a) is applied to forward simulations with a Mie scattering code for different observation geometries, frequently used to remotely sense stratospheric aerosols, i.e., satellite-based occultation (stellar, lunar or solar) measurements and lidar measurements.…”

“…Stratospheric-aerosol particle size information is also retrieved from limb-scatter measurements with the OSIRIS (Optical Spectrograph and InfraRed Imager System; e.g., Bourassa et al, 2008) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography; e.g., Malinina et al, 2018) instruments. The limb-scatter geometry is not considered in the present study, because it requires accurate treatment of surface reflection and multiple scattering, which is beyond the scope of the treatment used here.…”

“…According to Robock (2015) the variation of the stratospheric-aerosol particle size as a consequence of volcanic injections into the stratosphere is one of the key open science questions of current stratospheric-aerosol research. Considering the relatively large differences between stratospheric-aerosol particle size estimates reported in the limited number of available studies (McLinden et al, 1999;Bingen et al, 2003Bingen et al, , 2004aDeshler et al, 2003;Bourassa et al, 2008;Deshler, 2008;Malinina et al, 2018;Ugolnikov et al, 2018;, it appears fair to state that particle size parameters of stratospheric aerosols even under background conditions are not well established. For example, stratospheric-aerosol particle size retrievals -for volcanically quiescent periods -from satellite occultation measurements tend to yield relatively large median radii of up to several hundred nanometers (e.g., Bingen et al, 2003Bingen et al, , 2004aWrana, 2019;Wrana et al, 2020), whereas retrievals based on several other measurement techniques typically lead to significantly smaller median radii (e.g., McLinden et al, 1999;Bourassa et al, 2008;Malinina et al, 2018;Ugolnikov et al, 2018;.…”

Location controls the findings of ground-based PSC observations

“…Emphasised are the established PSC observatories Esrange, Sweden (Blum et al, 2005), Eureka, Canada (Donovan et al, 1997), Ny Ålesund, Svalbard (Massoli et al, 2006), and Sodankylä, Finland (Müller et al, 2001) in the Arctic and Belgrano II (Córdoba-Jabonero et al, 2013), Dumont d'Urville (David et al, 1998;Santecesaria et al, 2001), McMurdo (Adriani et al, 2004;Snels et al, 2019), and Syowa (Shibata et al, 2003) in the Antarctic. Also highlighted are stations with a record of lidar measurements that are not specifically dedicated to PSC observations: Alomar (Langenbach et al, 2019), Iqualuit, and Summit (Neely et al, 2013) in the Arctic and Davis in the Antarctic.…”