Ground-based Backscatter Lidar Research Articles

Extreme smoke event over the high Arctic

The intense western Canadian fires of August 2017 resulted in a (10-year) extreme, high-Arctic smoke event. The primary measurements employed to monitor smoke events were fine mode (FM) aerosol optical depths (AODs) derived from the measured AOD spectra of two AEROCAN/AERONET (CIMEL) sunphotometers at Eureka, Nunavut, Canada. The FM AOD attribution is argued to be a necessary condition for the presence of smoke. Various supporting information, including the correlation with smoke proxy (CO) retrievals, the high frequency (rapid diurnal variation) and the high amplitude nature of the FM AODs, ground-based backscatter lidar profiles, the redundancy of the double CIMEL retrievals, satellite remote sensing, aerosol modeling and backtrajectories indicated that the peak event was likely due to smoke from extreme pyroCb fires in British Columbia.The hypothesis that the FM AOD peak event was an extreme event was tested for a derived ensemble of fine mode events and their peaks over the 10-year sampling period. The results confirmed the hypothesis at the 0.001 level of significance. Important indicators that the 10-year ensemble of FM AOD events did indeed represent smoke were their high frequency and high amplitude FM nature, their occurrence during the Boreal forest fire and agricultural fire seasons in Canada and Asia, and their strong correlation with CO abundances retrieved from FTIR measurements (when sufficient FM AOD and CO statistics were available).In the process of accumulating climatological-scale, monthly-binned fine mode AOD statistics, we found moderate correlations with forest fire or agricultural fire emissions from the Boreal North American, Boreal Asia or Central Asia regions as well as with CO retrievals at Eureka. We argued that confounding factors constraining the monthly binned fine mode AOD vs emissions correlations were associated with the monthly-binned meteorological dynamics (with notable, event-level, exceptions) while confounding factors constraining fine mode AOD vs CO correlations included the different physio-optical nature of those smoke proxies (solar attenuation by fine mode particle scattering versus solar attenuation by molecular absorption). We also employed historic (2005-2010) AHSRL (Arctic High Spectral Resolution Lidar) profiles to estimate an optically averaged smoke plume height of ~3 to 3 ½ km during the spring and summer seasons.

Lidar Measurements Supporting the Ocular Hazard Distance Calculation Using Atmospheric Attenuation

A series of lidar measurements has been performed at the Vidsel Test Range, Vidsel, situated in the inland of the very northern part of Sweden, as a part of an assessment of reducing the laser hazard distance using atmospheric attenuation within the calculations of nominal ocular hazard distance (NOHD). The question was “How low is the atmospheric attenuation as function of height in this area, using a wavelength of 1064 nm?” The work included building a ground based backscatter lidar, performing a series of measurements and analyzing the results. The measurements were performed during June to November, 2014, with the objective to measure at clear air and good weather situations.The lidar measurements at 1064 nm showed a very low atmospheric attenuation as a function of height to altitudes of at least 10 km at several occasions. The lowest limit of backscatter coefficient possible to measure with this instrument is 0.3·10-7 m-1 sr-1 . Assuming a lidar ratio varying between 30 – 100 sr, this was leading to an extinction coefficient of about 0.9 - 3·10-6 m-1 . The atmospheric attenuation reduces the laser hazard distance with about 50 – 56 % depending on the lidar ratio. A recommendation is to monitor the atmospheric attenuation at the occasions when the method to the reduced laser hazard distance using atmospheric attenuation is used.

Haze event monitoring and investigation in Penang Island, Malaysia using a ground-based backscatter Lidar

During 24th July 2013 to 1st August 2013, a haze event struck Penang Island, causing the visibility to decrease and increase in Air Pollution Index (API). A ground-based backscatter Lidar, operate at 355 nm which was setup at the roof top of the School of Physics, Universiti Sains Malaysia. It was used to monitor and investigate the haze event. For this work, we studied the daytime variation of the aerosol intensity, distribution, planetary boundary layer (PBL) height and the aerosol optical depth (AOD) values during these days. We found that the aerosol are very intense during the first two days of the haze event and slowly decline as time passed. Finally the haze event died off on 1st August 2013. As for daily aerosol distribution, aerosols are generally more intense during the afternoon. Its intensity is slightly lower in the morning and evening. Similar trends were observed for AOD values as they increase from morning to afternoon and slowly decrease in the evening. Most aerosols are found contained below the PBL which generally found at around 1000 – 2000 m in height.

Lidar characterization of the Arctic atmosphere during ASTAR 2007: four cases studies of boundary layer, mixed-phase and multi-layer clouds

Abstract. During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR), which was conducted in Svalbard in March and April 2007, tropospheric Arctic clouds were observed with two ground-based backscatter lidar systems (micro pulse lidar and Raman lidar) and with an airborne elastic lidar. In the time period of the ASTAR 2007 campaign, an increase in low-level cloud cover (cloud tops below 2.5 km) from 51% to 65% was observed above Ny-Ålesund. Four different case studies of lidar cloud observations are analyzed: With the ground-based Raman lidar, a layer of spherical particles was observed at an altitude of 2 km after the dissolution of a cloud. The layer probably consisted of small hydrated aerosol (radius of 280 nm) with a high number concentration (around 300 cm−3) at low temperatures (−30 °C). Observations of a boundary layer mixed-phase cloud by airborne lidar and concurrent airborne in situ and spectral solar radiation sensors revealed the localized process of total glaciation at the boundary of different air masses. In the free troposphere, a cloud composed of various ice layers with very different optical properties was detected by the Raman lidar, suggesting large differences of ice crystal size, shape and habit. Further, a mixed-phase double layer cloud was observed by airborne lidar in the free troposphere. Local orography influenced the evolution of this cloud. The four case studies revealed relations of cloud properties and specific atmospheric conditions, which we plan to use as the base for numerical simulations of these clouds.

Comparison between Backscatter Lidar and Radiosonde Measurements of the Diurnal and Nocturnal Stratification in the Lower Troposphere

AbstractA collection of boundary layer heights has been derived from measurements performed by a ground-based backscatter lidar in Neuchâtel, Switzerland (47.000°N, 6.967°E, 485 m ASL). A dataset of 98 cases have been collected during 2 yr. From these data, 61 are noon and 37 are midnight cases. The following two different schemes were used to retrieve the mixed layer depth and the height of the residual layer from the measurements: the gradient and variance methods. The obtained values were compared with those derived from the potential temperature profiles as computed from radiosonde data. For nocturnal cases, the height of the first aerosol layer above the residual layer was also compared to the corresponding potential temperature value. Correlation coefficients between lidar and radiosonde in both convective and stable conditions are between 0.88 and 0.97.

Lidar remote sounding of cirrus clouds and comparison of simulated fluxes with surface and METEOSAT observations

Three cirrus cloud cases have been remotely sounded near Paris by a ground-based backscatter lidar and broadband radiometers. Some cirrus properties (optical depth, emissivity, height) are derived from these measurements and used to compare radiative transfer calculations to surface and METEOSAT observations of broadband irradiances. For a useful comparison, the three cirrus cases were selected to have different morphologies and optical properties: June 29, 1993—thin cirrus cloud (thickness 1.5 km, optical depth 0.22); September 6, 1993—thick cirrus cloud (thickness 5 km, optical depth 2.7); and November 16, 1993—inhomogeneous and geometrically thick cirrus cloud (thickness 3.5–6.5 km) but optically thin (optical depth 0.82). At surface, the differences between measurements and model range from 1.5 to 4 Wm −2 for longwave fluxes, and from 20 to 70 Wm −2 for shortwave fluxes. At the top of the atmosphere, the differences between METEOSAT measurements and model are in fair agreement for longwave fluxes (up to 50 Wm −2). However, unexpected high differences are found for shortwave fluxes (up to 144 Wm −2) due to cirrus clouds heterogeneities and uncertainties in their microphysical properties and especially the occurrence of high reflectivity due to horizontally oriented ice crystals at the cloud top, which are not taken into account by the Model presently.

Significance of multiple scattering from tropospheric aerosols for ground-based backscatter lidar measurements

The influence of multiple scattering on the retrieval of extinction coefficients of tropospheric aerosols from ground-based backscatter lidar measurements is numerically modeled. In a first step, lidar returns are computed by means of a Monte Carlo code for model atmospheres with different aerosol types and different extinction coefficient profiles. In so doing, synthetic lidar signals with and without multiple scattering can be simulated. In a second step, both types of signal are inverted by the most frequently used analytical solution, which, however, is based on the single-scatter assumption. From a comparison of the results, the error of the retrieved aerosol-extinction profiles can be quantitatively determined. It was found that the contribution of multiply scattered photons to the lidar signals is typically below 10% and never exceeds 20%. The relative errors of the retrieved aerosol-extinction profile in the planetary boundary layer are still smaller; they were determined to be less than 3% for all aerosol types, even for extinction coefficients as large as 3.9 km(-1). Thus, for ground-based lidar measurements and typical meteorological conditions, errors caused by neglecting multiple scattering are by far less significant than other errors in lidar data evaluation.

Stratospheric aerosols: observations, trends, and effects

The stratospheric aerosol layer has been investigated by ground based backscatter lidar at Garmisch-Partenkirchen since 1976. A conversion of particle backscatter to particle extinction, mass and surface area via an aerosol model allows to study climatic implications of volcanic events and heterogeneous chemistry effects on the stratospheric ozone layer. The observations show that a volcanically perturbed period lasting from 1979 until 1986 was preceeded and followed by volcanically quiescent periods. A comparison of these two background periods reveals enhanced backscattering in 1989/90 with respect to the 1977/79 level. A comparison of IFU aerosol data with ozone data obtained by the German Weather Service at Hohenpeissenberg through balloon soundings indicates an anticorrelation during the maximum of the volcanic perturbation in 1982/83 following the El Chichón eruption of April 1982.