F. Rocadenbosch

A long Saharan dust event over the western Mediterranean: Lidar, Sun photometer observations, and regional dust modeling

A long Saharan dust event affected the western Mediterranean in the period 12– 28 June 2002. Dust was present mainly between 1- and 5-km height affecting most parts of the Iberian Peninsula and reaching western/central Europe. Intensive backscatter lidar observations over Barcelona (Spain) and Sun photometer data from two stations (El Arenosillo, Spain, and Avignon, France) are used to evaluate different configurations the Dust Regional Atmospheric Modeling (DREAM) system. DREAM currently operates dust forecasts over the Mediterranean region (http://www.bsc.es/projects/earthscience/ DREAM/) considering four particle size bins while only the first two are relevant for long-range transport analysis since their life time is larger than 12 hours. A more detailed bin method is implemented, and two different dust distributions at sources are compared to the operational version. Evaluations are performed at two wavelengths (532 and 1064 nm). The dust horizontal and vertical structure simulated by DREAM shows very good qualitative agreement when compared to SeaWIFS satellite images and lidar height-time displays over Barcelona. When evaluating the modeled aerosol optical depth (AOD) against Sun photometer data, significant improvements are achieved with the use of the new detailed bin method. In general, the model underpredicts the AOD for increasing A ° ngstro¨m exponents because of the influence of anthropogenic pollution in the boundary layer. In fact, the modeled AOD is highly anticorrelated with the observed A ° ngstro¨m exponents. Avignon shows higher influence of small anthropogenic aerosols which explains the better results of the model at the wavelength of 1064 nm over this location. The uncertainties of backscatter lidar inversions (20–30%) are in the same order of magnitude as the differences between the model experiments. Better model results are obtained when comparing to lidar because most of the anthropogenic effect is removed.

Aerosol lidar intercomparison in the framework of the EARLINET project. 3. Raman lidar algorithm for aerosol extinction, backscatter, and lidar ratio

An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.

Aerosol lidar intercomparison in the framework of the EARLINET project. 2.Aerosol backscatter algorithms

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.

Aerosol closure study by lidar, Sun photometry, and airborne optical counters during DAMOCLES field campaign at El Arenosillo sounding station, Spain

We present a comparison of aerosol properties derived from in situ and remote sensing instruments during DAMOCLES campaign, aimed at investigating the equivalence between the instrumentation and methodologies employed by several Spanish groups to study atmospheric aerosols at a regional background site. The complete set of instruments available during this closure experiment allowed collecting a valuable high-resolution aerosol measurement data set. The data set was augmented with airborne in situ measurements carried out in order to characterize aerosol particles during the midday of 29 June 2006. This work is focused on aerosol measurements using different techniques of high-quality instruments (ground-based remote sensing and aircraft in situ) and their comparisons to characterize the aerosol vertical profiles. Our results indicate that the variability between the detected aerosol layers was negligible in terms of aerosol optical properties and size distributions. Relative differences in aerosol extinction coefficient profiles were less than 20% at 355 and 532 nm and less than 30% at 1064 nm, in the region with high aerosol concentration. Absolute differences in aerosol optical depth (AOD) were below 0.01 at 532 and 1064 nm and less than 0.02 at 355 nm, less than the uncertainties assumed in the AOD obtained from elastic lidar. Columnar values of the lidar ratio revealed some discrepancies with respect to the in situ aircraft measurements, caused fundamentally by the lack of information in the lowest part of the boundary layer.

Intense dust and extremely fresh biomass burning outbreak in Barcelona, Spain: characterization of their optical properties and estimation of their direct radiative forcing

An extremely fresh smoke plume (<5 h) was transported over Barcelona on 23 July 2009, just 5 h after an intense Saharan dust event finalized. Both events were observed by sun-photometer, lidar and satellite systems. Results indicate surprisingly large absorption of mixed dust particles (SSA ~ 0.83 ± 0.04) with lower SSA than that observed for smoke (0.86 ± 0.04) particles at 440 nm. Our investigation shows that dust particles may have mixed during their transport with anthropogenic and smoke particles. Dust and smoke layers are observed between 1-6 and 1-4 km, with associated lidar ratios at 532 nm of 51 and 36 sr, respectively. Due to low SSAs and moderate surface albedos, shortwave (SW) radiative forcing calculations reveal that a large part of the solar energy losses at the surface is gained by the atmosphere for each aerosol. Here, dust particles produced a positive instantaneous forcing at TOA ( + 8 W m−2 at 12 UT), while the smoke produced a negative forcing of − 13 W m−2 at 17 UT. The associated SW heating rate is calculated to be around 2-3 K day−1 for both dust and smoke aerosols.

Backscatter Error Bounds for the Elastic Lidar Two-Component Inversion Algorithm

Total backscatter-coefficient inversion error bounds for the two-component lidar inversion algorithm (so-called Fernalds or Klett-Fernald-Sasanos method) are derived in analytical form in response to the following three error sources: 1) the measurement noise; 2) the user uncertainty in the backscatter-coefficient calibration; and 3) the aerosol extinction-to-backscatter ratio. The following two different types of error bounds are presented: 1) approximate error bounds using first-order error propagation and 2) exact error bounds using a total-increment method. Both error bounds are formulated in explicit analytical form, which is of advantage for practical physical sensitivity analysis and computational implementation. A Monte Carlo approach is used to validate the error bounds at 355-, 532-, and 1064-nm wavelengths.

Intercomparison of spanish advanced lidars in the framework of EARLINET

To extend and reinforce the action of the EARLINET- ASOS project, a nucleus of Spanish advanced lidars was created. Four systems were intercompared satisfactorily in terms of backscatter coefficients at two elastic wavelengths.

Planetary Boundary Layer Height and Wind Field Characterization by Means of a Lidar at the Teide Observatory in the Canary Islands

A lidar field campaign was performed between 30th June and 4th July 2007 in the Teide Observatory in the Canarian island of Tenerife to characterize the atmosphere of this astrophysical observation site in terms of nocturnal boundary layer height and wind fields. The nocturnal boundary layer height was found lower than 810 m in all cases and the aerosol optical thickness lower than 0.005 and 0.03 at 1064 and 532 nm, respectively. Wind fields could hardly be retrieved because of faint signals and very weak wind velocities observed during the campaign.

Multi-wavelength aerosol LIDAR signal pre-processing: practical considerations

Elastic lidars provide range-resolved information about the aerosol content in the atmosphere. Nevertheless, a number of pre-processing techniques need to be used before performing the inversion of the detected signal: range-correction, time-averaging, photoncounting channel dead-time correction, overlap correction, Rayleigh-fitting and gluing of both channels.