This paper presents the first results about the assimilation of CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization)
extinction coefficient measurements onboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite in the MOCAGE
(MOdèle de Chimie Atmosphérique à Grande Echelle) chemistry transport model of Météo-France. This assimilation module is an extension of the aerosol optical depth (
In this study we focus on the desert dust outbreak which happened during late June 2012
over the Mediterranean Basin (MB) during the TRAQA campaign. The comparison with the AERONET (Aerosol Robotic Network)
Compared to MODIS (Moderate-resolution Imaging Spectroradiometer)
The comparison of in situ aircraft and balloon measurements to both modelled and assimilated outputs shows that the CALIOP lidar assimilation highly improves the model aerosol field.
The evaluation with the LOAC (Light Optical Particle Counter) measurements indicates that the aerosol vertical
profiles are well simulated by the direct model but with a general underestimation of the aerosol number concentration, especially in the altitude range 2–5
Analysis of the vertical distribution of the desert aerosol concentration shows that the aerosol dust transport
event is well captured by the model but with an underestimated intensity. The assimilation of CALIOP observations
allows the improvement of the geographical representation of the event within the model as well as its intensity
by a factor of 2 in the altitude range 1–5
Aerosols play an important role in the atmospheric system of our planet. They have a significant
impact on the Earth's radiation budget by direct scattering/absorption of sunlight and by
changing cloud properties
Global observations of tropospheric aerosols have been performed
from several satellite instruments including radiometers and lidars
since the late 1970s (the reader may refer to
Most of the aerosols related to air quality and pollution are
found in the lower troposphere or boundary layer
Chemical data assimilation consists in combining in an optimal way observations provided by instruments and
a priori knowledge about a physical system such as model
output. The observations act as constraints for the models and thus can be used to overcome model deficiencies
The assimilation of different aerosol components has been conducted in the framework of many studies including
AOD
In this study, we present the assimilation module of lidar measurements
in the CTM of Météo-France, MOCAGE (MOdèle de Chimie Atmosphérique à Grande Echelle). The assimilation system coupled to the MOCAGE CTM was initially developed and used for atmospheric gases, predominately ozone (
We consider the extinction coefficient measurements from the CALIOP
lidar onboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite. We focus on the African dust event that occurred in late June–early July 2012 over the
Mediterranean Basin (MB) during the
TRAQA (TRAnsport à longue distance et Qualité de l'Air dans le bassin méditerranéen) field campaign held between 26 and 11 July 2012 (see Sect.
This study aims principally to
present the lidar assimilation module as well as the first results dealing with the assimilation
of CALIOP observations in terms of extinction coefficient into the MOCAGE CTM and evaluate the impact of lidar assimilation on the 3D tropospheric aerosol distribution at
regional scale during this large-scale event. The lidar measurements from the CALIOP instrument are assimilated into the MOCAGE CTM using the variational 3D-FGAT (first guess at appropriate time) method.
The impact of the CALIOP extinction coefficient assimilation on the aerosol distribution has been evaluated
using a set of independent data including AERONET (AErosol RObotic NETwork), MODIS, aircraft as well as
balloon measurements.
The paper outline is as follows. In Sect.
The CALIPSO satellite is a partnership between NASA (National Aeronautics and Space Administration)
and the French Space Agency, CNES (Centre National d’Etudes Spatiales). It was launched on 28 April 2006 with the cloud profiling radar system on the CloudSat satellite.
It flew in the international “A-Train” constellation for coincident Earth observations until 13 September 2018, when CALIPSO began lowering its orbit from 705 to 688
The CALIPSO satellite provides new insight into the role that clouds and atmospheric aerosols (airborne particles)
play in regulating Earth's weather, climate, and air quality
In this study, the quality controls of the selected aerosol profiles
of CALIOP observations to be assimilated have to be consistent with the following criteria
It should be noted that the vertical resolution of the CALIOP observations is much higher than that of the model. Before the assimilation, each CALIOP profile is adjusted to the model resolution. We first choose a vertical grid that best fits the model. This vertical grid is projected onto each profile of the CALIOP data in such a way that each level corresponds to the middle of the layer. The intermediate levels are then averaged inside each layer. Thus, the profile best corresponds to that of the model while keeping as much as possible of the vertical information.
We will thus see the ability of the CALIOP aerosol observations to constrain the MOCAGE model and to provide added value when assessed against independent observations.
MOCAGE
Variation range of different primary aerosol bins within the MOCAGE model.
The assimilation system is MOCAGE-Valentina
The assimilation system used in this study is the same as described in
In this formulation, there is no need for the explicit specification of the inverse matrix
The minimisation of the cost function with the preconditioned form gives, as a result, an increment of the analysis in the space of variable
The background error covariance matrix
The correlation matrix
The horizontal correlation (
The vertical correlation is modelled using a Gaussian function in terms of the logarithm of the pressure.
Thus, the vertical correlation (
In data assimilation, the covariance matrices
Different validation exercises indicate that the CALIOP observations are situated within a range between 10 % and 25 % in comparison
to different independent data
Note that for better model–observation comparison and memory optimisation, the assimilation cycle (assimilation window) is generally divided into
For aerosols, the modelled prognostic variable and observations are usually not the same physical quantity. In MOCAGE, the prognostic
variable is the aerosol mass concentration of each bin, and the quantities that could be assimilated within this assimilation system
are the aerosol optical depth and the lidar backscatter/extinction profiles.
For assimilation, it is necessary to choose the control variable
In the literature we can find different choices for the control variable for the assimilation of different aerosol
parameters. For more information about the different approaches concerning the choice of the control variable
for the aerosol assimilation, the reader could be referred to
For our assimilation system, we chose to use the 3D total aerosol concentration as the control variable
as in
In MOCAGE-Valentina, we keep the relative contribution of each bin constant in terms of their mass during the
assimilation cycle. Bulk aerosol observations do not have any information on the contributions of different aerosol types. The validation of this approach has been done in
The information on the aerosol vertical profile can be obtained from lidar observations. Incorporating this information into MOCAGE-Valentina is an important improvement in the model. For the assimilation of lidar profiles, it is necessary to develop an observation operator which links the total concentration in the model space with observed lidar quantities in the observation space.
The observation operator transforms the control variable in terms of total aerosol concentration into the lidar extinction coefficient observed quantity. First, the lidar profile observation operator within the MOCAGE-PALM assimilation system sums all individual species in order to calculate the total concentration. Second, it solves the lidar equation by taking into account the contributions of aerosols, gases and Rayleigh scattering. In order to make a connection between total aerosol mass and lidar-observed quantities, the relative mass contributions among aerosol species and sizes (bins) are considered constant in the tangent-linear and adjoint operators (during an assimilation cycle).
To calculate the increment at the end of the cycle, the same relative mass contribution determined before the assimilation is used to convert the total concentration into all the aerosol bins. The observation operator simulates measurements of an elastic backscatter lidar.
By using 3D total concentration as the control variable, we develop the system which is able to efficiently
assimilate
We extend our study to the lidar measurements derived from the CALIOP instrument onboard the CALIPSO satellite and we focus on the TRAQA campaign for which we have access to a wide range of datasets comprising AOD, in situ measurements from the aircraft and the LOAC balloon observations. We study the case of a desert dust transport from Africa to the MB. The added value of the assimilation of CALIOP measurements will be assessed in terms of the improvement of the representation of the desert aerosol within the MOCAGE model during this event.
The MODIS instruments onboard the two EOS (Earth Observing System) satellites Terra (since 2000) and Aqua (since 2002) observe atmospheric aerosols and provide information about aerosol distribution
on global coverage at horizontal resolutions of 10 and 3
The MODIS
The AERONET project is a federation of ground-based remote-sensing aerosol networks. It uses CIMEL Sun/sky radiometers that make measurements within 340–1020
TRAQA is a scientific experiment within the MISTRALS (Mediterranean Integrated STudies at Regional And Local Scales) programme
(
The objectives of the TRAQA field campaign were to study transport, ageing and mixing
of the pollution occurring in the MB
In this study, we will focus on this desert dust outbreak to evaluate the added value of the CALIOP observations within the assimilation system compared to the free model run.
During TRAQA, the ATR-42 aircraft was equipped with the PCASP instrument.
It is an aerosol spectrometer that measures the concentration and the particle size distribution of aerosols
at high frequency in 30 channels distributed over the diameter range 0.1–3
The LOAC
We assimilate the extinction coefficient measurements from the CALIOP instrument into MOCAGE during the TRAQA campaign period.
The objective is to assess the added value of CALIOP analyses compared to the model free run.
Both fields are compared to different datasets presented in Sect.
For this assimilation experiment, the domain in which the assimilation takes place, called the control domain,
is defined with a resolution of
To evaluate the impact of CALIOP lidar measurements on the modelled field, we analyse the behaviour of the assimilation diagnostics in terms of observation minus analysis (OMA)
and OMF. Figure
Histograms of the assimilation diagnostics in terms of
In this section, we present a first evaluation of the extinction coefficient assimilated product with respect to the MOCAGE free run by comparing both fields with the MODIS-independent observations in terms of AOD.
Figure
Comparison of aerosol optical depth obtained by both the MOCAGE free-run model
Table
Statistics (correlation, bias and RMSE) between MODIS observations and MOCAGE free run/assimilation during the TRAQA campaign between 20 June and 11 July 2012.
In Fig.
Many previous studies have highlighted the existence of biases between the CALIOP and MODIS observations
In this section, we exploit the AOD in situ observations from AERONET to quantify the added value of the CALIOP assimilated field in comparison to the model free run. We therefore use all available
AERONET AOD L2 data collected during the period of study from different stations located within
the assimilation domain. Figure
Map of the AERONET stations used for the validation of CALIOP assimilation. The colour code presents the number of observations in each station used within the whole period of study.
Figure
Time series of AOD at 550
The stations located in the western part of the MB and Spain are marked by
a dust episode of relatively high amplitude illustrated by high AOD values (around 27 June). The stations in Spain recorded the event earlier than the stations in France, where it happened a few days later.
This event is clearly highlighted by high AOD values.
The localisation as well as the duration of this event are well represented by both the model free run and the CALIOP analyses over all the stations of comparison. Nevertheless, the AOD values from the free model run are
underestimated over all the stations compared to AERONET measurements.
The assimilated field corrects this underestimation regarding the AOD amplitude since the
agreement between CALIOP analyses and AERONET data is better than that of the free-run model. Table
Scatter plots of AOD where colours represent the number of counts between
the independent observations of AERONET and the two simulations: the direct free model run
Correlation, bias and RMSE between AERONET observations and MOCAGE free/assimilation runs.
In this section, we evaluate in detail the performance of the CALIOP lidar assimilated field by comparing the results of assimilation and the MOCAGE model with the
aerosol concentrations from in situ measurements onboard the instrumented aircraft. We therefore use measurements of the PCASP instrument that
embarked onboard the ATR-42 aircraft to measure the total concentration for particle diameters above 100
(1) Time evolution of aerosol number concentration (in
Flight A was performed on 29 June from 05:00 to 09:00 UTC from Toulouse to Corsica.
This flight coincides with the beginning of the establishment of the desert dust event over southern
France with incursions into eastern and north-eastern Spain and the western part of Italy (see Fig.
During flight B (on 29 June between 10:00 and 15:00), the desert dust event is well established, with an air mass of desert dust spreading over the MB from the eastern and north-eastern coasts of Spain to the coasts of Corsica and Italy (Fig.
These examples illustrate again the ability of the CALIOP assimilation to improve the model and then
to reproduce the aircraft in situ measurements in terms of aerosol concentrations. The assimilation of lidar aerosol data from CALIOP into MOCAGE improves the results
by enhancing the aerosol number concentration by about a factor of 2 getting closer to the aircraft measurements. Nevertheless, the minimum values of concentrations, close to zero, are not well reproduced by the MOCAGE model.
The general underestimation of MOCAGE and the assimilation compared to the independent aircraft measurements during the TRAQA aircraft
campaign is likely due to the horizontal resolution of the MOCAGE CTM (resolution of
During the TRAQA campaign, the LOAC flew onboard three balloons, all launched from Martigues (5.05
Vertical profile of aerosol number concentration in
The comparison of LOAC profiles to those resulting from the assimilation of AOD and CALIOP lidar extinction coefficient observations
(Fig. 9 of
In this section, we evaluate the impact of assimilating the observations from the CALIOP instrument on the desert aerosol vertical distribution.
First, we evaluate the capability of both the model free run and the assimilated field to
reproduce the CALIOP observations. Figure
In a second step, we evaluate the added value of CALIOP observation assimilation to better represent the desert dust plume.
Figure
Figure
Figure
This part of the orbit highlights an air mass of desert dust above the MB in the altitude range between 1 and 5
The aim of this paper is to present and describe the assimilation of lidar observations from the CALIOP
(Cloud-Aerosol Lidar with Orthogonal Polarization) instrument
in the chemistry-transport model (CTM) of Météo-France, MOCAGE (MOdèle de Chimie Atmosphérique à Grande Échelle). We presented the first results of the assimilation of the
extinction coefficient observations of the CALIOP lidar
instrument onboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite during the TRAQA (TRAnsport à longue distance et Qualité de
l’Air dans le bassin méditerranéen) field campaign. The assimilation system used in this study is an extension of the assimilation system developed
for aerosol optical depth (AOD) already presented by
In this study, we have evaluated the added value of the assimilation of the CALIOP extinction coefficient observations
to better document a desert dust transport event compared to the model free run.
The CALIOP assimilation product has been evaluated against different independent datasets:
Note also that the represented aerosol species in this study do not consider the secondary aerosols, which can be the main component of the fine fraction. The lack of secondary aerosols may partly explain the negative biases generally observed in this study.
Space-borne aerosol lidar observations have been revealed to be useful for better understanding the aerosol properties in the atmosphere
Despite the fact that satellite nadir-view active sensors such as CALIOP have limited spatial coverage compared
to passive sensors, the global observations of aerosol vertical distribution from lidars have contributed to improving the quality of atmospheric aerosol observations
We also plan to study the added value of measurements from passive and active probes during volcanic eruption events. This is a very important theme for Météo-France since it is part of the VAAC (Volcanic Ash Advisory Center), whose responsibility extends over a large part of Europe, Asia and Africa.
As a perspective of this work, we will consider simultaneously
assimilating the observations from passive and active sensors by carrying out an initial de-biasing of both observation datasets.
A much more ambitious solution will consist in assimilating satellite radiances directly in a global model using an integrated approach. Assimilation of satellite radiances, i.e. in numerical weather prediction assimilation systems, has proven to be an essential component for improving the forecast skills, particularly for global models
The MOCAGE model as well as its assimilation system are property of Météo-France and not allowed to be shared publicly. The volume of the model and the assimilation analyses used in this paper is large, but for scientific purpose subsets can be made available upon request. The PCASP and LOAC TRAQA data are available on the ChArMEx database at
LEA prepared the paper with contributions from all the authors. BS, AP and LEA implemented the assimilation module within the MOCAGE model. LEA, BS, VM, NF, and JLA designed the experiment and provided scientific guidance. LEA, BS and NF prepared the observations to be assimilated. LEA performed the analysis of MODIS and AERONET data. LEA and BS performed the analysis of the aircraft data. LEA, BS, VM, NF and JLA performed the analyses of the model assimilation runs.
The authors declare that they have no conflict of interest.
This article is part of the special issue “CHemistry and AeRosols Mediterranean EXperiments (ChArMEx) (ACP/AMT inter-journal SI)”. It is not associated with a conference.
This work is funded in France by the Centre National de Recherches Météorologiques (CNRM) of Météo-France and the Centre National de Recherches Scientifiques (CNRS). We would also like to thank NASA and the ICARE, the French atmospheric composition database (CNES and CNRS-INSU), for providing the AERONET and CALIOP data.
This research has been supported by Météo-France as well as by CNRS and CNES through the EECLAT programme.
This paper was edited by Matthias Beekmann and reviewed by two anonymous referees.