In this study we attempted to better quantify radiative effects of dust over the Arabian Peninsula and their dependence on input parameters. For this purpose we have developed a stand-alone column radiation transport model coupled with the Mie, T-matrix and geometric optics calculations and driven by reanalysis meteorological fields and atmospheric composition. Numerical experiments were carried out for a wide range of aerosol optical depths, including extreme values developed during the dust storm on 18–20 March 2012. Comprehensive ground-based observations and satellite retrievals were used to estimate aerosol optical properties, validate calculations and carry out radiation closure. The broadband surface albedo, fluxes at the bottom and top of the atmosphere as well as instantaneous dust radiative forcing were estimated both from the model and observations. Diurnal cycle of the shortwave instantaneous dust direct radiative forcing was studied for a range of aerosol and surface characteristics representative of the Arabian Peninsula. Mechanisms and parameters responsible for diurnal variability of the radiative forcing were evaluated. We found that intrinsic variability of the surface albedo and its dependence on atmospheric conditions, along with anisotropic aerosol scattering, are mostly responsible for diurnal effects.
Mineral dust is an important and integral part of the Earth system.
Dust aerosol perturbs radiation balance by changing optical properties
of the atmosphere
The world's biggest deserts are the major source regions of dust
The Arabian Peninsula is the third largest source region of dust after
North Africa and Central and East Asia, accounting for about 12 % of
total emissions
Daytime cycles of dust impact have also been studied using the observations
from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) and
Geostationary Earth Radiation Budget (GERB) instruments on Meteosat-9,
by
The complexity of the mineral dust radiative effect is associated
with several factors. Dust aerosol is optically active in both shortwave
(SW) and longwave (LW) ranges. It is one of the most absorbing aerosols
after black carbon
The current study aims at better quantification of the clear-sky mineral
dust instantaneous direct radiative forcing (DRF) and its diurnal
cycle over the Arabian Peninsula. This region is less studied and
lacks in situ observations, even though it represents one of the major
sources of dust and occupies a significant part of the dust belt area
Column stand-alone radiative transfer models are widely used for detailing
the radiative impact of dust aerosol meteorological input profiles from ERA-Interim products or GCMs output gas component profiles from observations and from chemistry and transport
model outputs spectral aerosol optical property profiles derived from Aeronet products cloud property profiles (not used in this study) surface spectral optical properties from airplane observations, MODIS
land products and parameterizations
In-depth details of each component and treatment of aerosols are given
in the following sections.
RRTM has been extensively validated and is known to be used in various
applications including the Integrated Forecast System at the European
Centre for Medium-Range Weather Forecasts (ECMWF) and the Weather
Research and Forecasting Model (WRF-ARW) at the National Center for
Atmospheric Research (NCAR). The LW module, RRTM_LW k-distributions are obtained directly from a line-by-line radiative
transfer model. Modeled molecular absorbers are water vapor, carbon dioxide, ozone,
methane and oxygen; additional sources of extinction are Rayleigh
scattering in SW and nitrous oxide, nitrogen and halocarbons in LW.
Aerosol scattering effects are taken into account in both SW and LW. RRTM_SW error with respect to line-by-line calculations is
1 RRTM_LW error with respect to line-by-line calculations is
1.5
The meteorological characteristics required to drive RRTM were taken
from the ECMWF reanalysis (ERA-Interim) data set. ERA-Interim data
were obtained from the ECMWF Data Server with 0.125 by 0.125
The aerosol optical properties in shortwave (extinction
Particles from the coarse and fine modes might have different refractive
indices. We assume that aerosol is represented by one dominant type
(dust, justified further). We assume that two modes are externally
mixed, thus optical properties of the mixture could be obtained according
to e.g.,
To define aerosol size distribution, we use effective radius and standard
deviation of the fine and coarse modes from Aeronet inversion products
Extinction profile of the dust at 532
According to CALIPSO, the ratio of the “not dust” and “dust”
successful retrievals (screened) in the column between 0 and 5
It is known that surface albedo is extremely important for calculation
of the dust radiative effect
The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument
aboard Terra and Aqua satellites views the entire Earth's surface
every 1 to 2 days, acquiring data in 36 spectral bands. The MODIS
multidate and multiangular remotely sensed surface reflectances are
used to derive MODIS Bidirectional Reflectance Distribution Function
(BRDF)/Albedo product MCD43 based on the Ross-thick–Li-sparse reciprocal
model
For ocean surface we adopted parameterization provided by
LW surface emissivity is mostly defined by the surface type. Over
land it also depends on the season and vegetation and is thus best
observed from space. Daily land surface emissivities in LW were obtained
by combining MODIS level 3 MOD11C1 and MYD11C1 products. LW emissivity
for seawater was obtained from the Aster Spectral Library
This formulation of the surface albedo (both for land and ocean) introduces
nonlinearity in the radiation transfer calculations, since surface
albedo itself depends on
Observations over the Arabian Peninsula are scarce. Below we discuss the set of measurements that we were able to retrieve and employ in our study.
From 1995 until 2003, the King Abdulaziz City for Science and Technology
(KACST) and the National Renewable Energy Laboratory (NREL) cooperated
to establish a 12-station network of high-quality radiation monitoring
installations across the Kingdom of Saudi Arabia. The Solar Village
site served as the network operations center, calibration facility,
and data retrieval and quality assessment center. One- and five-minute
data are collected by a suite of instruments compatible with the BSRN
specifications, including upwelling and downwelling longwave and shortwave
fluxes
In the scope of collaboration with the WHOI (Woods Hole Oceanographic
Institution), a fully instrumented shore-side tower was deployed at
the KAUST campus in 2009
To test the simulated radiation fluxes at TOA we used satellite observations.
Instantaneous footprint-level (20
In this section we discuss clear-sky radiative transfer calculations conducted for different locations over the Arabian Peninsula and the sensitivity studies. In each case mineral dust DRF is calculated as a difference between perturbed (P) and control (C) experiments, where P experiments account for dust aerosol and C experiments do not. Both C and P experiment calculations are carried out using the same meteorology and atmospheric composition.
In this section we conduct calculations for two specific locations: in the central Arabian desert at Solar Village and in the semi-desert area at the coastal plain of KAUST campus.
The first case study focuses on the 9–12 August 2002 DRF during fair-weather
AOD conditions, based on Aeronet measurements at the Solar Village
site established in 1999. For this case measurements of both the surface
incident and reflected shortwave fluxes are available. They were used
to estimate broadband surface albedo and compare it with the one derived
from the model runs based on the MODIS BRDF/Albedo product. Description
of the experiments and corresponding abbreviations are provided in
Table
Deviations from the default setup of the experiments conducted for the Solar Village case.
The second case study deals with a major dust outbreak that occurred
over the Arabian Peninsula during March 2012. The storm was observed
by Aeronet at the KAUST campus site established in February 2012.
The storm front first arrived on 18 March, causing strong AOD growth
up to
Fluxes from satellite retrieval products, ground-based observations
and the model used in this study span different spectral ranges as
summarized in Table
In order to quantitatively compare time series of computed and observed
quantities (
Spectral ranges of the satellite products, ground observations and RRTM model. Wavelengths are given in microns.
This case study is characterized by naturally cloud-free conditions
and relatively low column AOD shown in Fig.
Figure
Surface downwelling fluxes at Solar Village. The top panel presents
SW surface downwelling perturbed experiment direct (P dir, black stars)
and diffuse (P dif, purple stars), in situ measured direct (Obs dir,
red circles) and diffuse (Obs dif, blue circles) fluxes and Aeronet
SDA column AOD at 500
Due to positive bias in the surface downwelling flux (mostly due to
diffuse components, Fig.
Figure
Three days accumulated broadband SW albedo at Solar Village derived from ground-based measurements (Obs, red stars) and perturbed experiment based on MODIS BRDF (P, blue circles).
SW (top panel) and LW (bottom panel) computed surface upwelling fluxes with approximated surface temperature (P, blue) and prescribed from ERA-Interim (PE, black) and in situ measurements (Obs, red) at Solar Village. Aeronet SDA column AOD at 500 nm (top panel, green) is plotted against the right vertical axis.
TOA upwelling fluxes at Solar Village. The top (SW) and bottom (LW) panels present fluxes computed with approximated surface temperature (P, blue) and prescribed from ERA-Interim (PE, cyan) and satellite-inferred fluxes (CERES, red). Cloudy area percent coverage derived from CERES product (top panel, green) is plotted against the right vertical axis.
Given a good agreement (with uncertainties close to instrumental)
of the surface (downwelling and upwelling) and TOA (upwelling) fluxes
and thus fairly accurate radiation closure, we focus on calculating
the mineral dust radiative forcings. We first define total downward
minus upward flux as
Figure
SW (top panel) and LW (bottom panel)
The time span of the numerical experiments for the KAUST case consists
of several days of fair-weather AOD followed by the major dust outbreak
that occurred over the Arabian Peninsula on 18–20 March 2012 and several
days of recovery. Figure
SW (top panel) and LW (bottom panel) surface downwelling fluxes at
KAUST for the perturbed experiment (P, blue) and in situ measurements (Obs,
red). Aeronet SDA column AOD at 500
Strong reflection of the SW radiation back to space was observed during
the storm. The top panel in Fig.
For this case study, GERB cloud-screened SW and LW TOA DRF are available
for comparison and are shown in Fig.
SW (top panel) and LW (bottom panel) perturbed experiment (P, black) and satellite-inferred (GERB, red and CERES, blue) TOA upwelling fluxes at KAUST. Cloudy area percent coverage derived from CERES product (top panel, green) is plotted against the right vertical axis.
SW (top panel) and LW (bottom panel) TOA DRF at KAUST derived from the model (RRTM, blue) and satellite retrieval (GERB, red).
In this section we focus on the sensitivity of the SW DRF diurnal
cycle at the TOA and BOA and atmospheric absorption by aerosol, i.e.,
Unlike the Solar Village and KAUST case studies, in this section we
conduct sensitivity analysis to model parameters rather than considering
a specific time period. Size distribution statistics are derived from
Aeronet Level 2.0 inversion product as a function of total column
AOD over the Arabian Peninsula (Fig.
Fine- and course-mode AOD (blue and red, respectively, left panel) at 674
Obtained statistics were fitted as a function of total column AOD
covering the range from 0 to 3 as shown in Fig.
AOD at 674
In order to estimate the diurnal cycle of AOD we use AOD statistics
derived from Aeronet AOT Level 2.0 product. Figure
We discuss below the sensitivity of the TOA SW DRF diurnal cycle computed
for different RIs, surface albedos and AODs. In Fig.
SW TOA DRF over ocean, coastal plain and desert surface albedo (top to bottom) for B09, B15 and B27 refractive indices (left to right) as a function of column AOD and local daytime (hours). Mean local daytime values (red) are projected on the DRF-AOD plane.
SW TOA DRF shown in Fig.
For the white body surface albedo case (blue curves),
If we turn on the aerosol absorption (right two columns in Fig.
For the white body albedo case, since the Anisotropic scattering by dust significantly contributes to the diurnal
cycle of the TOA and BOA DRF and explains the MMM structure. Higher surface albedo modulates Stronger absorption by dust significantly contributes to the diurnal
cycle of the
Special cases of the SW DRF
diurnal cycle for the black surface (black), desert (red) and white
surface (blue) surface albedos.
Broadband SW albedo for desert (blue), coastal plain (red) and ocean (black), with diurnal cycle (lines with stars) and fixed solar zenith angle (circles) averaged over the range of considered optical depths.
In the previous section we considered SW DRF dependence on the parameters
that do not have a diurnal cycle. In this section we quantify the
effect of the albedo diurnal cycle on the SW TOA DRF. Airplane observations
of the albedo usually are done at nadir. So, we define the reference
albedo for the solar zenith angle at local noon, i.e.,
Contribution
In this section we focus on the daily mean dust DRF, discuss the contribution
of the SW, LW and NET (SW plus LW) effects and their sensitivity to
the surface albedo, and aerosol absorption efficiency. Similarly to
Sect.
Daily mean dust DRF for B09, B15 and
B27 RI (indicated by the vertical dash line in each column) and ocean,
coastal plain and desert (left to right) surface albedo over the Arabian
Peninsula. Each bar represents three diagnostic variables:
A column radiation transfer model was used to investigate dust instantaneous
direct radiative forcing over the Arabian Peninsula for a range of
optical depths covering fair-weather and dust storm conditions. According
to the CALIPSO product, dust is a dominant aerosol in this area. We calculated
the forcing of the dust aerosol over ocean, coastal plain and desert
surfaces, accounting for non-sphericity of dust aerosol and using a
range of plausible refractive indices suggested by
Compiling aerosol statistics using Aeronet data, we found similar
scaling patterns between the Arabian Peninsula and North Africa. In
particular, coarse- and fine-mode AODs at 674
We found that for typical conditions over the Arabian Peninsula, the relative difference in daily mean SW fluxes is about 0.5–1.5 % in experiments where dust aerosol is treated as a mixture of randomly oriented spheroids and as a mixture of spheres. Process analysis for black and white surface albedo, non-absorbing dust and isotropic scattering revealed that dust anisotropic scattering controls the diurnal variability of the SW BOA and TOA DRF. This emphasizes the importance of the assumptions about particle shape and thus the phase function to correctly capture the maximum and minimum of the SW TOA DRF diurnal cycle. Due to prevailing forward scattering by dust aerosol, BOA DRF is more sensitive to changes in single scattering albedo or absorption by dust, than TOA DRF. This also implies, that diurnal variations of the TOA DRF are less sensitive to changes in atmospheric absorption by dust than BOA DRF.
Daily mean dust DRF over the Arabian Peninsula for a 0.5 AOD at 674
Several sources of the dust forcing uncertainty are currently known, which include refractive index, number size distribution and surface albedo. The treatment of the surface albedo is often oversimplified and albedo itself is assumed to be fixed. We found that intrinsic variability of the surface albedo and its dependence on the atmospheric conditions are important factors to be taken into account, especially for the desert surfaces, where daily mean TOA DRF is close to zero.
The main results could be formulated as follows.
Dust is a major aerosol over the Arabian Peninsula and its coarse
mode mostly contributes to the total column AOD compared to fine mode. The developed model allows relatively accurate radiation
closure to be carried out. The calculated fluxes are in a good agreement with best available
observations. Dust DRF is estimated and compares well to the independently derived
satellite values. Dust TOA DRF has strong diurnal cycle over desert, but three extrema
structures are present over any surface. Anisotropic scattering by dust significantly contributes to the diurnal
cycle of the SW TOA and BOA DRF. Diurnal intrinsic variability of the surface albedo has a strong impact
on the dust DRF diurnal cycle.
We thank the PI Naif Al-Abbadi for his effort in establishing and maintaining the Solar Village Aeronet site, David Doelling for his expertise in CERES products, Susan Strahan for providing gas composition of the atmosphere, Tom Farrar for establishing the meteorological tower at KAUST, Alexei Lyapustin for the thoughtful discussions about the MODIS land products and surface albedo, Oleg Dubovik for sharing the software package for calculating the optical properties of the spheroids and the Supercomputing Laboratory for computer time at King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia. Research reported in this publication was supported by KAUST. Helen Brindley and Jamie Banks are supported by KAUST CRG-1-2012-STE-IMP grant. Edited by: M. Dameris