The Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) Experiment was carried out in the environs of Manaus, Brazil, in the central region of the Amazon basin for 2 years from ...1 January 2014 through 31 December 2015. The experiment focused on the complex interactions among vegetation, atmospheric chemistry, and aerosol production on the one hand and their connections to aerosols, clouds, and precipitation on the other. The objective was to understand and quantify these linked processes, first under natural conditions to obtain a baseline and second when altered by the effects of human activities. To this end, the pollution plume from the Manaus metropolis, superimposed on the background conditions of the central Amazon basin, served as a natural laboratory. The present paper, as the introduction to the special issue of GoAmazon2014/5, presents the context and motivation of the GoAmazon2014/5 Experiment. The nine research sites, including the characteristics and instrumentation of each site, are presented. The sites range from time point zero (T0) upwind of the pollution, to T1 in the midst of the pollution, to T2 just downwind of the pollution, to T3 furthest downwind of the pollution (70 km). In addition to the ground sites, a low-altitude G-159 Gulfstream I (G-1) observed the atmospheric boundary layer and low clouds, and a high-altitude Gulfstream G550 (HALO) operated in the free troposphere. During the 2-year experiment, two Intensive Operating Periods (IOP1 and IOP2) also took place that included additional specialized research instrumentation at the ground sites as well as flights of the two aircraft. GoAmazon2014/5 IOP1 was carried out from 1 February to 31 March 2014 in the wet season. GoAmazon2014/5 IOP2 was conducted from 15 August to 15 October 2014 in the dry season. The G-1 aircraft flew during both IOP1 and IOP2, and the HALO aircraft flew during IOP2. In the context of the Amazon basin, the two IOPs also correspond to the clean and biomass burning seasons, respectively. The Manaus plume is present year-round, and it is transported by prevailing northeasterly and easterly winds in the wet and dry seasons, respectively. This introduction also organizes information relevant to many papers in the special issue. Information is provided on the vehicle fleet, power plants, and industrial activities of Manaus. The mesoscale and synoptic meteorologies relevant to the two IOPs are presented. Regional and long-range transport of emissions during the two IOPs is discussed based on satellite observations across South America and Africa. Fire locations throughout the airshed are detailed. In conjunction with the context and motivation of GoAmazon2014/5 as presented in this introduction, research articles including thematic overview articles are anticipated in this special issue to describe the detailed results and findings of the GoAmazon2014/5 Experiment.
Broadband solar and terrestrial irradiance measurements of high temporal resolution are needed to study inhomogeneous clouds or surfaces and to derive vertical profiles of heating/cooling rates at ...cloud top. An efficient method to enhance the temporal resolution of slow-response measurements of broadband terrestrial irradiance using pyrgeometer is introduced. It is based on the deconvolution theorem of Fourier transform to restore amplitude and phase shift of high frequent fluctuations. It is shown that the quality of reconstruction depends on the instrument noise, the pyrgeometer response time and the frequency of the oscillations. The method is tested in laboratory measurements for synthetic time series including a boxcar function and periodic oscillations using a CGR-4 pyrgeometer with response time of 3 s. The originally slow-response pyrgeometer data were reconstructed to higher resolution and compared to the predefined synthetic time series. The reconstruction of the time series worked up to oscillations of 0.5 Hz frequency and 2 W m−2 amplitude if the sampling frequency of the data acquisition is 16 kHz or higher. For oscillations faster than 2 Hz, the instrument noise exceeded the reduced amplitude of the oscillations in the measurements and the reconstruction failed. The method was applied to airborne measurements of upward terrestrial irradiance from the VERDI (Vertical Distribution of Ice in Arctic Clouds) field campaign. Pyrgeometer data above open leads in sea ice and a broken cloud field were reconstructed and compared to KT19 infrared thermometer data. The reconstruction of amplitude and phase shift of the deconvoluted data improved the agreement with the KT19 data. Cloud top temperatures were improved by up to 1 K above broken clouds of 80–800 m size (1–10 s flight time) while an underestimation of 2.5 W m−2 was found for the upward irradiance over small leads of about 600 m diameter (10 s flight time) when using the slow-response data. The limitations of the method with respect to instrument noise and digitalization of measurements by the data acquisition are discussed.
Based on airborne spectral imaging observations, three-dimensional (3-D) radiative effects between Arctic boundary layer clouds and highly variable Arctic surfaces were identified and quantified. A ...method is presented to discriminate between sea ice and open water under cloudy conditions based on airborne nadir reflectivity gamma lambda measurements in the visible spectral range. In cloudy cases the transition of gamma lambda from open water to sea ice is not instantaneous but horizontally smoothed. In general, clouds reduce gamma lambda above bright surfaces in the vicinity of open water, while gamma lambda above open sea is enhanced. With the help of observations and 3-D radiative transfer simulations, this effect was quantified to range between 0 and 2200 m distance to the sea ice edge (for a dark-ocean albedo of alpha water = 0.042 and a sea-ice albedo of alpha ice = 0.91 at 645 nm wavelength). The affected distance Delta L was found to depend on both cloud and sea ice properties. For a low-level cloud at 0-200 m altitude, as observed during the Arctic field campaign VERtical Distribution of Ice in Arctic clouds (VERDI) in 2012, an increase in the cloud optical thickness tau from 1 to 10 leads to a decrease in Delta L from 600 to 250 m. An increase in the cloud base altitude or cloud geometrical thickness results in an increase in Delta L; for tau = 1/10 Delta L = 2200 m/1250 m in case of a cloud at 500-1000 m altitude. To quantify the effect for different shapes and sizes of ice floes, radiative transfer simulations were performed with various albedo fields (infinitely long straight ice edge, circular ice floes, squares, realistic ice floe field). The simulations show that Delta L increases with increasing radius of the ice floe and reaches maximum values for ice floes with radii larger than 6 km (500-1000 m cloud altitude), which matches the results found for an infinitely long, straight ice edge. Furthermore, the influence of these 3-D radiative effects on the retrieved cloud optical properties was investigated. The enhanced brightness of a dark pixel next to an ice edge results in uncertainties of up to 90 and 30 % in retrievals of tau and effective radius reff, respectively. With the help of Delta L, an estimate of the distance to the ice edge is given, where the retrieval uncertainties due to 3-D radiative effects are negligible.
Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice ...in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 μm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.
In a numerical study, influences of relative humidity (RH) on aerosol optical properties and direct radiative forcing in the surface boundary layer at Xinken (XK) in Pearl River Delta of China are ...investigated based on the observed particle hygroscopic growth (
f
g
(
D
p
,
RH
)
) and the relevant aerosol physical and chemical properties. The model is validated by comparing the simulated ambient extinction coefficients with those measured by a Raman LIDAR. At 550
nm, the scattering coefficient increases with a factor of 1.54 and 2.31 at a RH increase from 30% to 80% and 90%, respectively. This ratio is mainly controlled by
f
g
, and generally increases with increasing wavelength and decreases with increasing particle effective diameter. From RH 30% to 90%, the average single scattering albedo (
ω
0
) changes from 0.77 to 0.88, 0.91 to 0.97, and 0.86 to 0.94 for internal, respectively, external mixtures, and the retrieved mixing state of the elemental carbon (EC) at XK. The assumption that the absorption coefficient does not change upon humidification alone can introduce an overestimation of
ω
0
of 0.02 (
∼
5
%
) at RH 90% for an internal mixture. However, accounting for the EC mixing state at XK, this error is only about 0.3%. The mean upscatter fraction (
β
¯
) is evaluated as 0.233 at RH 30% with a 30% decrease to 0.191 at RH 90%. The estimation of aerosol direct radiative forcing (
Δ
F
R
) in the surface boundary layer at XK strongly depends on RH and the EC mixing state. The critical
ω
0
at XK is estimated to be about 0.77–0.80 at 550
nm. Assuming an internal or coated mixture of EC, no pronounced
Δ
F
R
is observed when
RH
<
60
%
, whereas a cooling effect arises while
RH
>
60
%
. Under the actual EC mixing conditions at XK, the effect of
Δ
F
R
is cooling. Over 40% of this cooling effect is contributed by water at RH 80% and
Δ
F
R
at RH 90% exceeds that at RH 30% by about a factor of 2.7.
Helicopter-borne observations of the impact of turbulent mixing and cloud microphysical properties in shallow trade wind cumuli are presented. The measurements were collected during the Cloud, ...Aerosol, Radiation and Turbulence in the Trade Wind Regime over Barbados (CARRIBA) project. Basic meteorological parameters (3D wind vector, air temperature, and relative humidity), cloud condensation nuclei concentrations, and cloud microphysical parameters (droplet number, size distribution, and liquid water content) are measured by the Airborne Cloud Turbulence Observation System (ACTOS), which is fixed by a 160-m-long rope underneath a helicopter flying with a true airspeed of approximately 20 m s super(-1). Clouds at different evolutionary stages were sampled. A total of 300 clouds are classified into actively growing, decelerated, and dissolving clouds. The mixing process of these cloud categories is investigated by correlating the cloud droplet number concentration and cubed droplet mean volume diameter. A significant tendency to more inhomogeneous mixing with increasing cloud lifetime is observed. Furthermore, the mixing process and its effects on droplet number concentration, droplet size, and cloud liquid water content are statistically evaluated. It is found that, in dissolving clouds, liquid water content and droplet number concentration are decreased by about 50% compared to actively growing clouds. Conversely, the droplet size remains almost constant, which can be attributed to the existence of a humid shell around the cloud that prevents cloud droplets from rapid evaporation after entrainment of premoistened air. Moreover, signs of secondary activation are found, which results in a more difficult interpretation of observed mixing diagrams.
The CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) project, focused on high resolution and collocated measurements of thermodynamic, turbulent, ...microphysical, and radiative properties of trade wind cumuli over Barbados, is introduced. The project is based on two one-month field campaigns in November 2010 (climatic wet season) and April 2011 (climatic dry season). Observations are based on helicopter-borne and ground-based measurements in an area of 100 km2 off the coast of Barbados. CARRIBA is accompanied by long-term observations at the Barbados Cloud Observatory located at the East coast of Barbados since early in 2010 and which provides a longer-term context for the CARRIBA measurements. The deployed instrumentation and sampling strategy are presented together with a classification of the meteorological conditions. The two campaigns were influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, and the African continent which led to distinct aerosol conditions. Pristine conditions with low aerosol particle number concentrations of ~100 cm3 were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparably high number concentrations of ~ 500 cm3. The biomass burning aerosol was originating from both the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Two days with different aerosol and cloud microphysical properties but almost identical meteorological conditions have been analyzed in detail. The differences in the droplet number concentration and droplet sizes appear not to show any significant change for turbulent cloud mixing, but the relative roles of droplet inertia and sedimentation in initiating coalescence, as well as the cloud reflectivity, do change substantially.
Within the German Leibniz‐network OCEANET project, ship‐based lidar and microwave remote sensing as well as spectral zenith radiance observations with the COmpact RAdiation measurements System ...(CORAS) were performed. During three cruises latitudes between 50°N and 50°S were covered. A new spectral retrieval method to derive the cloud optical thickness τ and the droplet effective radius reff using CORAS measurements is developed. The method matches CORAS measurements of ratios of spectral transmissivity at six wavelengths with modeled transmissivities. This retrieval is fast and accurate and thus suitable for operational purposes. The new approach circumvents ambiguities of existing cloud retrievals and reduces the influence of measurement uncertainties. It is applied to homogenous and heterogeneous liquid water and cirrus clouds. In boundary layer liquid water clouds, the retrieved effective radius was more variable, whereas in the cirrus it was rather constant. Furthermore, the liquid water path LWP was derived and compared to data from a microwave radiometer. The new retrieval tends to overestimate LWP for thick liquid water clouds but slightly underestimate LWP for thin clouds. The presented method cannot be applied to mixed‐phase clouds. The maximum retrieval of τ and reff for liquid water clouds is 80 in τ and 30 μm in reff, respectively; for cirrus clouds the limitations of the retrieval are 10 in τ and 60 μm in reff.
Key Points
Microphysical cloud properties are retrieved with a multispectral retrievalSpectral zenith transmissivity of clouds is characterizedLiquid water clouds and ice clouds are characterized
The sensitivity of passive remote sensing measurements to retrieve microphysical parameters of convective clouds, in particular their thermodynamic phase, is investigated by three-dimensional (3-D) ...radiative transfer simulations. The effects of different viewing geometries and vertical distributions of the cloud microphysical properties are investigated. Measurement examples of spectral solar radiance reflected by cloud sides (passive) in the near-infrared (NIR) spectral range are performed together with collocated lidar observations (active). The retrieval method to distinguish the cloud thermodynamic phase (liquid water or ice) exploits different slopes of cloud side reflectivity spectra of water and ice clouds in the NIR. The concurrent depolarization backscattering lidar provides geometry information about the cloud distance and height as well as the depolarization.
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