Cloud Radiative Forcing of the Arctic Surface: The Influence of Cloud Properties, Surface Albedo, and Solar Zenith Angle

Extended and intense wildfires occurred in Northern Canada and, unexpectedly, on the Greenlandic West coast during summer 2017. The thick smoke plume emitted into the atmosphere was transported to the high Arctic, producing one of the largest impacts ever observed in the region. Evidence of Canadian and Greenlandic wildfires was recorded at the Thule High Arctic Atmospheric Observatory (THAAO, 76.5&#176;N, 68.8&#176;W, www.thuleatmos-it.it) by a suite of instruments managed by ENEA, INGV, Univ. of Florence, and NCAR. Ground-based observations of the radiation budget have allowed quantification of the surface radiative forcing at THAAO.&#160;Excess biomass burning chemical tracers such as CO, HCN, H2CO, C2H6, and NH3 were&#160; measured in the air column above Thule starting from August 19 until August 23. The aerosol optical depth (AOD) reached a peak value of about 0.9 on August 21, while an enhancement of wildfire compounds was&#160; detected in PM10. The measured shortwave radiative forcing was -36.7 W/m2 at 78&#176; solar zenith angle (SZA) for AOD=0.626.MODTRAN6.0 radiative transfer model (Berk et al., 2014) was used to estimate the aerosol radiative effect and the heating rate profiles at 78&#176; SZA. Measured temperature profiles, integrated water vapour, surface albedo, spectral AOD and aerosol extinction profiles from CALIOP onboard CALIPSO were used as model input. The peak&#160; aerosol heating rate (+0.5 K/day) was&#160; reached within the aerosol layer between 8 and 12 km, while the maximum radiative effect (-45.4 W/m2) is found at 3 km, below the largest aerosol layer.The regional impact of the event that occurred on August 21 was investigated using a combination of atmospheric radiative transfer modelling with measurements of AOD and ground surface albedo from MODIS. The aerosol properties used in the radiative transfer model were constrained by in situ measurements from THAAO. Albedo data over the ocean have been obtained from Jin et al. (2004). Backward trajectories produced through HYSPLIT simulations (Stein et al., 2015) were also employed to trace biomass burning plumes.The radiative forcing efficiency (RFE) over land and ocean was derived, finding values spanning from -3 W/m2 to -132 W/m2, depending on surface albedo and solar zenith angle. The fire plume covered a vast portion of the Arctic, with large values of the daily shortwave RF (< -50 W/m2) lasting for a few days. This large amount of aerosol is expected to influence cloud properties in the Arctic, producing significant indirect radiative effects.

Download Full-text

Cloud Radiative Forcing at Summit, Greenland

Journal of Climate ◽

10.1175/jcli-d-15-0076.1 ◽

2015 ◽

Vol 28 (15) ◽

pp. 6267-6280 ◽

Cited By ~ 49

Author(s):

Nathaniel B. Miller ◽

Matthew D. Shupe ◽

Christopher J. Cox ◽

Von P. Walden ◽

David D. Turner ◽

...

Keyword(s):

Zenith Angle ◽

Annual Cycle ◽

Radiative Forcing ◽

Solar Zenith Angle ◽

Critical Role ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Energy Budget ◽

Liquid Water Path ◽

Cloud Radiative Forcing

Abstract The surface energy budget plays a critical role in determining the mass balance of the Greenland Ice Sheet, which in turn has significant implications for global sea levels. Nearly three years of data (January 2011–October 2013) are used to characterize the annual cycle of surface radiative fluxes and cloud radiative forcing (CRF) from the central Greenland Ice Sheet at Summit Station. The annual average CRF is 33 W m−2, representing a substantial net cloud warming of the central Greenland surface. Unlike at other Arctic sites, clouds warm the surface during the summer. The surface albedo is high at Summit throughout the year, limiting the cooling effect of the shortwave CRF and thus the total CRF is dominated by cloud longwave warming effects in all months. All monthly mean CRF values are positive (warming), as are 98.5% of 3-hourly cases. The annual cycle of CRF is largely driven by the occurrence of liquid-bearing clouds, with a minimum in spring and maximum in late summer. Optically thick liquid-bearing clouds [liquid water path (LWP) > 30 g m−2] produce an average longwave CRF of 85 W m−2. Shortwave CRF is sensitive to solar zenith angle and LWP. When the sun is well above the horizon (solar zenith angle < 65°), a maximum cloud surface warming occurs in the presence of optically thin liquid-bearing clouds. Ice clouds occur frequently above Summit and have mean longwave CRF values ranging from 10 to 60 W m−2, dependent on cloud thickness.

Download Full-text

Sensitivity of Arctic sulfate aerosol and clouds to changes in future surface seawater dimethylsulfide concentrations

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-6419-2019 ◽

2019 ◽

Vol 19 (9) ◽

pp. 6419-6435 ◽

Cited By ~ 4

Author(s):

Rashed Mahmood ◽

Knut von Salzen ◽

Ann-Lise Norman ◽

Martí Galí ◽

Maurice Levasseur

Keyword(s):

Sea Ice ◽

Radiative Forcing ◽

Arctic Region ◽

Sulfate Aerosol ◽

The Arctic ◽

Surface Seawater ◽

Cloud Radiative Forcing ◽

Future Global Warming ◽

Arctic Surface ◽

The Arctic Region

Abstract. Dimethylsulfide (DMS), outgassed from ocean waters, plays an important role in the climate system, as it oxidizes to methane sulfonic acid (MSA) and sulfur dioxide (SO2), which can lead to the formation of sulfate aerosol. Newly formed sulfate aerosol resulting from DMS oxidation may grow by condensation of gases, in-cloud oxidation, and coagulation to sizes where they may act as cloud condensation nuclei (CCN) and influence cloud properties. Under future global warming conditions, sea ice in the Arctic region is expected to decline significantly, which may lead to increased emissions of DMS from the open ocean and changes in cloud regimes. In this study we evaluate impacts of DMS on Arctic sulfate aerosol budget, changes in cloud droplet number concentration (CDNC), and cloud radiative forcing in the Arctic region under current and future sea ice conditions using an atmospheric global climate model. Given that future DMS concentrations are highly uncertain, several simulations with different surface seawater DMS concentrations and spatial distributions in the Arctic were performed in order to determine the sensitivity of sulfate aerosol budgets, CDNC, and cloud radiative forcing to Arctic surface seawater DMS concentrations. For any given amount and distribution of Arctic surface seawater DMS, similar amounts of sulfate are produced by oxidation of DMS in 2000 and 2050 despite large increases in DMS emission in the latter period due to sea ice retreat in the simulations. This relatively low sensitivity of sulfate burden is related to enhanced sulfate wet removal by precipitation in 2050. However simulated aerosol nucleation rates are higher in 2050, which results in an overall increase in CDNC and substantially more negative cloud radiative forcing. Thus potential future reductions in sea ice extent may cause cloud albedos to increase, resulting in a negative climate feedback on radiative forcing in the Arctic associated with ocean DMS emissions.

Download Full-text

Reassessment of the common concept to derive the surface cloud radiative forcing in the Arctic: Consideration of surface albedo – cloud interactions

10.5194/acp-2019-534 ◽

2019 ◽

Cited By ~ 2

Author(s):

Johannes Stapf ◽

André Ehrlich ◽

Evelyn Jäkel ◽

Christof Lüpkes ◽

Manfred Wendisch

Keyword(s):

Radiative Forcing ◽

Surface Albedo ◽

Global Climate Models ◽

Cooling Effect ◽

The Arctic ◽

Radiative Energy ◽

Cloud Radiative Forcing ◽

Airborne Measurements ◽

Solar Cooling ◽

Snow And Ice

Abstract. The concept of cloud radiative forcing (CRF) is commonly used to quantify the warming or cooling effect due to clouds on the radiative energy budget (REB). In the Arctic, radiative interactions between micro- and macrophysical properties of clouds and the surface influence the CRF and complicate its estimate obtained from observations or models. In this study the individual components and processes related to the surface CRF are analysed separately using simulations and measurement from low-level airborne observations of the REB in the heterogeneous springtime marginal sea ice zone (MIZ). The measurements were obtained during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign. The effect of changing surface albedo, due to the presence of clouds, and its dependence on cloud optical thickness was found to be relevant for the estimation of the solar CRF. A method to correct this albedo effect by retrieving the cloud-free surface albedo from observations under cloudy conditions is proposed. The application of this new concept to ACLOUD data shows, that the estimated average solar cooling effect by clouds almost doubles over snow and ice covered surfaces (−63 W m−2 instead of −33 W m−2), if surface albedo-cloud interactions are considered. Concerning the seasonal cycle of the surface albedo, this effect would potentially enhance solar cooling in periods where cold snow and ice dominate the surface and weaken the cooling by optical thin clouds and surface albedos commonly found during the summertime Arctic melting season. These findings suggest, that the surface albedo-cloud interaction needs to be represented in global climate models and in long-term observations to obtain a realistic estimate of the solar CRF and a reasonable representation of cloud radiative feedback mechanisms in the Arctic and to quantify the role of clouds in Arctic amplification.

Download Full-text

Sensitivity of Arctic sulfate aerosol and clouds to changes in future surface seawater dimethylsulfide concentrations

10.5194/acp-2018-876 ◽

2018 ◽

Cited By ~ 1

Author(s):

Rashed Mahmood ◽

Knut von Salzen ◽

Ann-Lise Norman ◽

Martí Galí ◽

Maurice Levasseur

Keyword(s):

Sea Ice ◽

Radiative Forcing ◽

Arctic Region ◽

Sulfate Aerosol ◽

The Arctic ◽

Surface Seawater ◽

Cloud Radiative Forcing ◽

Future Global Warming ◽

Arctic Surface ◽

The Arctic Region

Abstract. Dimethylsulfide (DMS), outgassed from ocean waters, plays an important role in the climate system, as it oxidizes to methane sulfonic acid (MSA) and sulfur dioxide (SO2), which can lead to the formation of sulfate aerosol. Newly formed sulfate aerosol resulting from DMS oxidation may grow by condensation of gases, in-cloud oxidation, and coagulation to sizes where they may act as cloud condensation nuclei (CCN) and influence cloud properties. Under future global warming conditions, sea-ice in the Arctic region is expected to decline significantly, which may lead to increased emissions of DMS from the open ocean and changes in cloud regimes. In this study we evaluate impacts of DMS on Arctic sulfate aerosol budget, changes in cloud droplet number concentration (CDNC), and cloud radiative forcing in the Arctic region under current and future sea ice conditions using an atmospheric global climate model. Given that future DMS concentrations are highly uncertain, several simulations with different surface seawater DMS concentrations and spatial distributions in the Arctic were performed in order to determine the sensitivity of sulfate aerosol budgets, CDNC, and cloud radiative forcing to Arctic surface seawater DMS concentrations. For any given amount and distribution of Arctic surface seawater DMS, similar amounts of sulfate are produced by oxidation of DMS in 2000 and 2050 despite large increases in DMS emission in the latter period due to sea ice retreat in the simulations. This relatively low sensitivity of sulfate burden is related to enhanced sulfate wet removal by precipitation in 2050. However simulated aerosol nucleation rates are higher in 2050, which results in an overall increase in CDNC and substantially more negative cloud radiative forcing. Thus potential future reductions in sea ice extent may cause cloud albedos to increase, resulting in a negative climate feedback on radiative forcing in the Arctic associated with ocean DMS emissions.

Download Full-text

Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo–cloud interactions

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-9895-2020 ◽

2020 ◽

Vol 20 (16) ◽

pp. 9895-9914 ◽

Cited By ~ 1

Author(s):

Johannes Stapf ◽

André Ehrlich ◽

Evelyn Jäkel ◽

Christof Lüpkes ◽

Manfred Wendisch

Keyword(s):

Sea Ice ◽

Radiative Forcing ◽

Surface Albedo ◽

The Arctic ◽

Radiative Energy ◽

Cloud Radiative Forcing ◽

Airborne Measurements ◽

Ice Surfaces ◽

The Impact ◽

Snow And Ice

Abstract. The concept of cloud radiative forcing (CRF) is commonly applied to quantify the impact of clouds on the surface radiative energy budget (REB). In the Arctic, specific radiative interactions between microphysical and macrophysical properties of clouds and the surface strongly modify the warming or cooling effect of clouds, complicating the estimate of CRF obtained from observations or models. Clouds tend to increase the broadband surface albedo over snow or sea ice surfaces compared to cloud-free conditions. However, this effect is not adequately considered in the derivation of CRF in the Arctic so far. Therefore, we have quantified the effects caused by surface-albedo–cloud interactions over highly reflective snow or sea ice surfaces on the CRF using radiative transfer simulations and below-cloud airborne observations above the heterogeneous springtime marginal sea ice zone (MIZ) during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign. The impact of a modified surface albedo in the presence of clouds, as compared to cloud-free conditions, and its dependence on cloud optical thickness is found to be relevant for the estimation of the shortwave CRF. A method is proposed to consider this surface albedo effect on CRF estimates by continuously retrieving the cloud-free surface albedo from observations under cloudy conditions, using an available snow and ice albedo parameterization. Using ACLOUD data reveals that the estimated average shortwave cooling by clouds almost doubles over snow- and ice-covered surfaces (−62 W m−2 instead of −32 W m−2), if surface-albedo–cloud interactions are considered. As a result, the observed total (shortwave plus longwave) CRF shifted from a warming effect to an almost neutral one. Concerning the seasonal cycle of the surface albedo, it is demonstrated that this effect enhances shortwave cooling in periods when snow dominates the surface and potentially weakens the cooling by optically thin clouds during the summertime melting season. These findings suggest that the surface-albedo–cloud interaction should be considered in global climate models and in long-term studies to obtain a realistic estimate of the shortwave CRF to quantify the role of clouds in Arctic amplification.

Download Full-text

The Influence of Solar Zenith Angle and Cloud Type on Cloud Radiative Forcing at the Surface in the Arctic

Journal of Climate ◽

10.1175/1520-0442-12.1.147 ◽

1999 ◽

Vol 12 (1) ◽

pp. 147-158 ◽

Cited By ~ 35

Author(s):

Peter J. Minnett

Keyword(s):

Zenith Angle ◽

Radiative Forcing ◽

Cloud Cover ◽

Solar Zenith Angle ◽

Surface Radiation ◽

Infrared Heating ◽

Radiation Budget ◽

The Arctic ◽

Cloud Type ◽

Surface Radiation Budget

Abstract Measurements of the long- and shortwave incident radiation taken from the USCGC Polar Sea during a research cruise to the Northeast Water Polynya during the summer of 1993 are analyzed together with observations of cloud type and amount to determine the effects of summertime Arctic clouds on the surface radiation budget. It is found that the solar zenith angle is critical in determining whether clouds heat or cool the surface. For large solar zenith angles (>∼80°) the infrared heating effect of clouds is greater than the reduction in insolation caused by clouds, and the surface is heated by the presence of cloud. For smaller zenith angles, cloud cover cools the surface, and for intermediate zenith angles, the surface radiation budget is insensitive to the presence of, or changes in, cloud cover.

Download Full-text

Parameterization of sea-salt optical properties and physics of the associated radiative forcing

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-4787-2008 ◽

2008 ◽

Vol 8 (16) ◽

pp. 4787-4798 ◽

Cited By ~ 25

Author(s):

J. Li ◽

X. Ma ◽

K. von Salzen ◽

S. Dobbie

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Zenith Angle ◽

Radiative Forcing ◽

Solar Zenith Angle ◽

Surface Albedo ◽

Sea Salt ◽

Salt Loading ◽

Ambient Relative Humidity ◽

Order Of Magnitude

Abstract. The optical properties of sea-salt aerosol have been parameterized at shortwave and longwave wavelengths. The optical properties were parameterized in a simple functional form in terms of the ambient relative humidity based on Mie optical property calculations. The proposed parameterization is tested relative to Mie calculations and is found to be accurate to within a few percent. In the parameterization, the effects of the size distribution on the optical properties are accounted for in terms of effective radius of the sea-salt size distribution. This parameterization differs from previous works by being formulated directly with the wet sea-salt size distribution and, to our knowledge, this is the first published sea-salt parameterization to provide a parameterization for both shortwave and longwave wavelengths. We have used this parameterization in a set of idealized 1-D radiative transfer calculations to investigate the sensitivity of various attributes of sea-salt forcing, including the dependency on sea-salt column loading, effective variance, solar angle, and surface albedo. From these sensitivity tests, it is found that sea-salt forcings for both shortwave and longwave spectra are linearly related to the sea-salt loading for realistic values of loadings. The radiative forcing results illustrate that the shortwave forcing is an order of magnitude greater than the longwave forcing results and opposite in sign, for various loadings. Forcing sensitivity studies show that the influence of effective variance for sea-salt is minor; therefore, only one value of effective variance is used in the parameterization. The dependence of sea-salt forcing with solar zenith angle illustrates an interesting result that sea-salt can generate a positive top-of-the-atmosphere result (i.e. warming) when the solar zenith angle is relatively small (i.e. <30°). Finally, it is found that the surface albedo significantly affects the shortwave radiative forcing, with the forcing diminishing to zero as the surface albedo tends to unity.

Download Full-text