scholarly journals Spectral and Broadband Longwave Downwelling Radiative Fluxes, Cloud Radiative Forcing, and Fractional Cloud Cover over the South Pole

2005 ◽  
Vol 18 (20) ◽  
pp. 4235-4252 ◽  
Author(s):  
Michael S. Town ◽  
Von P. Walden ◽  
Stephen G. Warren
Keyword(s):  
Water Vapor ◽  
Radiative Forcing ◽  
Cloud Cover ◽  
South Pole ◽  
The South ◽  
Near Surface ◽  
Cloud Radiative Forcing ◽  
Clear Sky ◽  
Fractional Cloud ◽  
Visual Observations

Abstract Annual cycles of downwelling broadband infrared radiative flux and spectral downwelling infrared flux were determined using data collected at the South Pole during 2001. Clear-sky conditions are identified by comparing radiance ratios of observed and simulated spectra. Clear-sky fluxes are in the range of 110–125 W m−2 during summer (December–January) and 60–80 W m−2 during winter (April–September). The variability is due to day-to-day variations in temperature, strength of the surface-based temperature inversion, atmospheric humidity, and the presence of “diamond dust” (near-surface ice crystals). The persistent presence of diamond dust under clear skies during the winter is evident in monthly averages of clear-sky radiance. About two-thirds of the clear-sky flux is due to water vapor, and one-third is due to CO2, both in summer and winter. The seasonal constancy of this approximately 2:1 ratio is investigated through radiative transfer modeling. Precipitable water vapor (PWV) amounts were calculated to investigate the H2O/CO2 flux ratio. Monthly mean PWV during 2001 varied from 1.6 mm during summer to 0.4 mm during winter. Earlier published estimates of PWV at the South Pole are similar for winter, but are 50% lower for summer. Possible reasons for low earlier estimates of summertime PWV are that they are based either on inaccurate hygristor technology or on an invalid assumption that the humidity was limited by saturation with respect to ice. The average fractional cloud cover derived from the spectral infrared data is consistent with visual observations in summer. However, the wintertime average is 0.3–0.5 greater than that obtained from visual observations. The annual mean of longwave downwelling cloud radiative forcing (LDCRF) for 2001 is about 23 W m−2 with no apparent seasonal cycle. This is about half that of the global mean LDCRF; the low value is attributed to the small optical depths and low temperatures of Antarctic clouds.

scholarly journals Cloud Cover over the South Pole from Visual Observations, Satellite Retrievals, and Surface-Based Infrared Radiation Measurements

2007 ◽  
Vol 20 (3) ◽  
pp. 544-559 ◽  
Author(s):  
Michael S. Town ◽  
Von P. Walden ◽  
Stephen G. Warren
Keyword(s):  
Radiative Forcing ◽  
Cloud Cover ◽  
Longwave Radiation ◽  
South Pole ◽  
The South ◽  
Cloud Properties ◽  
Satellite Retrievals ◽  
Polar Day ◽  
Very High ◽  
Visual Observations

Abstract Estimates of cloud cover over the South Pole are presented from five different data sources: routine visual observations (1957–2004; Cvis), surface-based spectral infrared (IR) data (2001; CPAERI), surface-based broadband IR data (1994–2003; Cpyr), the Extended Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder (APP-x) dataset (1994–99; CAPP-x), and the International Satellite Cloud Climatology Project (ISCCP) dataset (1994–2003; CISCCP). The seasonal cycle of cloud cover is found to range from 45%–50% during the short summer to a relatively constant 55%–65% during the winter. Relationships between Cpyr and 2-m temperature, 10-m wind speed and direction, and longwave radiation are investigated. It is shown that clouds warm the surface in all seasons, 0.5–1 K during summer and 3–4 K during winter. The annual longwave cloud radiative forcing is 18 W m−2 for downwelling radiation and 10 W m−2 for net radiation. The cloud cover datasets are intercompared during the time periods in which they overlap. The nighttime bias of Cvis is worse than previously suspected, by approximately −20%; CISCCP shows some skill during the polar day, while CAPP-x shows some skill at night. The polar cloud masks for the satellite data reviewed here are not yet accurate enough to reliably derive surface or cloud properties over the East Antarctic Plateau. The best surface-based source of cloud cover in terms of the combination of accuracy and length of record is determined to be Cpyr. The use of the Cpyr dataset for further tests of satellite retrievals and for tests of polar models is recommended.

A Climatology of Midlatitude Continental Clouds from the ARM SGP Central Facility. Part II: Cloud Fraction and Surface Radiative Forcing

2006 ◽  
Vol 19 (9) ◽  
pp. 1765-1783 ◽  
Author(s):  
Xiquan Dong ◽  
Baike Xi ◽  
Patrick Minnis
Keyword(s):  
Water Vapor ◽  
Radiative Forcing ◽  
Cloud Cover ◽  
Surface Albedo ◽  
Cloud Fraction ◽  
Total Cloud Cover ◽  
Cloud Radiative Forcing ◽  
Low Clouds ◽  
High Clouds ◽  
Central Facility

Abstract Data collected at the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility (SCF) are analyzed to determine the monthly and hourly variations of cloud fraction and radiative forcing between January 1997 and December 2002. Cloud fractions are estimated for total cloud cover and for single-layered low (0–3 km), middle (3–6 km), and high clouds (>6 km) using ARM SCF ground-based paired lidar–radar measurements. Shortwave (SW) and longwave (LW) fluxes are derived from up- and down-looking standard precision spectral pyranometers and precision infrared radiometer measurements with uncertainties of ∼10 W m−2. The annual averages of total and single-layered low-, middle-, and high-cloud fractions are 0.49, 0.11, 0.03, and 0.17, respectively. Both total- and low-cloud amounts peak during January and February and reach a minimum during July and August; high clouds occur more frequently than other types of clouds with a peak in summer. The average annual downwelling surface SW fluxes for total and low clouds (151 and 138 W m−2, respectively) are less than those under middle and high clouds (188 and 201 W m−2, respectively), but the downwelling LW fluxes (349 and 356 W m−2) underneath total and low clouds are greater than those from middle and high clouds (337 and 333 W m−2). Low clouds produce the largest LW warming (55 W m−2) and SW cooling (−91 W m−2) effects with maximum and minimum absolute values in spring and summer, respectively. High clouds have the smallest LW warming (17 W m−2) and SW cooling (−37 W m−2) effects at the surface. All-sky SW cloud radiative forcing (CRF) decreases and LW CRF increases with increasing cloud fraction with mean slopes of −0.984 and 0.616 W m−2 %−1, respectively. Over the entire diurnal cycle, clouds deplete the amount of surface insolation more than they add to the downwelling LW flux. The calculated CRFs do not appear to be significantly affected by uncertainties in data sampling and clear-sky screening. Traditionally, cloud radiative forcing includes not only the radiative impact of the hydrometeors, but also the changes in the environment. Taken together over the ARM SCF, changes in humidity and surface albedo between clear and cloudy conditions offset ∼20% of the NET radiative forcing caused by the cloud hydrometeors alone. Variations in water vapor, on average, account for 10% and 83% of the SW and LW CRFs, respectively, in total cloud cover conditions. The error analysis further reveals that the cloud hydrometeors dominate the SW CRF, while water vapor changes are most important for LW flux changes in cloudy skies. Similar studies over other locales are encouraged where water and surface albedo changes from clear to cloudy conditions may be much different than observed over the ARM SCF.

scholarly journals Subgrid-scale variability of clear-sky relative humidity and forcing by aerosol-radiation interactions in an atmosphere model

2017 ◽  
Author(s):  
Paul Petersik ◽  
Marc Salzmann ◽  
Jan Kretzschmar ◽  
Ribu Cherian ◽  
Daniel Mewes ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Radiative Forcing ◽  
Cloud Cover ◽  
Specific Humidity ◽  
Subgrid Scale ◽  
Hygroscopic Growth ◽  
Anthropogenic Aerosols ◽  
Clear Sky ◽  
Fractional Cloud ◽  
Atmosphere Model

Abstract. Atmosphere models with resolutions of several tens of kilometres take subgrid-scale variability of the total specific humidity qt into account by using a uniform probability density function (PDF) to predict fractional cloud cover. However, usually only mean relative humidity RH or mean clear-sky relative humidity RHcls is used to compute hygroscopic growth of soluble aerosol particles. In this study, a stochastic parameterization of subgrid-scale variability of RHcls is applied. For this, we sample the subsaturated part of the uniform RH-PDF from the cloud cover scheme for application in association with the hygroscopic growth parameterization in the ECHAM6-HAM2 atmosphere model. Due to the non-linear dependence of the hygroscopic growth on RH, this causes an increase in aerosol hygroscopic growth. Aerosol optical depth (AOD) increases by a global mean of 0.009 (∼ 7.8 % in comparison to the control run). Especially over the tropics AOD is enhanced with a mean of about 0.013. The ability of the model to simulate AOD is slightly improved with respect to satellite data from MODIS-Aqua. Due to the increase in AOD, net top of the atmosphere clear-sky solar radiation decreases by −0.22 W m−2 (∼ −0.08 %). Finally, the effective radiative forcing due to aerosol-radiation interactions under clear-sky conditions (ERFaricls) changes from −0.29 W m−2 to −0.45 W m−2 by about 57 %. The reason for this very disproportionate effect is that anthropogenic aerosols are disproportionally hygroscopic.

scholarly journals Dry Bias in Satellite-Derived Clear-Sky Water Vapor and Its Contribution to Longwave Cloud Radiative Forcing

2006 ◽  
Vol 19 (21) ◽  
pp. 5570-5580 ◽  
Author(s):  
Byung-Ju Sohn ◽  
Johannes Schmetz ◽  
Rolf Stuhlmann ◽  
Joo-Young Lee
Keyword(s):  
Water Vapor ◽  
Radiative Forcing ◽  
Active Regions ◽  
Tropical Regions ◽  
Cloud Radiative Forcing ◽  
Cloud Data ◽  
Clear Sky ◽  
Cloud Development ◽  
Sampling Problem ◽  
Sky Conditions

Abstract In this paper, the amount of satellite-derived longwave cloud radiative forcing (CRF) that is due to an increase in upper-tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky conditions is assessed. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear-sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear-sky longwave CRF. Water vapor amounts in the 200–500-mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor Microwave Temperature-2 Profiler and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere. It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40%–50% drier over convectively active regions. Results indicate that up to 12 W m−2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.

scholarly journals Surface cloud radiative forcing in the South of Portugal

2013 ◽  
Author(s):  
M. J. Costa ◽  
V. Salgueiro ◽  
D. Santos ◽  
D. Bortoli ◽  
A. M. Silva ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
The South ◽  
Cloud Radiative Forcing

scholarly journals Variability of the Daily-Mean Shortwave Cloud Radiative Forcing at the Surface at a Midlatitude Site in Southwestern Europe

2014 ◽  
Vol 27 (20) ◽  
pp. 7769-7780 ◽  
Author(s):  
Vanda Salgueiro ◽  
Maria João Costa ◽  
Ana Maria Silva ◽  
Daniele Bortoli
Keyword(s):  
Seasonal Variation ◽  
Radiative Forcing ◽  
Quantitative Relationship ◽  
Mean Values ◽  
Cloud Radiative Forcing ◽  
Radiative Forcings ◽  
Black And White ◽  
Clear Sky ◽  
Surface Measurements ◽  
Multifilter Rotating Shadowband Radiometer

Abstract The shortwave cloud radiative forcing is calculated from surface measurements taken in Évora from 2003 to 2010 with a multifilter rotating shadowband radiometer (MFRSR) and with an Eppley black and white pyranometer. A new approach to estimate the clear-sky irradiance based on radiative transfer calculations is also proposed. The daily-mean values of the cloud radiative forcing (absolute and normalized) as well as their monthly and seasonal variabilities are analyzed. The study shows greater variability of radiative forcing during springtime with respect to the other seasons. The mean daily cloudy periods have seasonal variation proportional to the seasonal variation of the cloud radiative forcing, with maximum values also occurring during springtime. The minimum values found for the daily-mean cloud radiative forcing are −139.5 and −198.4 W m−2 for MFRSR and Eppley data, respectively; the normalized values present about 40% of sample amplitude, both for MFRSR and Eppley. In addition, a quantitative relationship between the MFRSR and Eppley cloud radiative forcings applicable to other locations is proposed.

scholarly journals Cloud radiative forcing intercomparison between fully coupled CMIP5 models and CERES satellite data

2014 ◽  
Vol 32 (7) ◽  
pp. 793-807 ◽  
Author(s):  
M. Calisto ◽  
D. Folini ◽  
M. Wild ◽  
L. Bengtsson
Keyword(s):  
Radiative Forcing ◽  
Cloud Cover ◽  
Coupled Model ◽  
Cmip5 Models ◽  
Radiative Fluxes ◽  
Cloud Radiative Forcing ◽  
Summer Hemisphere ◽  
Intercomparison Project ◽  
Annual Means ◽  
The Tropics

Abstract. In this paper, radiative fluxes for 10 years from 11 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and from CERES satellite observations have been analyzed and compared. Under present-day conditions, the majority of the investigated CMIP5 models show a tendency towards a too-negative global mean net cloud radiative forcing (NetCRF) as compared to CERES. A separate inspection of the long-wave and shortwave contribution (LWCRF and SWCRF) as well as cloud cover points to different shortcomings in different models. Models with a similar NetCRF still differ in their SWCRF and LWCRF and/or cloud cover. Zonal means mostly show excessive SWCRF (too much cooling) in the tropics between 20° S and 20° N and in the midlatitudes between 40 to 60° S. Most of the models show a too-small/too-weak LWCRF (too little warming) in the subtropics (20 to 40° S and N). Difference maps between CERES and the models identify the tropical Pacific Ocean as an area of major discrepancies in both SWCRF and LWCRF. The summer hemisphere is found to pose a bigger challenge for the SWCRF than the winter hemisphere. The results suggest error compensation to occur between LWCRF and SWCRF, but also when taking zonal and/or annual means. Uncertainties in the cloud radiative forcing are thus still present in current models used in CMIP5.

scholarly journals Spatial Distribution of Microparticles Within Antarctic Snow-Fall

1982 ◽  
Vol 3 ◽  
pp. 300-306 ◽  
Author(s):  
Lonnie G. Thompson ◽  
Ellen Mosley Thompson
Keyword(s):  
Size Distribution ◽  
Snow Accumulation ◽  
Distribution Data ◽  
South Pole ◽  
The South ◽  
Near Surface ◽  
Ross Ice Shelf ◽  
Particle Accumulation ◽  
Base Camp ◽  
Antarctic Snow

The concentration and size distribution of water insoluble microparticles were measured in 2 332 snow and firn samples from (a) two sites on the Antarctic Peninsula, (b) the Byrd station strain network, West Antarctica, (c) the Q-13 and base camp sites on the Ross Ice Shelf, and (d) the South Pole and Dome C sites in East Antarctica. These near-surface microparticle studies indicate that, while the number of particles per unit volume of sample remains fairly uniform from site to site, the annual particle accumulation is greatest at locations nearest the coast and decreases rapidly with distance inland. The relationship between particle accumulation and distance from the coast is analogous to a relationship between snow accumulation and distance from the coast. Ten times more particles are deposited annually at stations within 50 km of the coast than at the South Pole and Dome C sites. The size distribution data reveal that, with the possible exception of the Q-13 site, the particulates deposited in Antarctica are well-sorted, indicating little contribution from local sources.

scholarly journals Temporal heterogeneity in aerosol characteristics and the resulting radiative impacts at a tropical coastal station – Part 2: Direct short wave radiative forcing

2007 ◽  
Vol 25 (11) ◽  
pp. 2309-2320 ◽  
Author(s):  
S. Suresh Babu ◽  
K. Krishna Moorthy ◽  
S. K. Satheesh
Keyword(s):  
Radiative Forcing ◽  
Monsoon Season ◽  
Short Wave ◽  
Coastal Station ◽  
Near Surface ◽  
Aerosol Radiative Forcing ◽  
Clear Sky ◽  
Synoptic Conditions ◽  
Post Monsoon Season ◽  
Sky Conditions

Abstract. Seasonal distinctiveness in the microphysical and optical properties of columnar and near-surface (in the well mixed region) aerosols, associated with changes in the prevailing synoptic conditions, were delineated based on extensive (spread over 4 years) and collocated measurements at the tropical coastal location, Trivandrum (8.55° N; 76.97° E, 3 m a.m.s.l.), and the results were summarized in Part 1 of this two-part paper. In Part 2, we use these properties to develop empirical seasonal aerosol models, which represent the observed features fairly accurately, separately for winter monsoon season (WMS, December through March), inter-monsoon season (IMS, April and May), summer monsoon season (SMS, June through September) and post monsoon season (PMS, October and November). The models indicate a significant transformation in the aerosol environment from an anthropogenic-dominance in WMS to a natural-dominance in SMS. The modeled aerosol properties are used for estimating the direct, short wave aerosol radiative forcing, under clear-sky conditions. Our estimates show large seasonal changes. Under clear sky conditions, the daily averaged short-wave TOA forcing changes from its highest values during WMS, to the lowest values in SMS; this seasonal change being brought-in mainly by the reduction in the abundance and the mass fraction (to the composite) of black carbon aerosols and of accumulation mode aerosols. The resulting atmospheric forcing varies from the highest, (47 to 53 W m−2) in WMS to the lowest (22 to 26 W m−2) in SMS.

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