Cloud Radiative Forcing at Summit, Greenland

Abstract. The impact of changes in aerosol and cloud droplet concentration (Na and Nd) on the radiative forcing of stratocumulus-topped boundary layers (STBLs) has been widely studied. How these impacts compare to those due to variations in meteorological context has not been investigated in a systematic fashion for non-drizzling overcast stratocumulus. In this study we examine the impact of observed variations in meteorological context and aerosol state on daytime, non-drizzling overcast stratiform evolution, and determine how resulting changes in cloud properties compare. Using large-eddy simulation (LES) we create a model base case of daytime southeast Pacific coastal stratocumulus, spanning a portion of the diurnal cycle (early morning to near noon) and constrained by observations taken during the VOCALS (VAMOS Ocean-Atmosphere-Land Study) field campaign. We perturb aerosol and meteorological properties around this base case to investigate the stratocumulus response. We determine perturbations in the cloud top jumps in potential temperature θ and total water mixing ratio qt from ECMWF Re-analysis Interim data, and use a set of Nd values spanning the observable range. To determine the cloud response to these meteorological and aerosol perturbations, we compute changes in liquid water path (LWP), bulk optical depth (τ) and cloud radiative forcing (CRF). We find that realistic variations in the thermodynamic jump properties can elicit a response in the cloud properties of τ and shortwave (SW) CRF that are on the same order of magnitude as the response found due to realistic changes in aerosol state (i.e Nd). In response to increases in Nd, the cloud layer in the base case thinned due to increases in evaporative cooling and entrainment rate. This cloud thinning somewhat mitigates the increase in τ resulting from increases in Nd. On the other hand, variations in θ and qt jumps did not substantially modify Nd. The cloud layer thickens in response to an increase in the θ jump and thins in response to an increase in the qt jump, both resulting in a τ and SW CRF response comparable to those found from perturbations in Nd. Longwave CRF was not substantially altered by the perturbations we tested. We find that realistic variations in meteorological context can elicit a response in CRF and τ on the same order of magnitude as, and at times larger than, that response found due to realistic changes in aerosol state. We estimate the limits on variability of cloud top jump properties required for accurate observation of aerosol SW radiative impacts on stratocumulus, and find strict constraints: less than 1 K and 1 g kg−1 in the early morning hours, and order 0.1 K and 0.1 g kg−1 close to solar noon. These constraints suggest that accurately observing aerosol radiative impacts in stratocumulus may be challenging as co-variation of meteorological properties may obfuscate aerosol-cloud interactions.

[1] Although shallow cumuli are common over large areas of the globe, their impact on the surface cloud radiative forcing (CRF) has not been carefully evaluated. This study addresses this shortcoming by analyzing data collected during conditions with single-layer shallow cumuli over eight summers (2000 through 2007) at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. During periods with clouds, the average shortwave and longwave CRF at the surface are −45.5 W m−2 (out of 612 W m−2 estimated for clear-sky conditions) and +15.9 W m−2 (out of −105.2 W m−2 estimated for clear-sky conditions), respectively. Instances of cloud-induced enhancement of the shortwave irradiance over that estimated for clear skies are observed approximately 20% of the time and are caused by spatial and temporal inhomogeneity of cumuli. Such enhancement is responsible for occurrences of positive shortwave CRF with instantaneous values as large as +75 W m−2. The total amount of shortwave and longwave energy deposited at the surface over a period of time depends nonlinearly on the fractional sky cover, and the largest values of the deposited energy occur for intermediate cloud amounts between 0.4 and 0.6.

A seasonal analysis of aerosol-cloud-radiation interaction over Indian region during 2000–2017

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.

A dataset of 9 years in duration (2009–17) of clouds and radiation was obtained at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Cloud radiative forcings (CRF) were derived from the dataset and related to cloud cover and temperature. Also, the data were compared with RCM output. Results indicate that there is a seasonal cycle (i.e., winter, spring, summer, and autumn) in longwave (CRF-LW: 48.3, 34.4, 30.8, and 38.7 W m−2) and shortwave (CRF-SW: −23.6, −60.9, −67.8, and −32.9 W m−2) forcings at CESAR. Total CRF is positive in winter and negative in summer. The RCM has a cold bias with respect to the observations, but the model CRF-LW corresponds well to the observed CRF-LW as a result of compensating errors in the difference function that makes up the CRF-LW. The absolute value of model CRF-SW is smaller than the observed CRF-SW in summer, mostly because of albedo differences. The majority of clouds from above 2 km are present at the same time as low clouds, so the higher clouds have only a small impact on CRF whereas low clouds dominate their values. CRF-LW is a function of fractional cloudiness. CRF-SW is also a function of fractional cloudiness, if the values are normalized by the cosine of solar zenith angle. Expressions for CRF-LW and CRF-SW were derived as functions of temperature, fractional cloudiness, and solar zenith angle, indicating that CRF is the largest when fractional cloudiness is the highest but is also large for low temperature and high sun angle.

[1] A new hybrid classification algorithm to objectively identify Deep Convective Systems (DCSs) in radar and satellite observations has been developed. This algorithm can classify the convective cores (CC), stratiform rain (SR) area and nonprecipitating anvil cloud (AC) from the identified DCSs through an integrative analysis of ground-based scanning radar and geostationary satellite data over the Southern Great Plains. In developing the algorithm, AC is delineated into transitional, thick, and thin components. While there are distinct physical/dynamical differences among these subcategories, their top-of-atmosphere (TOA) radiative fluxes are not significantly different. Therefore, these anvil subcategories are grouped as total anvil, and the radiative impact of each DCS component on the TOA radiation budget is quantitatively estimated. We found that more DCSs occurred during late afternoon, producing peak AC fraction right after sunset. AC covers 3 times the area of SR and almost an order of magnitude larger than CC. The average outgoing longwave (LW) irradiances are almost identical for CC and SR, while slightly higher for AC. Compared to the clear-sky average, the reflected shortwave (SW) fluxes for the three DCS components are greater by a factor of 2–3 and create a strong cooling effect at TOA. The calculated SW and LW cloud radiative forcing (CRF) of AC contribute up to 31% of total NET CRF, while CC and SR contribute only 4 and 11%, respectively. The hybrid classification further lays the groundwork for studying the life cycle of DCS and improvements in geostationary satellite IR-based precipitation retrievals.

This study explores connections between process-level modeling of convection and global climate model (GCM) simulated clouds and cloud feedback to global warming through a set of perturbed-physics and perturbed sea surface temperature experiments. A bulk diagnostic approach is constructed, and a set of variables is derived and demonstrated to be useful in understanding the simulated relationship. In particular, a novel bulk quantity, the convective precipitation efficiency or equivalently the convective detrainment efficiency, is proposed as a simple measure of the aggregated properties of parameterized convection important to the GCM simulated clouds. As the convective precipitation efficiency increases in the perturbed-physics experiments, both liquid and ice water path decrease, with low and middle cloud fractions diminishing at a faster rate than high cloud fractions. This asymmetry results in a large sensitivity of top-of-atmosphere net cloud radiative forcing to changes in convective precipitation efficiency in this limited set of models. For global warming experiments, intermodel variations in the response of cloud condensate, low cloud fraction, and total cloud radiative forcing are well explained by model variations in response to total precipitation (or detrainment) efficiency. Despite significant variability, all of the perturbed-physics models produce a sizable increase in precipitation efficiency to warming. A substantial fraction of the increase is due to its convective component, which depends on the parameterization of cumulus mixing and convective microphysical processes. The increase in convective precipitation efficiency and associated change in convective cloud height distribution owing to warming explains the increased cloud feedback and climate sensitivity in recently developed Geophysical Fluid Dynamics Laboratory GCMs. The results imply that a cumulus scheme using fractional removal of condensate for precipitation and inverse calculation of the entrainment rate tends to produce a lower climate sensitivity than a scheme using threshold removal for precipitation and the entrainment rate formulated inversely dependent on convective depth.

Effects of Clouds and Aerosols on Surface Radiation Budget Inferred from DOE AMF at Shouxian, China

Measurements and models show that enhanced aerosol concentrations can modify macro- and micro-physical properties of clouds. Here, we examine the effect of aerosols on continental mesoscale convective cloud systems during the Indian summer monsoon and find that these aerosol–cloud interactions have a net cooling effect at the surface and the top-of-atmosphere. Long-term (2002–2016) satellite data provide evidence of aerosol-induced cloud invigoration effect (AIvE) during the Indian summer monsoon. The AIvE leads to enhanced formation of thicker stratiform anvil clouds at higher altitudes. These AIvE-induced stratiform anvil clouds are also relatively brighter because of the presence of smaller sized ice particles. As a result, AIvE-induced increase in shortwave cloud radiative forcing is much larger than longwave cloud radiative forcing leading to the intensified net cooling effect of clouds over the Indian summer monsoon region. Such aerosol-induced cooling could subsequently decrease the surface diurnal temperature range and have significant feedbacks on lower tropospheric turbulence in a warmer and polluted future scenario.

Abstract. Surface radiative cooling in polar regions can generate persistent stability in the atmospheric boundary layer. Stable layers below clouds can decouple the cloud layer from the near-surface environment. Under these conditions, surface aerosol measurements are not necessarily representative of the near-cloud or intra-cloud aerosol populations. To better understand the variability in the vertical structure of aerosol properties over the central Greenland Ice Sheet, in situ measurements of aerosol particle size distributions up to cloud base were made at Summit Station in July and August 2023. These measurements identified distinct vertical aerosol layers between the surface- and cloud-base-associated thermodynamic decoupling layers. Such decoupling layers occur 49 % of the time during the summer in central Greenland, suggesting that surface aerosol measurements are insufficient for describing the cloud-relevant aerosol population half of the time. Experience during this first measurement season demonstrated the ability of a tethered-balloon platform to operate effectively under icing conditions and at low surface pressure (< 680 hPa). The results presented here illustrate the value of vertically resolved in situ measurements of aerosol properties in developing a nuanced understanding of the aerosol effects on cloud properties in polar regions.

The influence of anthropogenic aerosols, in the form of ship exhaust effluent, on the microphysics and radiative properties of marine stratocumulus is studied using data gathered from the U.K. Met. Office C-130 and the University of Washington C-131A aircraft during the Monterey Area Ship Track (MAST) experiment in 1994. During the period of MAST, stratocumulus clouds were studied during 11 flights and a wide range of levels of background pollution was observed. The impact of the aerosol emitted from the ships was found to be very dependent on the background cloud microphysical conditions. In clouds of continental influence, the susceptibility of the cloud to further aerosol emissions was low, with a correspondingly weak microphysics and radiation signature in the ship tracks. In clean clouds, changes in droplet concentration of a factor of 2, and reductions in droplet size of up to 50%, were measured. These changes in the microphysics had significant impacts on the cloud radiative forcing. Furthermore, as a result of the cloud droplet size being reduced, in some cases the drizzle was suppressed in the clean clouds, resulting in an increase in liquid water path in the polluted ship track environment. The impact of this combined change in liquid water path and droplet radius was to increase the cloud radiative forcing by up to a factor of 4.

We compare radiation budgets derived from different Earth Radiation Budget Experiment (ERBE) archives over the tropical oceans (30S and 30N) from 1985 to 1989. Two ERBE data sets are used. One is taken from the Earth Radiation Budget Satellite (ERBS), and the other from the combined ERBS, National Oceanic and Atmospheric Administration (NOAA) 9, and NOAA 10 satellites. The domain‐mean all‐sky outgoing longwave radiation (OLR) derived from the combined data set shows a notable change in early 1987 when NOAA 10 replaced NOAA 9. This change is also found in longwave (LW) cloud radiative forcing (CRF), all‐sky shortwave (SW) radiation, and SWCRF. The ERBS, however, does not show such changes.We also examine the sensitivity of cloud–radiation interaction to the sea surface temperature (SST) of the tropical oceans. In general, each component of radiative feedbacks derived from the two ERBE data have the same sign, although they show a certain degree of discrepancy in the magnitude. The discrepancy is more notable for averaged quantities over the entire tropical oceans, particularly over the subtropics where convective activities are relatively weak. The combined data show a larger sensitivity of LWCRF and SWCRF to SST than those of the ERBS, consistent with the above results. The response of clouds to an increase in SST has a net cooling effect when using the combined data but has a net heating effect when using the ERBS data (−0.80 W m−2 K−1 versus 0.48 W m−2 K−1). Most of the discrepancies of the net CRF between the two ERBE data sets can be accounted for by the difference in the sensitivity of all‐sky OLR (4.52 W m−2 K−1 versus 1.73 W m−2 K−1). Copyright © 2002 Royal Meteorological Society.

The Greenland ice sheet (GrIS) has been losing mass at an accelerating rate in recent decades due to warming, and understanding the underlying mechanisms, such as the impacts of clouds, is essential. Using spaceborne data, this study investigates the spatial distribution of ice clouds and liquid-bearing clouds (LBCs) over the GrIS and their surface radiative forcing effects during summer daytime from 2006 to 2017, along with their characteristics during the North Atlantic Oscillation (NAO) events. Due to the perennial high-albedo surface, both ice and LBCs have a less important shortwave radiative cooling effect than in other environments. Based on the spatial variation pattern of clouds with the NAO index, the GrIS can be divided into three regions: the western, central, and eastern GrIS. During the positive NAO, the westerly wind strengthens in the western region, which causes the fraction of both ice clouds and LBCs to increase, and the cloud radiative effect at the surface increases by 2.07 W m−2; the temperature decreases in the central region, the fraction of ice clouds increases, the fraction of LBCs decreases, and the net radiative forcing is −2.05 W m−2; and sinking airflow is generated in the eastern region, both ice cloud and LBCs decrease, and the net cloud radiative effect at the surface is −1.34 W m−2. The spatial and temporal variations in clouds in different phases over the GrIS are closely related to the NAO, and the response of clouds to changes in the atmospheric circulation field during the NAO varies in different regions of the GrIS. Significance Statement This study investigates the associations between the North Atlantic Oscillation and the spatiotemporal variations in clouds, including both cloud fraction and cloud phases, and further examines the impacts of these changes on the surface radiation balance. The findings can help us improve our understanding of cloud variability and the corresponding influence on surface radiation over the GrIS, which are essential for better prediction of ice coverage over this region and for more efficient protection of ecosystems located there.

The Yangtze River Valley (YRV) experienced an unprecedented heatwave in midsummer of 2022. Still, the detailed physical processes involved in the influence of abnormal large-scale atmospheric circulation on the heatwave remain unexplored. Here, we show that the positive meridional gradient of anomalous atmospheric moisture at the middle-lower troposphere and associated extreme dry air advection over the YRV are vital prerequisites for forming the 2022 YRV heatwave. The 2022 YRV heatwave is dominated by interannual variability, contributing 72.7% to the total temperature anomalies. Diagnosis of the surface heat budget equation indicates that the surface cloud radiative forcing is the most critical process in driving the 2022 YRV heatwave, which is dominated by the positive surface short-wave cloud radiative forcing associated with the suppressed precipitation and the middle-low clouds. The suppressed precipitation is induced by the vertical dynamical processes of anomalous moisture advection caused by the abnormal descending flows over the YRV, which are driven by the negative advection of anomalous latent heat energy by climatological meridional wind (anomalous dry air advection) according to the atmospheric moist static energy equation. Simulations from the Lagrangian model FLEXPART further indicate that the moisture anomaly over the north of YRV mainly originated from the surface evaporation in the YRV, implying that there is a positive land-air feedback during the life cycle of the YRV heatwave. Our study enriches the mechanism understanding of the 2022 YRV heatwave from the perspective of surface energy budget and land-air feedback.

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear‐sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid‐latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid‐latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than 10 W m−2.