Longwave Cloud Radiative Forcing Research Articles

Towards Unified Online-Coupled Aerosol Parameterization for the Brazilian Global Atmospheric Model (BAM): Aerosol–Cloud Microphysical–Radiation Interactions

In this work, we report the ongoing implementation of online-coupled aerosol–cloud microphysical–radiation interactions in the Brazilian global atmospheric model (BAM) and evaluate the initial results, using remote-sensing data for JFM 2014 and JAS 2019. Rather than developing a new aerosol model, which incurs significant overheads in terms of fundamental research and workforce, a simplified aerosol module from a preexisting global aerosol–chemistry–climate model is adopted. The aerosol module is based on a modal representation and comprises a suite of aerosol microphysical processes. Mass and number mixing ratios, along with dry and wet radii, are predicted for black carbon, particulate organic matter, secondary organic aerosols, sulfate, dust, and sea salt aerosols. The module is extended further to include physically based parameterization for aerosol activation, vertical mixing, ice nucleation, and radiative optical properties computations. The simulated spatial patterns of surface mass and number concentrations are similar to those of other studies. The global means of simulated shortwave and longwave cloud radiative forcing are comparable with observations with normalized mean biases ≤11% and ≤30%, respectively. Large positive bias in BAM control simulation is enhanced with the inclusion of aerosols, resulting in strong overprediction of cloud optical properties. Simulated aerosol optical depths over biomass burning regions are moderately comparable. A case study simulating an intense biomass burning episode in the Amazon is able to reproduce the transport of smoke plumes towards the southeast, thus showing a potential for improved forecasts subject to using near-real-time remote-sensing fire products and a fire emission model. Here, we rely completely on remote-sensing data for the present evaluation and restrain from comparing our results with previous results until a complete representation of the aerosol lifecycle is implemented. A further step is to incorporate dry deposition, in-cloud and below-cloud scavenging, sedimentation, the sulfur cycle, and the treatment of fires.

Distinct response of near surface air temperature to clouds in North China

AbstractUsing the daily 2 m maximum temperature (Tmax), 2 m minimum temperature (Tmin) and cloud cover data measured at ground sites of the China Meteorological Administration in North China from 2000 to 2017, this study investigates the influence of clouds on the daily temperature range (DTR) defined as the difference between Tmax and Tmin. As expected, the cloud cover shows the similar averaged spatial distribution and monthly variation with Tmin. Surprisingly, it also shows the similar average spatial distribution and monthly variation with Tmax, suggesting the more important roles of regions (latitude) and seasons associated with the variations of land surface temperature, which is further related to solar radiation absorbed and surface heat capacity. By comparing monthly variations of temperature between cloudy and clear skies, we find that clouds can weaken Tmax and increase Tmin, and thus decrease DTR. As a result, the spatial distribution of DTR is opposite to the cloud cover. The clouds have relatively stronger impact on Tmin and DTR over mountain region, which is most likely caused by the stronger longwave cloud radiative forcing associated with higher cloud tops over the mountain region.

A study on the anomalous TOA net radiative warming by clouds in a sub-region within the Indian summer monsoon region

Indian summer monsoon clouds generally exert a net radiative cooling at top of atmosphere. In contrast, clouds over a sub-region comprising parts of South peninsular India, Sri Lanka and adjoining Bay of Bengal (SISB) inside the Indian monsoon region exert a net radiative warming. In this work, an attempt is made to understand the reasons behind the anomalous radiative warming found over SISB. Ten-year (2000–2009) top of atmosphere radiative flux data from Clouds and the Earth’s Radiant Energy System payloads onboard Aqua and Terra satellites and cloud data from International Satellite Cloud Climatology Project during the peak Indian summer monsoon season of July–August are analyzed to understand the underlying causes. Top of Atmosphere net radiative forcing is positive and as high as ∼15 Wm−2 over SISB during the peak Indian summer monsoon season.Cloud cover amounts over SISB with net radiative warming and north Bay of Bengal with net radiative cooling are compared. Cloud cover amounts of high-level cirrostratus and deep convective cumulonimbus clouds are less by 65% and 87% respectively over SISB when compared to north Bay of Bengal. SISB is located on the leeward side of the Western Ghats mountain chain with descending motion. Adiabatic compression and associated warming of descending air leads to dehydration of air parcel. The upper tropospheric tropical easterly jet sweeps the deep convective cloud tops of the Indian monsoon and advect them to large distance as upper level thin cirrus family of clouds. When these clouds reach SISB with descending motion, they thin out by melting and evaporation or sublimation as the air mass is compressed and heated adiabatically. Over SISB, pressure vertical velocity at 500 hPa is moderately inversely correlated to cloud cover amount of cirrostratus and cumulonimbus clouds. Cloud cover amount of these two clouds is significantly correlated to shortwave cloud radiative forcing and longwave cloud radiative forcing over SISB. When cloud cover amount of cirrostratus and cumulonimbus clouds are low (<∼15% and <∼5% respectively), magnitude of longwave cloud radiative forcing is mostly higher than the magnitude of shortwave cloud radiative forcing which results in net radiative warming. Reduction in cloud cover amount of cirrostratus and cumulonimbus clouds by subsidence appears to be the cause behind the observed warming over SISB. When the magnitude of shortwave cloud radiative forcing is below (above) ∼55 Wm−2, longwave cloud radiative forcing is more (less) than the magnitude of shortwave cloud radiative forcing which results in net warming (cooling). Net radiative warming is found in 65% of grid points over SISB.

Influence of Thermodynamic State Changes on Surface Cloud Radiative Forcing in the Arctic: A Comparison of Two Approaches Using Data From AFLUX and SHEBA

AbstractThe cloud radiative forcing (CRF) quantifies the warming or cooling effects of clouds. To derive the CRF, reference values of net (downward minus upward) irradiances in cloud‐free conditions are required. There are two groups of techniques to estimate these reference values; one is based on radiative transfer modeling, and a second group uses measurements in cloud‐free situations. To compare both approaches, we first look at a case study from the airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX) campaign, where a moving cloud field with a sharp edge separating a cloudy boundary layer from an adjacent evolving cloud‐free area was probed. These data enabled the quantification of the impact of changing atmospheric and surface properties relevant for the reference net irradiances in cloud‐free conditions. The systematically higher surface albedo below clouds compared to cloud‐free conditions, results in a 11 W·m−2 smaller shortwave cooling effect by clouds estimated from the radiative transfer approach compared to the measurement‐based one. Due to the transition of thermodynamic parameters between the cloudy and cloud‐free atmospheric states, a 20 W·m−2 stronger warming effect is estimated by the radiative transfer approach. In a second step, radiative transfer simulations based on radiosoundings from the Surface Heat Budget of the Arctic Ocean campaign are used to quantify the impact of the vertical profiles of thermodynamic properties on the CRF. The largest difference between the longwave CRF estimated by the two methods is found in autumn with up to 25 W·m−2.

Impacts of Greenland Block Location on Clouds and Surface Energy Fluxes Over the Greenland Ice Sheet

AbstractClouds and blocking activity have been implicated as causes of increased Greenland Ice Sheet (GrIS) melt in the 21st century. Although Greenland blocks (i.e., long‐lasting, mostly stationary anticyclones) generally reduce cloud cover and move warm air over Greenland, the elevated GrIS perturbs air and moisture transport in complex ways, implying a need to better understand how blocks affect cloud and surface energy flux anomaly patterns. In this study, we use a combination of daily MODIS cloud data and meteorological and energy flux data from MERRA‐2 reanalysis to better understand how Greenland block location, separated into four equal‐area quadrants, affects regional cloud and surface energy flux spatial patterns in the summer months of 2002–2018. Overall, cloud fraction and cloud water path reductions are approximately four times greater during northern block days than southern block days. Net cloud radiative forcing anomalies are negative for all Greenland block locations because negative longwave cloud radiative forcing anomalies exceed positive shortwave cloud radiative forcing changes. However, greater cloud cover reductions during northern block days produce more negative net cloud radiative forcing anomalies than southern block days. Greenland‐average (i.e., latitude‐weighted average of all GrIS grids) net surface energy flux anomalies range from +7 to +12W/m2 for all block quadrants. While net shortwave energy anomalies dominate the total surface energy response during western Greenland block days, sensible heating is responsible for approximately half of positive total surface energy change during eastern Greenland block days.

Evaluation of climate simulations produced with the Brazilian global atmospheric model version 1.2

This paper presents an evaluation of climate simulations produced by the Brazilian Global Atmospheric Model version 1.2 (BAM-1.2) of the Center for Weather Forecast and Climate Studies (CPTEC). The model was run over the 1975–2017 period at two spatial resolutions, corresponding to ~ 180 and ~ 100 km, both with 42 vertical levels, following most of the Atmospheric Model Intercomparison Project (AMIP) protocol. In this protocol, observed sea surface temperatures (SSTs) are used as boundary conditions for the atmospheric model. Four ensemble members were run for each of the two resolutions. A series of diagnostics was computed for assessing the model’s ability to represent the top of the atmosphere (TOA) radiation, atmospheric temperature, circulation and precipitation climatological features. The representation of precipitation interannual variability, El Nino-Southern Oscillation (ENSO) precipitation teleconnections, the Madden and Julian Oscillation (MJO) and daily precipitation characteristics was also assessed. The model at both resolutions reproduced many observed temperature, atmospheric circulation and precipitation climatological features, despite several identified biases. The model atmosphere was found to be more transparent than the observations, leading to misrepresentation of cloud-radiation interactions. The net cloud radiative forcing, which produces a cooling effect on the global mean climate at the TOA, was well represented by the model. This was found to be due to the compensation between both weaker longwave cloud radiative forcing (LWCRF) and shortwave cloud radiative forcing (SWCRF) in the model compared to the observations. The model capability to represent inter-annual precipitation variability at both resolutions was found to be linked to the adequate representation of ENSO teleconnections. However, the model produced weaker than observed convective activity associated with the MJO. Light daily precipitation over the southeast of South America and other climatologically similar regions was diagnosed to be overestimated, and heavy daily precipitation underestimated by the model. Increasing spatial resolution helped to slightly reduce some of the diagnosed biases. The performed evaluation identified model aspects that need to be improved. These include the representation of polar continental surface and sea ice albedo, stratospheric ozone, low marine clouds, and daily precipitation features, which were found to be larger and last longer than the observed features.

Direct observations of the multi-year seasonal mean diurnal variations of TOA cloud radiative forcing over tropics using Megha-Tropiques-ScaRaB/3

Diurnal variation of cloud radiative forcing (CRF) is a major factor that controls the global radiation balance. This study presents multi-year seasonal mean diurnal variations of longwave cloud radiative forcing (LWCRF) and daytime shortwave cloud radiative forcing (SWCRF) at the top of atmosphere over tropics, derived from the broadband radiation measurements made by ScaRaB/3 onboard the low-inclination Megha-Tropiques satellite. The largest LWCRF (60–80 Wm−2) occurs over the oceanic regions of the east equatorial Indian Ocean and the western Pacific during all seasons, as well as the South Pacific Convergence Zone, the northeast Bay of Bengal, Amazon region, central and southern Africa and north Indian landmass (monsoon trough) during the local summer. Diurnal variations of 15–25 Wm−2 in LWCRF (20–35% of the mean) are observed with peak values occurring at 18–21 local time (LT) over continents and 00–06 LT over oceans. The minimum LWCRF occurs at 09–12 LT throughout the tropics. Over convective regions, SWCRF maximizes at 12–15 LT (− 220 to − 300 Wm−2) and has a higher magnitude over continents due to early convection occurrence, indicating the importance of diurnal phase. Certain specific features including the CRF associated with the double inter-tropical convergence zone, day-night changes in net CRF, and the effect of El Ni $$\stackrel{\sim }{\mathrm{n}}$$ o on CRF are also presented. The net CRF and its zonal variations are strikingly similar during the normal and El Ni $$\stackrel{\sim }{\mathrm{n}}$$ o periods because the changes in LWCRF and SWCRF are mutually compensated.

Early predictors of seasonal Arctic sea-ice volume loss: the impact of spring and early-summer cloud radiative conditions

AbstractClouds play an important role in the Arctic surface radiative budget, impacting the seasonal evolution of Arctic sea-ice cover. We explore the large-scale impacts of springtime and early summer (March through July) cloud and radiative fluxes on sea ice by comparing these fluxes to seasonal ice volume losses over the central Arctic basin, calculated for available observational years 2004–2007 (ICESat) and 2011–2017 (CryoSat-2). We also supplement observation data with sea-ice volume computed from the Pan-Arctic Ice–Ocean Modeling and Assimilation System (PIOMAS) during summer months. We find that the volume of sea ice lost over the melt season is most closely related to observed downwelling longwave radiation in March and early summer (June and July) longwave cloud radiative forcing, which together explain a large fraction of interannual variability in seasonal sea-ice volume loss (R2= 0.71,p= 0.007). We show that downwelling longwave fluxes likely impact the timing of melt onset near the sea-ice edge, and can limit the magnitude of ice thickening from March to April. Radiative fluxes in June and July are likely critical to seasonal volume loss because modeled data show the greatest ice volume reductions occur during these months.

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

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.

Responses of Cloud-Radiative Forcing to Strong El Niño Events over the Western Pacific Warm Pool as Simulated by CAMS-CSM

Cloud-radiative forcing (CRF) at the top of the atmosphere (TOA) over the western Pacific warm pool (WP) shows unique characteristics in response to El Nino events. In this region, the responses of CRF to El Nino events have been a useful metric for evaluating climate models. Satellite data are used to analyze the CRF anomalies to El Nino events simulated by the new and old versions of the Climate System Model of the Chinese Academy of Meteorological Sciences (CAMS-CSM), which has participated in the Atmospheric Model Intercomparison Project (AMIP). Here, simulations for super El Nino years, El Nino years, and normal years are compared with observations. The results show that the mean values of both longwave CRF (LWCRF) and shortwave CRF (SWCRF) in CAMS-CSM are weaker than the observations for each category of El Nino events. Compared with the old version of CAMS-CSM, the decrease in LWCRF during El Nino events is well simulated by the new version of CAMS-CSM. However, both new and old models cannot reproduce the anomalous SWCRF in El Nino events. The biases in the CRF response to El Nino events are attributed to the biases in the cloud vertical structure because of a weaker crash of the Walker circulation in CAMS-CSM. Due to the modification of the conversion rate from cloud droplets to raindrops in the cumulus convection scheme, the new version of CAMS-CSM has better CRF skills in normal years, but biases in El Nino events still exist in the new version. Improving the response of the Walker circulation to El Nino events is key to higher skills in simulating the cloud radiative responses.

How changing cloud water to rain conversion profile impacts on radiation and its linkage to a better Indian summer monsoon rainfall simulation

Effect of modifying cloud water to rain conversion factor above freezing level on radiation flux is studied. The linkage of cloud-radiation interaction with a better Indian summer monsoon (ISM) rainfall simulation has also been investigated. Two simulations were performed with Climate Forecast System version 2 (CFSv2), one with default revised simplified Arakawa-Schubert (RSAS) convection scheme and another with modified RSAS (RSAS-mod) in which the cloud water to precipitation conversion factor is revised. It is observed that both top of the atmosphere (TOA) and surface energy imbalances are improved in the RSAS-mod simulation. The major contribution seems to come from the enhanced reflected shortwave radiation at the TOA. In RSAS-mod, more shortwave radiation is getting reflected at the TOA and thus less shortwave is available to reach at the surface. It has happened as a consequence of enhanced planetary albedo in RSAS-mod. It is also observed that high cloud cover has increased in RSAS-mod and there is a decrease in low cloud cover. As a result of increased high cloud cover, outgoing longwave radiation has decreased significantly in RSAS-mod. It is also found from the multivariate empirical orthogonal function (MV-EOF) analysis that the composite of longwave cloud radiative forcing corresponding to phases 4 and 5 of boreal summer intraseasonal oscillation (BSISO), which signifies the active phase of ISM, is better simulated in RSAS-mod. This has helped to simulate a better seasonal rainfall.

Impact of Subgrid Variation of Water Vapor on Longwave Radiation in a General Circulation Model

AbstractMost general circulation models compute radiation fluxes by assuming that water vapor is uniform within individual grid layers, which leads to an underestimation of satellite‐observed longwave (LW) cloud radiative forcing (LWCF). To fix this problem, we calculated water vapor content separately for clear and cloudy portions and used them to compute LW radiation. The impacts of this modification were examined by comparing two global simulations with and without the modification (NEW and OLD, respectively). Global‐annual mean LWCF from NEW was 1.8 W m−2 higher than that of OLD, thus remedying a long‐standing negative bias of LWCF. This improvement is a combined result of more clear‐sky and less all‐sky upward LW flux at the top of the atmosphere than OLD. Large increases in LWCF and clear‐sky LW flux occurred in the tropical deep convection and midlatitude storm track regions where upper‐ and middle‐level clouds are abundant. Although only the LW radiation scheme was modified, global‐annual mean shortwave cloud radiative forcing also increased, particularly in the vicinity of the eastern subtropical marine stratocumulus decks through radiative feedback processes. With this improved treatment, it may be possible to tune general circulation models in a more flexible and physical way without introducing compensating errors.

Cloud Radiative Feedbacks during the ENSO Cycle Simulated by CAMS-CSM

This study evaluated the simulated cloud radiative feedbacks (CRF) during the El Nino–Southern Oscillation (ENSO) cycle in the latest version of the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM). We conducted two experimental model simulations: the Atmospheric Model Intercomparison Project (AMIP), forced by the observed sea surface temperature (SST); and the preindustrial control (PIcontrol), a coupled run without flux correction. We found that both the experiments generally reproduced the observed features of the shortwave and longwave cloud radiative forcing (SWCRF and LWCRF) feedbacks. The AMIP run exhibited better simulation performance in the magnitude and spatial distribution than the PIcontrol run. Furthermore, the simulation biases in SWCRF and LWCRF feedbacks were linked to the biases in the representation of the corresponding total cloud cover and precipitation feedbacks. It is interesting to further find that the simulation bias originating in the atmospheric component was amplified in the PIcontrol run, indicating that the coupling aggravated the simulation bias. Since the PIcontrol run exhibited an apparent mean SST cold bias over the cold tongue, the precipitation response to the SST anomaly (SSTA) changes during the ENSO cycle occurred towards the relatively warmer western equatorial Pacific. Thus, the corresponding cloud cover and CRF shifted westward and showed a weaker magnitude in the PIcontrol run versus observational data. In contrast, the AMIP run was forced by the observational SST, hence representing a more realistic CRF. Our results demonstrate the challenges of simulating CRF in coupled models. This study also underscores the necessity of realistically representing the climatological mean state when simulating CRF during the ENSO cycle.

LASG Global AGCM with a Two-moment Cloud Microphysics Scheme: Energy Balance and Cloud Radiative Forcing Characteristics

Cloud dominates influence factors of atmospheric radiation, while aerosol-cloud interactions are of vital importance in its spatiotemporal distribution. In this study, a two-moment (mass and number) cloud microphysics scheme, which significantly improved the treatment of the coupled processes of aerosols and clouds, was incorporated into version 1.1 of the IAP/LASG global Finite-volume Atmospheric Model (FAMIL1.1). For illustrative purposes, the characteristics of the energy balance and cloud radiative forcing (CRF) in an AMIP-type simulation with prescribed aerosols were compared with those in observational/reanalysis data. Even within the constraints of the prescribed aerosol mass, the model simulated global mean energy balance at the top of the atmosphere (TOA) and at the Earth’s surface, as well as their seasonal variation, are in good agreement with the observational data. The maximum deviation terms lie in the surface downwelling longwave radiation and surface latent heat flux, which are 3.5 W m-2 (1%) and 3 W m-2 (3.5%), individually. The spatial correlations of the annual TOA net radiation flux and the net CRF between simulation and observation were around 0.97 and 0.90, respectively. A major weakness is that FAMIL1.1 predicts more liquid water content and less ice water content over most oceans. Detailed comparisons are presented for a number of regions, with a focus on the Asian monsoon region (AMR). The results indicate that FAMIL1.1 well reproduces the summer-winter contrast for both the geographical distribution of the longwave CRF and shortwave CRF over the AMR. Finally, the model bias and possible solutions, as well as further works to develop FAMIL1.1 are discussed.

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

The present study uses 18 years (March 2000–May 2017) of satellite–derived relevant parameters to examine the impact of aerosols on cloud properties, radiative fluxes and ACI (aerosol-cloud interaction) over Indian region. The study includes consideration of shortwave cloud radiative forcing (SWCRF), longwave cloud radiative forcing (LWCRF) and net cloud radiative forcing (NetCRF) from CERES (Clouds and Earth's Radiant Energy System). Also, aerosol optical depth (AOD) and cloud properties such as ice/liquid CER (Cloud Effective Radius), CF (Cloud Fraction), COD (Cloud Optical Depth), CTP (Cloud Top Pressure) and ice/liquid CWP (Cloud Water Path; i.e., ICWP and LCWP) are also considered. Moderate to high aerosol loading and the significant increasing trend of AOD is observed over several parts of Indian region depending upon the seasons. CF is found to be moderate to higher during monsoon months with increasing trend over several parts of the country. Optically thicker high-level clouds have low SWCRF value; whereas, middle-level and low-level clouds have low to moderate SWCRF value. In the majority of cases, Twomey effect is observed whereas in some scenarios Anti-Twomey effect is seen.

Long-term cloud fraction biases in CMIP5 GCMs over India during monsoon season

Using 24 years of cloud fraction (CF) data from the International Satellite Cloud Climatology Project (ISCCP) observations and their corresponding simulators in general circulation models (GCMs) from the Coupled Model Intercomparison Project phase 5 (CMIP5), we have analyzed cloud biases and their role on radiation over the Indian region (65–100° E and 5–40° N) for the monsoon season of June to September. The present study reports the spatial patterns of CFs and their biases in GCMs compared to observations. It is found that the simulated CFs are highly underestimated up to ~ 40%. Mean of total CF from ISCCP observations is 75% with at least 10% difference with simulated CFs. For high-topped clouds, this difference is about 3–4%. Except for high-topped clouds, other cloud types are not simulated realistically by CMIP5 models used in this study. Further, we investigated the individual cloud types classified based on cloud optical depth and cloud top pressure. We found that, in general, individual cloud types are poorly simulated by models, although some (Max Planck Institute Earth System Model, Low Resolution and Hadley Centre Global Environmental Model, version 2, Earth System) models convincingly simulate high-topped thin clouds. To assess the impact of cloud biases on the simulated radiative forcings, we studied shortwave and longwave cloud radiative forcings from CERES (Clouds and the Earth’s Radiant Energy System) observations and CMIP5 GCMs. It is noticed that the spatial patterns of biases in radiative forcings are similar to the patterns of biases in CFs for high-topped clouds, specifically over the oceanic regions. We find that the biases in cloud radiative forcings could potentially be caused due to the inefficacy of CMIP5 models in simulating high-topped anvil clouds (high-topped cirrus/stratocirrus clouds). The present study confirms that the uncertainty in simulating cloud fractions over the Indian region is still a prominent issue to be addressed in general circulation models.

Suppression of Cold Weather Events over High-Latitude Continents in Warm Climates

Recent studies, using Lagrangian single-column atmospheric models, have proposed that in warmer climates more low clouds would form as maritime air masses advect into Northern Hemisphere high-latitude continental interiors during winter (DJF). This increase in low cloud amount and optical thickness could reduce surface radiative cooling and suppress Arctic air formation events, partly explaining both the warm winter high-latitude continental interior climate and frost-intolerant species found there during the Eocene and the positive lapse-rate feedback in future Arctic climate change scenarios. Here the authors examine the robustness of this low-cloud mechanism in a three-dimensional atmospheric model that includes large-scale dynamics. Different warming scenarios are simulated under prescribed CO2 and sea surface temperature, and the sensitivity of winter temperatures and clouds over high-latitude continental interior to mid- and high-latitude sea surface temperatures is examined. Model results show that winter 2-m temperatures on extreme cold days increase about 50% faster than the winter mean temperatures and the prescribed SST. Low cloud fraction and surface longwave cloud radiative forcing also increase in both the winter mean state and on extreme cold days, consistent with previous Lagrangian air-mass studies, but with cloud fraction increasing for different reasons than proposed by previous work. At high latitudes, the cloud longwave warming effect dominates the shortwave cooling effect, and the net cloud radiative forcing at the surface tends to warm high-latitude land but cool midlatitude land. This could contribute to the reduced meridional temperature gradient in warmer climates and help explain the greater warming of winter cold extremes relative to winter mean temperatures.

Time Evolution of the Cloud Response to Moisture Intrusions into the Arctic during Winter

AbstractNorthward fluxes of moisture and sensible heat into the Arctic affect the atmospheric stability, sea ice and snow cover, clouds, and surface energy budget. Intense moisture fluxes into the Arctic are called moisture intrusions; some can lead to basinwide increases in downward longwave radiation (DLR) at the surface, called downward infrared (IR) events. Using the ERA-Interim reanalysis from 1990 to 2016, this study investigated the time evolution of cloud amount and cloud properties and their impact on the surface radiation fluxes in response to Arctic moisture intrusions and downward IR events during winter for better understanding of the Arctic moisture intrusions. A composite analysis revealed several key features: moisture intrusions produce more clouds and higher cloud liquid and ice water content; positive cloud amount anomalies can persist for over 10 days over the Arctic Ocean during downward IR events; positive high-level and middle-level cloud anomalies are evident in the early stage, and positive low-level cloud anomalies are evident in the late stage. Greater clear-sky DLR and longwave cloud radiative forcing (CRF) over the Arctic Ocean accompany the greater all-sky DLR during the downward IR events. Greater clear-sky DLR can be attributed to higher air temperatures and higher total column water vapor, while greater longwave CRF is the result of larger cloud amount and cloud water content. Longwave CRF anomalies account for approximately 40% of the all-sky DLR anomalies.

Understanding Cloud and Convective Characteristics in Version 1 of the E3SM Atmosphere Model

AbstractThis study provides comprehensive insight into the notable differences in clouds and precipitation simulated by the Energy Exascale Earth System Model Atmosphere Model version 0 and version 1 (EAMv1). Several sensitivity experiments are conducted to isolate the impact of changes in model physics, resolution, and parameter choices on these differences. The overall improvement in EAMv1 clouds and precipitation is primarily attributed to the introduction of a simplified third‐order turbulence parameterization Cloud Layers Unified By Binormals (along with the companion changes) for a unified treatment of boundary layer turbulence, shallow convection, and cloud macrophysics, though it also leads to a reduction in subtropical coastal stratocumulus clouds. This lack of stratocumulus clouds is considerably improved by increasing vertical resolution from 30 to 72 layers, but the gain is unfortunately subsequently offset by other retuning to reach the top‐of‐atmosphere energy balance. Increasing vertical resolution also results in a considerable underestimation of high clouds over the tropical warm pool, primarily due to the selection for numerical stability of a higher air parcel launch level in the deep convection scheme. Increasing horizontal resolution from 1° to 0.25° without retuning leads to considerable degradation in cloud and precipitation fields, with much weaker tropical and subtropical short‐ and longwave cloud radiative forcing and much stronger precipitation in the intertropical convergence zone, indicating poor scale awareness of the cloud parameterizations. To avoid this degradation, significantly different parameter settings for the low‐resolution (1°) and high‐resolution (0.25°) were required to achieve optimal performance in EAMv1.

Aerosol-induced intensification of cooling effect of clouds during Indian summer monsoon

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.