Geometrical and Microphysical Properties of Clouds Formed in the Presence of Dust above the Eastern Mediterranean

In this work, collocated lidar–radar observations are used to retrieve the vertical profiles of cloud properties above the Eastern Mediterranean. Measurements were performed in the framework of the PRE-TECT experiment during April 2017 at the Greek atmospheric observatory of Finokalia, Crete. Cloud geometrical and microphysical properties at different altitudes were derived using the Cloudnet target classification algorithm. We found that the variable atmospheric conditions that prevailed above the region during April 2017 resulted in complex cloud structures. Mid-level clouds were observed in 38% of the cases, high or convective clouds in 58% of the cases, and low-level clouds in 2% of the cases. From the observations of cloudy profiles, pure ice phase occurred in 94% of the cases, mixed-phase clouds were observed in 27% of the cases, and liquid clouds were observed in 8.7% of the cases, while Drizzle or rain occurred in 12% of the cases. The significant presence of Mixed-Phase Clouds was observed in all the clouds formed at the top of a dust layer, with three times higher abundance than the mean conditions (26% abundance at −15 °C). The low-level clouds were formed in the presence of sea salt and continental particles with ice abundance below 30%. The derived statistics on clouds’ high-resolution vertical distributions and thermodynamic phase can be combined with Cloudnet cloud products and lidar-retrieved aerosol properties to study aerosol-cloud interactions in this understudied region and evaluate microphysics parameterizations in numerical weather prediction and global climate models.

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Constraining Neoproterozoic subtropical low-level clouds to assess the plausibility of near-Snowball Earth states

10.5194/egusphere-egu21-4742 ◽

2021 ◽

Author(s):

Christoph Braun ◽

Aiko Voigt ◽

Johannes Hörner ◽

Joaquim G. Pinto

Keyword(s):

Sea Ice ◽

Climate Models ◽

Global Climate ◽

Global Climate Models ◽

Mixed Phase ◽

Snowball Earth ◽

Stable Range ◽

Climate Regime ◽

Low Level ◽

Mixed Phase Clouds

Stable waterbelt climate states with close to global ice cover challenge the classical Snowball Earth hypothesis because they provide a robust explanation for the survival of advanced marine species during the Neoproterozoic glaciations (1000 &#8211; 541 Million years ago). Whether Earth&#8217;s climate stabilizes in a waterbelt state or rushes towards a Snowball state is determined by the magnitude of the ice-albedo feedback in the subtropics, where dark, bare sea ice instead of snow-covered sea ice prevails. For a given bare sea-ice albedo, the subtropical ice-albedo feedback and thus the stable range of the waterbelt climate regime is sensitive to the albedo over ice-free ocean, which is largely determined by shortwave cloud-radiative effects (CRE). In the present-day climate, CRE are known to dominate the spread of climate sensitivity across global climate models. We here study the impact of uncertainty associated with CRE on the existence of geologically relevant waterbelt climate regimes using two global climate models and an idealized energy balance model. We find that the stable range of the waterbelt climate regime is very sensitive to the abundance of subtropical low-level mixed-phase clouds. If subtropical cloud cover is low, climate sensitivity becomes so high as to inhibit stable waterbelt states.The treatment of mixed-phase clouds is highly uncertain in global climate models. Therefore we aim to constrain the uncertainty associated with their CRE by means of a hierarchy of global and regional simulations that span horizontal grid resolutions from 160 km to 300m, and in particular include large eddy simulations of subtropical mixed-phase clouds located over a low-latitude ice edge. In the cold waterbelt climate subtropical CRE arise from convective events caused by strong meridional temperature gradients and stratocumulus decks located in areas of large-scale descending motion. We identify the latter to dominate subtropical CRE and therefore focus our large eddy simulations on subtropical stratocumulus clouds. By conducting simulations with two extreme scenarios for the abundance of atmospheric mineral dust, which serves as ice-nucleating particles and therefore can control mixed-phase cloud physics, we aim to estimate the possible spread of CRE associated with subtropical mixed-phase clouds. From this estimate we may assess whether Neoproterozoic low-level cloud abundance may have been high enough to sustain a stable waterbelt climate regime.

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Northern Hemisphere Atmospheric Blocking Representation in Global Climate Models: Twenty Years of Improvements?

Journal of Climate ◽

10.1175/jcli-d-16-0242.1 ◽

2016 ◽

Vol 29 (24) ◽

pp. 8823-8840 ◽

Cited By ~ 56

Author(s):

Paolo Davini ◽

Fabio D’Andrea

Keyword(s):

Northern Hemisphere ◽

General Circulation ◽

Climate Models ◽

Global Climate ◽

Global Climate Model ◽

Weather Prediction ◽

Global Climate Models ◽

Main Concern ◽

Atmospheric Blocking ◽

Atlantic Sector

Abstract The correct simulation of midlatitude atmospheric blocking has always been a main concern since the earliest days of numerical modeling of Earth’s atmosphere. To this day blocking represents a considerable source of error for general circulation models from both a numerical weather prediction and a climate perspective. In the present work, 20 years of global climate model (GCM) developments are analyzed from the special point of view of Northern Hemisphere atmospheric blocking simulation. Making use of a series of equivalent metrics, three generations of GCMs are compared. This encompasses a total of 95 climate models, many of which are different—successive—versions of the same model. Results from model intercomparison projects AMIP1 (1992), CMIP3 (2007), and CMIP5 (2012) are taken into consideration. Although large improvements are seen over the Pacific Ocean, only minor advancements have been achieved over the Euro-Atlantic sector. Some of the most recent GCMs still exhibit the same negative bias as 20 years ago in this region, associated with large geopotential height systematic errors. Some individual models, nevertheless, have improved and do show good performances in both sectors. Negligible differences emerge among ocean-coupled or atmosphere-only simulations, suggesting weak relevance of sea surface temperature biases. Conversely, increased horizontal resolution seems to be able to alleviate the Euro-Atlantic blocking bias.

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Mesoscale Simulations of Australian Direct Normal Irradiance, Featuring an Extreme Dust Event

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-17-0091.1 ◽

2018 ◽

Vol 57 (3) ◽

pp. 493-515 ◽

Cited By ~ 7

Author(s):

S. K. Mukkavilli ◽

A. A. Prasad ◽

R. A. Taylor ◽

A. Troccoli ◽

M. J. Kay

Keyword(s):

Radiative Transfer ◽

Climate Models ◽

Global Climate ◽

Weather Prediction ◽

Wrf Model ◽

Global Climate Models ◽

Radiative Transfer Model ◽

Transfer Model ◽

Dust Event ◽

Direct Normal Irradiance

AbstractDirect normal irradiance (DNI) is the main input for concentrating solar power (CSP) technologies—an important component in future energy scenarios. DNI forecast accuracy is sensitive to radiative transfer schemes (RTSs) and microphysics in numerical weather prediction (NWP) models. Additionally, NWP models have large regional aerosol uncertainties. Dust aerosols can significantly attenuate DNI in extreme cases, with marked consequences for applications such as CSP. To date, studies have not compared the skill of different physical parameterization schemes for predicting hourly DNI under varying aerosol conditions over Australia. The authors address this gap by aiming to provide the first Weather and Forecasting (WRF) Model DNI benchmarks for Australia as baselines for assessing future aerosol-assimilated models. Annual and day-ahead simulations against ground measurements at selected sites focusing on an extreme dust event are run. Model biases are assessed for five shortwave RTSs at 30- and 10-km grid resolutions, along with the Thompson aerosol-aware scheme in three different microphysics configurations: no aerosols, fixed optical properties, and monthly climatologies. From the annual simulation, the best schemes were the Rapid Radiative Transfer Model for global climate models (RRTMG), followed by the new Goddard and Dudhia schemes, despite the relative simplicity of the latter. These top three RTSs all had 1.4–70.8 W m−2 lower mean absolute error than persistence. RRTMG with monthly aerosol climatologies was the best combination. The extreme dust event had large DNI mean bias overpredictions (up to 4.6 times), compared to background aerosol results. Dust storm–aware DNI forecasts could benefit from RRTMG with high-resolution aerosol inputs.

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Factors Limiting Convective Cloud-Top Height at the ARM Nauru Island Climate Research Facility

Journal of Climate ◽

10.1175/jcli3722.1 ◽

2006 ◽

Vol 19 (10) ◽

pp. 2105-2117 ◽

Cited By ~ 83

Author(s):

Michael P. Jensen ◽

Anthony D. Del Genio

Keyword(s):

Climate Models ◽

Global Climate ◽

Environmental Parameters ◽

Convective Cloud ◽

Global Climate Models ◽

Entrainment Rate ◽

Research Facility ◽

Climate Research ◽

Low Level ◽

Cumulus Congestus

Abstract Cumulus congestus clouds, with moderate shortwave albedos and cloud-top temperatures near freezing, occur fairly often in the Tropics. These clouds may play an important role in the evolution of the Madden–Julian oscillation and the regulation of relative humidity in the midtroposphere. Despite this importance they are not necessarily simulated very well in global climate models. Surface remote sensing observations and soundings from the Atmospheric Radiation Measurement (ARM) climate research facility at Nauru Island are coupled with a simple parcel model in order to address the following questions about these cloud types: 1) Which environmental factors play a role in determining the depth of tropical convective clouds? 2) What environmental parameters are related to entrainment rate in cumulus congestus clouds? The results presented herein suggest that at Nauru Island a drying of the midtroposphere is more likely to be responsible for limiting congestus cloud-top heights than is a stabilizing of the freezing level. It is also found that low-level CAPE and the RH profile account for the largest portion of the variance in cumulus congestus entrainment rates, consistent with the idea that entrainment rate depends on the buoyant production of turbulent kinetic energy. If the analysis is limited to cases where there is a sounding during the hour preceding the cumulus congestus observations, it is found that the low-level CAPE accounts for 85% of the total variance in entrainment rate.

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Could the cloud cover in future climate models be improved by incorporating the role of sea spray in surface heat fluxes?

10.5194/egusphere-egu2020-6316 ◽

2020 ◽

Author(s):

Yajuan Song ◽

Fangli Qiao ◽

Qi Shu ◽

Jiping Liu ◽

Ying Bao ◽

...

Keyword(s):

Latent Heat ◽

Cloud Cover ◽

Climate Models ◽

Climate Model ◽

Global Climate ◽

Heat Fluxes ◽

Global Climate Models ◽

Shortwave Radiation ◽

Sea Spray ◽

Low Level

Accurate cloud cover and radiative effect simulation remains a long-standing challenge for global climate models (GCMs). The Southern Ocean (SO) cloud cover is substantially underestimated by most GCMs. Therefore, too much shortwave radiation is absorbed by oceans, which causes an overly warm sea surface temperature (SST) bias over the SO. For the first time, sea spray effects on latent and sensible heat fluxes are considered in a climate model. The most notable sea spray impacts on heat fluxes occur over the SO, with anomalous latent heat fluxes up to -7.74 W m-2. Enhanced latent heat release lead to SST cooling. In addition, more clouds are formed over the SO to reflect excessive downward shortwave radiation, especially low-level clouds at 1.51% increments. Our results provide a feasible solution to mitigate the lack of low-level clouds and overly warm SST biases over the SO in GCMs.

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Why Do Global Climate Models Struggle to Represent Low-Level Clouds in the West African Summer Monsoon?

Journal of Climate ◽

10.1175/jcli-d-16-0451.1 ◽

2017 ◽

Vol 30 (5) ◽

pp. 1665-1687 ◽

Cited By ~ 29

Author(s):

Lisa Hannak ◽

Peter Knippertz ◽

Andreas H. Fink ◽

Anke Kniffka ◽

Gregor Pante

Keyword(s):

Climate Models ◽

Global Climate ◽

West African Monsoon ◽

High Elevation ◽

West African ◽

Global Climate Models ◽

The West ◽

Near Surface ◽

Low Level ◽

The Impact

Abstract Climate models struggle to realistically represent the West African monsoon (WAM), which hinders reliable future projections and the development of adequate adaption measures. Low-level clouds over southern West Africa (5°–10°N, 8°W–8°E) during July–September are an integral part of the WAM through their effect on the surface energy balance and precipitation, but their representation in climate models has received little attention. Here 30 (20) years of output from 18 (8) models participating in phase 5 of the Coupled Model Intercomparison Project (Year of Tropical Convection) are used to identify cloud biases and their causes. Compared to ERA-Interim reanalyses, many models show large biases in low-level cloudiness of both signs and a tendency to too high elevation and too weak diurnal cycles. At the same time, these models tend to have too strong low-level jets, the impact of which is unclear because of concomitant effects on temperature and moisture advection as well as turbulent mixing. Part of the differences between the models and ERA-Interim appear to be related to the different subgrid cloud schemes used. While nighttime tendencies in temperature and humidity are broadly realistic in most models, daytime tendencies show large problems with the vertical transport of heat and moisture. Many models simulate too low near-surface relative humidities, leading to insufficient low cloud cover and abundant solar radiation, and thus a too large diurnal cycle in temperature and relative humidity. In the future, targeted model sensitivity experiments will be needed to test possible feedback mechanisms between low clouds, radiation, boundary layer dynamics, precipitation, and the WAM circulation.

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Cirrus clouds in a global climate model with a statistical cirrus cloud scheme

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-16607-2009 ◽

2009 ◽

Vol 9 (4) ◽

pp. 16607-16682 ◽

Cited By ~ 3

Author(s):

M. Wang ◽

J. E. Penner

Keyword(s):

Climate Models ◽

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Circulation Model ◽

Global Climate Models ◽

Cirrus Clouds ◽

Cirrus Cloud ◽

Ice Crystal ◽

Low Level

Abstract. A statistical cirrus cloud scheme that accounts for mesoscale temperature perturbations is implemented into a coupled aerosol and atmospheric circulation model to better represent both cloud fraction and subgrid-scale supersaturation in global climate models. This new scheme is able to better simulate the observed probability distribution of relative humidity than the scheme that was implemented in an older version of the model. Heterogeneous ice nuclei (IN) are shown to affect not only high level cirrus clouds through their effect on ice crystal number concentration but also low level liquid clouds through the moistening effect of settling and evaporating ice crystals. As a result, the change in the net cloud forcing is not very sensitive to the change in ice crystal concentrations associated with heterogeneous IN because changes in high cirrus clouds and low level liquid clouds tend to cancel. Nevertheless, the change in the net radiative flux at the top of the atmosphere due to changes in IN is still large because of changes in the greenhouse effect of water vapor caused by the changes in ice crystal number concentrations. Changes in the magnitude of the assumed mesoscale temperature perturbations by 25% alter the ice crystal number concentrations and radiative fluxes by an amount that is similar to that from a factor of 10 change in the heterogeneous IN number concentrations.

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Climate change presents increased potential for very large fires in the contiguous United States

International Journal of Wildland Fire ◽

10.1071/wf15083 ◽

2015 ◽

Vol 24 (7) ◽

pp. 892 ◽

Cited By ~ 167

Author(s):

R. Barbero ◽

J. T. Abatzoglou ◽

N. K. Larkin ◽

C. A. Kolden ◽

B. Stocks

Keyword(s):

Climate Change ◽

Climate Models ◽

Fire Suppression ◽

Global Climate ◽

Global Climate Models ◽

Atmospheric Conditions ◽

Management Options ◽

Absolute Increase ◽

Southern Us ◽

Large Fires

Very large fires (VLFs) have important implications for communities, ecosystems, air quality and fire suppression expenditures. VLFs over the contiguous US have been strongly linked with meteorological and climatological variability. Building on prior modelling of VLFs (>5000 ha), an ensemble of 17 global climate models were statistically downscaled over the US for climate experiments covering the historic and mid-21st-century periods to estimate potential changes in VLF occurrence arising from anthropogenic climate change. Increased VLF potential was projected across most historically fire-prone regions, with the largest absolute increase in the intermountain West and Northern California. Complementary to modelled increases in VLF potential were changes in the seasonality of atmospheric conditions conducive to VLFs, including an earlier onset across the southern US and more symmetric seasonal extension in the northern regions. These projections provide insights into regional and seasonal distribution of VLF potential under a changing climate, and serve as a basis for future strategic and tactical fire management options.

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The relative contribution of secondary ice processes in Alpine mixed-phase clouds

10.5194/egusphere-egu21-14659 ◽

2021 ◽

Author(s):

Paraskevi Georgakaki ◽

Georgia Sotiropoulou ◽

Etienne Vignon ◽

Alexis Berne ◽

Athanasios Nenes

Keyword(s):

Climate Models ◽

Weather Prediction ◽

Supercooled Liquid ◽

Mixed Phase ◽

Swiss Alps ◽

Blowing Snow ◽

Temperature Range ◽

Relative Contribution ◽

Mixed Phase Clouds

In-situ observations of mixed-phase clouds (MPCs) forming over mountain tops regularly reveal that ice crystal number concentrations (ICNCs) are orders of magnitude higher than ice-nucleating particle concentrations. This discrepancy has often been attributed to the influence of surface processes such as blowing snow and airborne hoar frost. &#921;n-cloud secondary ice production (SIP) processes may also explain this discrepancy, but their contribution has received less attention. Here we explore the potential role of SIP processes on orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the mountain-top site of Jungfraujoch in the Swiss Alps using the Weather Research and Forecasting model (WRF). The Hallett-Mossop (H-M) mechanism, included in the default version of the Morrison scheme in WRF, is ruled out since the simulated clouds were outside the active temperature range for this process. This study investigates if the implementation of two additional SIP mechanisms in WRF, namely collisional break-up (BR) between ice hydrometeors and frozen droplet shattering (DS), can bridge the gap between observed and modeled ICNCs. DS is inefficient in the examined conditions due to a lack of sufficiently large raindrops to trigger this process. The BR mechanism is likely important in Alpine MPCs, but the process is activated only within seeder-feeder situations, when precipitation particles are seeding the low-level MPCs inducing their glaciation. At times when a cloud exists near the ground, blowing snow ice particles may be mixed among supercooled liquid droplets and thus contribute significantly to ice growth, but they cannot account for the observed ICNCs. Our findings indicate that outside the H-M temperature range, ice-seeding and blowing snow can initiate ice multiplication in the Alps through the BR mechanism, which is found to elevate the modeled ICNCs up to 3 orders of magnitude, providing a better agreement with in-situ measurements. This highlights the importance of considering both SIP and surface-based processes in weather-prediction and climate models.

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Impacts of Aerosol Concentration Variability on Turbulence in Convective Low Level Mixed-phase Clouds in the Arctic

10.5194/egusphere-egu2020-18426 ◽

2020 ◽

Author(s):

Jan Chylik ◽

Stephan Mertes ◽

Roel Neggers

Keyword(s):

Boundary Layer ◽

Climate Models ◽

Cloud Condensation Nuclei ◽

Mixed Phase ◽

The Arctic ◽

Microphysics Scheme ◽

Low Level ◽

Large Eddy ◽

Mixed Phase Clouds ◽

The Impact

Arctic mixed-phase clouds are still not properly represented in weather forecast and climate models. Recent field campaigns in the Arctic have successfully probed low level mixed-phase clouds, however it remains difficult to gain understanding of this complex system from observational datasets alone. Complementary high-resolution simulations, properly constrained by relevant measurements, can serve as a virtual laboratory that provides a deeper insight into a developing boundary layer in the Arctic. Our study focus on the impact of variability in cloud condensation nuclei (CCN) concentrations on the turbulence in Arctic mixed-phase clouds. Large-Eddy Simulations of convective mixed-phase clouds over open water were performed as observed during the ACLOUD campaign, which took place in Fram Strait west of Svalbard in May and June 2017. The Dutch Atmospheric Large Eddy Simulation (DALES) is used including a well-established double-moment mixed-phase microphysics scheme of Seifert & Beheng. The results highlight various impact mechanisms of CCN on the boundary layer thermodynamic state, turbulence, and clouds. Lower CCN concentrations generally lead to decreased turbulence near the cloud top. However, they can also enhance the turbulence in the lower part of the boundary layer due to increased amount of sublimation of ice hydrometeors. Further implications for the role of mixed-phase clouds in the Arctic Amplification will be discussed.

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