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

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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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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Understanding the Roles of Convective Trigger Functions in the Diurnal Cycle of Precipitation in the NCAR CAM5

Journal of Climate ◽

10.1175/jcli-d-20-0699.1 ◽

2021 ◽

pp. 1-52

Author(s):

Zeyu Cui ◽

Guang J. Zhang ◽

Yong Wang ◽

Shaocheng Xie

Keyword(s):

Great Plains ◽

Diurnal Cycle ◽

Climate Models ◽

Global Climate ◽

Convective Available Potential Energy ◽

Global Climate Models ◽

Peak Time ◽

Entrainment Rate ◽

Trigger Function ◽

Diurnal Cycle Of Precipitation

AbstractThe wrong diurnal cycle of precipitation is a common weakness of current global climate models (GCMs). To improve the simulation of the diurnal cycle of precipitation and understand what physical processes control it, we test a convective trigger function described in Xie et al. (2019) with additional optimizations in the NCAR Community Atmosphere Model version 5 (CAM5). The revised trigger function consists of three modifications: 1) replacing the Convective Available Potential Energy (CAPE) trigger with a dynamic CAPE (dCAPE) trigger, 2) allowing convection to originate above the top of planetary boundary layer (i.e., the unrestricted air parcel launch level - ULL), and 3) optimizing the entrainment rate and threshold value of the dynamic CAPE generation rate for convection onset based on observations. Results from 1°-resolution simulations show that the revised trigger can alleviate the long-standing GCM problem of too early maximum precipitation during the day and missing the nocturnal precipitation peak that is observed in many regions, including the US Southern Great Plains (SGP). The revised trigger also improves the simulation of the propagation of precipitation systems downstream of the Rockies and the Amazon region. A further composite analysis over the SGP unravels the mechanisms through which the revised trigger affects convection. Additional sensitivity tests show that both the peak time and the amplitude of the diurnal cycle of precipitation are sensitive to the entrainment rate and dCAPE threshold values.

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

Remote Sensing ◽

10.3390/rs13245001 ◽

2021 ◽

Vol 13 (24) ◽

pp. 5001

Author(s):

Eleni Marinou ◽

Kalliopi Artemis Voudouri ◽

Ioanna Tsikoudi ◽

Eleni Drakaki ◽

Alexandra Tsekeri ◽

...

Keyword(s):

Climate Models ◽

Eastern Mediterranean ◽

Global Climate ◽

Weather Prediction ◽

Global Climate Models ◽

Mixed Phase ◽

Atmospheric Conditions ◽

Low Level ◽

Microphysical Properties ◽

Mixed Phase Clouds

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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The Role of Mesoscale Cellular Convective Cloud Morphologies in Low Cloud Feedbacks

10.5194/egusphere-egu21-8516 ◽

2021 ◽

Author(s):

Isabel L. McCoy ◽

Daniel T. McCoy ◽

Robert Wood ◽

Paquita Zuidema ◽

Frida A.-M. Bender

Keyword(s):

Large Scale ◽

Climate Models ◽

Global Climate ◽

Convective Cloud ◽

Static Stability ◽

Reanalysis Data ◽

Global Climate Models ◽

Liquid Water Path ◽

Radiative Impacts ◽

Low Cloud

Mesoscale cellular convective (MCC) clouds occur in large-scale patterns over the ocean, are prevalent in sub-tropical cloud regions and mid-latitudes, and have important radiative impacts on the climate system. On average, closed MCC clouds have higher albedos than open or disorganized MCC clouds for the same cloud fraction which suggests differences in micro- and macro-physical characteristics between MCC morphologies. Marine cold air outbreaks (MCAOs) influence the development of open MCC clouds and the transition from closed to open MCC clouds in the mid-latitudes. A MCAO index, M, combines atmospheric surface forcing and static stability and can be used to examine global MCC morphology dependencies. MCC cloud morphology occurrence is also expected to shift with sea surface temperature (SST) changes as the climate warms. Analysis of MCC identifications (derived from a neural network classifier applied to MODIS satellite collection 6 liquid water path retrievals) and ECMWF ERA5 reanalysis data shows that closed MCC cloud occurrence shifts to open or disorganized MCC within an M-SST space. Global climate models (GCMs) predict that M will change regionally in strength as SSTs increase. Based on our derived MCC-M-SST relationship in the current climate, closed MCC occurrence frequency is expected to increase with a weakening of M but decrease with an increase in SSTs. This results in a shift to cloud morphologies with lower albedos. Cloud controlling factor analysis is used to estimate the resulting low cloud morphology feedback which is found to be spatially varied and between &#177;0.15 W m-2 K-1. Because the morphology feedback is estimated to be positive in the extra-tropics and is not currently represented in GCMs, this implies a higher climate sensitivity than GCMs currently estimate.

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Analyzing the Climate Sensitivity of the HadSM3 Climate Model Using Ensembles from Different but Related Experiments

Journal of Climate ◽

10.1175/2008jcli2533.1 ◽

2009 ◽

Vol 22 (13) ◽

pp. 3540-3557 ◽

Cited By ~ 113

Author(s):

Jonathan Rougier ◽

David M. H. Sexton ◽

James M. Murphy ◽

David Stainforth

Keyword(s):

Climate Sensitivity ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Global Climate ◽

Global Climate Models ◽

Model Parameters ◽

Entrainment Rate ◽

Cloud Parameters ◽

Hadley Centre

Abstract Global climate models (GCMs) contain imprecisely defined parameters that account, approximately, for subgrid-scale physical processes. The response of a GCM to perturbations in its parameters, which is crucial for quantifying uncertainties in simulations of climate change, can—in principle—be assessed by simulating the GCM many times. In practice, however, such “perturbed physics” ensembles are small because GCMs are so expensive to simulate. Statistical tools can help in two ways. First, they can be used to combine ensembles from different but related experiments, increasing the effective number of simulations. Second, they can be used to describe the GCM’s response in ways that cannot be extracted directly from the ensemble(s). The authors combine two experiments to learn about the response of the Hadley Centre Slab Climate Model version 3 (HadSM3) climate sensitivity to 31 model parameters. A Bayesian statistical framework is used in which expert judgments are required to quantify the relationship between the two experiments; these judgments are validated by detailed diagnostics. The authors identify the entrainment rate coefficient of the convection scheme as the most important single parameter and find that this interacts strongly with three of the large-scale-cloud parameters.

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