NUBESOL-2 (2020-2023)

Frontier conditions between cloud and aerosol: their treatment in radiative and atmospheric models

(PID2019-105901RB-I00 )

Confusion between cloud and aerosol is relatively common as it is essentially the same phenomenon, particles suspended in the atmosphere. Despite this, both from the point of view of observation and modeling, it continues to be treated as distinct phenomena. This is to the point that experimental characterization and radiative modeling methods remain very different for cloud and aerosol treatment. This has important implications for calculating radiative levels and forcings in both radiation-specific models and the codes used in atmospheric models.

The project NUBESOL-2 aims to evaluate the deviations and uncertainties that are introduced in the calculation of radiative fluxes and levels in atmospheric models as a consequence of treating separately, and not like a continuous phenomenon, clouds and aerosols. Thus, and first of all, we continue assessing the frequency of situations in which the distinction between cloud and aerosol is not obvious. The analisys is extended to methods applicable to both night and day conditions, specifically those based on cameras with night-time capturing capabilities, ceilometer measurements, and long-wave radiation measurements performed with pyrgeometers. This would reinforce the argument that these situations of confusion (or border) cannot be disregarded in the building of cloud climatologies or in the study of the energy balance in the atmosphere and the climate system.

The treatment of the continuous cloud-aerosol in radiative transfer models (such as SBDART, RRTM or libRadTran), in the solar and terrestrial bands, is also analyzed, comparing the results of the models with those derived from surface measurements. Spectral and broadband data obtained from campaigns and the measurement stations available to the group are preferably used, although third-party data are also considered.

Finally, the impact of the forced distinction between cloud and aerosol on the performance of the parameters of the radiative transfer in the meteorological models is evaluated. This is done for some of the schemes used in those models, in particular for those included in the WRF model or the ecRAD built into ECMWF models. As far as possible, technical or methodological proposals are made to correct or improve that performance.

Goals:

1. Reinforce the argument that the frequency and importance of situations in which the distinction between cloud and aerosol is not obvious cannot be disregarded in the knowledge of the atmosphere and climate. To this end, continue to evaluate the frequency with which these “border” situations occur, extending the analysis, with respect to previous projects, to methods applicable to both night and day conditions, such as those based on night-time image capture cameras, in measurements of ceilometer, and in measurements of radiation in the terrestrial band (long wave).

2. To evaluate the performance of the radiative transfer models for solar and terrestrial bands in the treatment of the border situations within the continuous cloud-aerosol. Compare the results of the models with those derived from measurements made on the surface, whether spectral or broadband, whether solar or terrestrial, in particular those obtained with the equipment available to the research group.

3. To evaluate the impact of the forced distinction between cloud and aerosol on the performance of radiation parameterizations in atmospheric models, using the the codes implemented in those models. Technical or methodological proposals will be made, as far as possible, to improve their performance and to correct the potential biases introduced by the separate cloud and aerosol treatment in models for predicting solar resource or ultraviolet radiation levels.

NUCLIER (2004-2007)

Clouds and climate change: climatological analysis of cloud cover in the Iberian Peninsula, and measurement and modeling of its radiative effects at the surface

(MEC, CGL2004-02325/CLI)

The NUCLIER project was the first in a series of projects dedicated to the study of clouds, aerosol, radiation, and climate. Clouds are a key factor in the processes that regulate the climate, given their influence on the energy balance and on the hydrological cycle. Determining to what extent clouds contribute to climate change is a difficult problem to solve due to the complexity of the processes in which they intervene, the large amount of information required, and the uncertainty associated with the available data. This project faced the study of clouds to contribute to those difficulties. It addressed the cloud climatology in the Iberian Peninsula, and the existence of significant trends in the amount and/or type of clouds present in recent decades. On the other hand, the possibility of adapting one-dimensional radiative transfer models to describe the radiative effects of clouds began to be studied. A database of solar radiation and atmospheric infrared radiation, and continuous and simultaneous observations of the sky was made; it was also sought a methodology for improving the correspondence between modeling estimates and experimental measurements.

Goals:

1. To obtain a climatological description of cloud cover in the Iberian Peninsula, including analysis on a daily, seasonal and annual scale and the construction of average calendars, based on surface observations (in more than 50 Iberian observatories and over a period that is at least 1960 to 1990) and from satellite (data from the ISCCP project).

2. To analyze the existence or not of statistically significant trends in cloud cover over different areas of the Peninsula, including total cloud cover and cloud type.

3. To build a database with measurements of global and diffuse solar radiation, ultraviolet radiation, atmospheric infrared radiation, and continuous and simultaneous observations of the state of the sky.

4. To stablish a methodology for the use and improvement of the correspondence between the estimates of one-dimensional radiative transfer models and experimental measurements, for all sky conditions (clear, partly cloudy and overcast).

NUCLIEREX (2007-2010)

Clouds and climate change: variability and trends in cloud cover and sunshine duration in the Iberian Peninsula, and measurement and modeling of the radiative effects of clouds at the surface

(CGL2007-62664/CLI)

The NUCLIEREX project was an extension of the NUCLIER project that improved the knowledge of the climatology of cloudiness and sunshine duration in the Iberian Peninsula, for the last half of the 20th century, and made it possible to set up the existing radiometric and sky condition observation station at a level equivalent to those of the worldwide BSRN network. The results of the NUCLIER project were strengthened, providing knowledge to reduce the uncertainties associated with cloudiness in contemporary climate change. Climatology was spread over time, digitizing and analyzing data since the beginning of the 20th century; and also in space, integrating the results obtained for the Iberian Peninsula within the European framework. An attempt was also made to explain the temporary changes detected in cloudiness and sunshine duration in relation to changes in synoptic patterns, low-frequency variability patterns (NAO, WeMO, and others), and other phenomena such as volcanism or changes in solar activity. On the other hand, the role that clouds play in the radiative transfer through the atmosphere was studied in depth. The radiative simulation of all sky conditions (clear, partially cloudy and overcast) continued to be studied and the effects on the solar and terrestrial bands were studied, with the aim of improving the agreement between modeling and measurement in the case of one-dimensional models, which are closer to the parameterizations involved in climate simulation models. In this sense, the introduction into the models of data obtained from a sky observation camera and a ceilometer was studied as well.

Goals:

1. To extend the climatology of cloudiness and sunshine duration obtained in the NUCLIER project both in time (considering the existence of longer series, beginning at the beginning of the 20th century) and in space, integrating the climatology of the Iberian Peninsula in the European context.

2. To explain the changes detected in cloud cover and sunshine duration from the changes observed in the synoptic patterns and low frequency circulation variability, and also considering other possible causes such as volcanic eruptions, air pollution, etc.

3. To incorporate information on the spatial distribution of the cloud field that can be obtained from images with hemispheric sky observation cameras, and on the vertical distribution obtained from detection with a ceilometer, into the one-dimensional and three-dimensional radiative modeling.

4. To improve the agreement between the results of the radiative modeling and the corresponding surface measurements, for all kinds of skies (both covered and partially cloudy) and for the different radiation bands (solar and terrestrial).

NUCLIERSOL (2011-2014)

Clouds and their effects on radiation: from the study of situations on a local scale to a global climate analysis. Interactions with atmospheric aerosol

(CGL2010-18546)

The NUCLIERSOL project followed the line started in previous projects, focusing on clouds from a climate perspective and from the point of view of their detection and observation, mainly with methods and instruments based on ground-based radiometry. In addition, the project sought to contribute to the knowledge of the interactions between clouds and aerosols, since they are of great importance for the radiative balance of the Earth-atmosphere system. The reduction of the uncertainties may lead to improving future climate projections. One of the main goals of the project was to increase understanding of the climate behavior of clouds by means of cloud cover observations, as well as related variables, such as insolation, evaporation tank or visibility and how they are simulated in global and regional climate models. On the other hand, the improvement in the description of sky conditions (cloud type and coverage, as well as, some related properties or characteristics) that can be obtained with ground-based instruments (cameras, ceilometers, pyranometers, pyrgeometers) was pursed. In both cases, the additional complexity of the presence of atmospheric aerosols, extremely variable in concentration and properties, was well considered because they can be the cause, for example, of the discrepancy between climate simulations and observations of cloudiness and solar radiation.

Goals:

1. To evaluate the simulations of cloudiness and solar radiation from various climate models, comparing with available historical data. This evaluation included global climate models (GCM) besides regional outputs through downscaling techniques. Additionally, the comparison at Iberian peninsula and Europe scale was performed in greater detail and then it was extended to global scale according to the available data.

2. To characterize the behavior of a Campbell-Stokes heliograph as a function of concurrent measurements of global, direct and diffuse solar irradiance, and to determine the possible influence of the aerosol load on the insolation. To accurately determine the temporal scale (hourly, daily, monthly) for which insolation can be considered an excellent estimator of cloudiness.

3. To integrate a set of sky observation instruments (hemispherical camera, ceilometer, radiometers in various spectral bands) to obtain a description of the cloud field (coverage, type, height,…) as well as physical and microphysical characteristics (optical depth, droplets size), and also the aerosol load present.

4. To establish the possible relationship and interaction that may exist between the amount and type of aerosols in the atmosphere and clouds and their evolution. The first approach was from a statistical point of view, based on available datasets of both meteors (clouds and aerosols) either from direct observations or from estimations of climate models and secondly, the analysis was based on observations recorded by our radiometric and sky observation instruments (or with equivalent data from other international measurement sites).

NUBESOL (2015-2018)

Exploring the cloud aerosol frontier using surface observations and radiative transfer models

(CGL2014-55976-R)

The NUBESOL project focused on the difficulty in distinguishing between aerosol and cloud in some situations. This statement is true if we refer to visual observations of the sky, but also when more modern techniques are used, such as all-sky cameras, ceilometers or satellite sensors. The difficulty arises from the fact that cloud and aerosol are denominations for two particular cases of a single phenomenon (suspension of particles in the air). Sometimes, the characteristics of this suspension are on the frontier between those corresponding to a cloud and those that correspond to an atmospheric aerosol. Although interactions between the particles that form an aerosol and a cloud have been described, from a microphysical point of view, and the feedbacks between clouds-aerosols-radiative forcing (the so-called indirect effects of aerosols) have also been described, the radiative effects in the transition zone of the aerosol-cloud continuum in these situations has been poorly studied. These effects should be better characterized to improve knowledge of climate, cloud and aerosol parameterizations and ultimately, climate models and projections.

The evolution of our line of research led us to set the objective of contributing to reduce the uncertainties that climate studies still present, in relation to clouds and aerosols, by describing them as particular cases of a continuum, to facilitate the characterization and parameterization of intermediate situations.

The NUBESOL project sought first to determine how often there are situations in which the distinction between cloud and aerosol is not evident, and to establish the importance of the intermediate region between cloud and aerosol in partially cloudy conditions. Secondly, it was studied to what extent it is possible and convenient to describe the cloud-aerosol system as a continuum, both in its properties and its spatial distribution, determining which characteristics or parameters are the most indicated. The combination of observations and radiative transfer simulations was proposed. Finally, we wanted to see to what extent the exclusive distinction between cloud and aerosol (instead of its treatment as a continuum) could be introducing a bias in the processing, both locally and globally of the estimates of the aerosol load and properties, of the attribution of dimming-brightening causes, of the estimations from satellite. The consequences of all these factors in the performance of global models were also analysed.

Goals:

1. To determine how often there are situations in which the distinction between cloud and aerosols were not obvious, in particular, when observations are made from surface using cameras, broadband or multi-filter radiometers, spectrophotometers, ceilometers, heliographs, etc. Additionally to highlight the relevance of intermediate situations between cloud and aerosols under partially cloudy conditions.

2. To study to what extent it is possible and convenient to describe the cloud-aerosol system as a continuum, both in its properties and in its spatial distribution. We paid special interest in partially cloudy sky conditions, determining which characteristics or parameters are the most suitable. We combined observations with radiative modeling (on various bands) to analyze the processing of the aerosol cloud-continuum.

3. To evaluate to what extent the exclusive distinction between cloud and aerosol (instead of its treatment as a continuum) could be introducing a bias in the processing, both locally and globally of the estimates of the aerosol load and properties, of the attribution of dimming-brightening causes, of the estimations from satellite. The consequences of all these factors in the performance of global models were also analysed.