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FMP contact with scientist

  • HildeMaassen
  • Oct 20, 2020
  • 7 min read

With the pitch advices Tom Seymour gave us I wrote several people including a scientist Marieke Dirksen who wrote about the situation in The Netherlands during the lockdown. She answered and we met online, of course, because of COVID.


Clouds are one of the most complicated topics to study Marieke told me; there is so much involved such as wind, water, the sun and aerosols. A lot of uncertainties and changing factors and therefor clouds are difficult to predict in the scheme of climate change. There is still a lot of mystery around clouds. What is a good sign around the the equator can be quite the opposite elsewhere.


Marieke was involved in a study of the amount of radiation that reached the earth's surface on sunny days during the lockdown period in the Netherlands. It turned out that 3% more sunlight reached the Earth's surface on sunny days. That was much more than expected. She still sounds puzzled and trouble).


If we look at the amount of solar radiation in the Netherlands in the past 50 years, we see that it has become sunnier in the Netherlands since the mid-1980s. The daily percentage has increased from 32 to 37 percent during that time. The improved air quality is most likely the reason for this, especially the emission of sulfur. It has almost become 0. But this could also explain the decrease in cloud cover, because sulfur acts as condensation nuclei for clouds. The duration of the sunshine, in turn, affects the weather: they seem to have led to higher temperatures, lower humidity and better visibility, especially in summer. In comparison: 3% in corona is a lot.


Marieke tells me that they had measured the sunny days. Why not even on cloudy days is the question. The reason is that we cannot always see all clouds with the naked eye and determining the cloud cover is done with the help of observations. There is a phenomenon of 'invisible clouds'. These are clouds made up of so small and so few particles that the naked eye cannot see the clouds, but they do affect the amount of solar radiation on the ground.


Traditionally, cloud cover has been estimated by observers with the naked eye. This is neither accurate nor objective and therefore the measurement data from the past cannot be used. The unit of measurement for this is octas, or the coverage ratio in eighths (0 = completely cloudless, 8 = completely cloudy).


Cloud cover can be measured in various ways:

1. An 'all-sky' camera or an extreme fisheye lens.

2. A ceilometer (instrument to measure the height of the clouds)

3. A Campbell-Stokes sunshine duration meter


Marieke explains about clouds and aerosols. Aerosols are small particles that can be solid, gas or liquid and that are in the air and thus clouds. They can be natural or human causes. During the lockdown, more sun rays could reach the earth's surface; in other words, the light was filtered less by those aerosols.


Of course it is not said that air pollution is therefore better for the environment because it warms the earth more. Marieke says that these measurements can represent a huge step forward for research into clouds. Predictions are mainly made on the basis of simulations with computer models. These are fed in Europe with data from measurements that are carried out at 7 locations. Cabauw is one such measuring station in the Netherlands. All measuring stations are linked together. The situation with the lockdowns and for example 30% less air traffic is so different from the normal situation that it can yield an enormous mountain of information.


Not only is there measuring equipment on land, but airplanes also help with the collection of data. Special so-called ASDAR equipment has been placed on airplanes since 1990. Smart aircraft selection based on flight paths has created a dense observing network over the oceans and areas that are sparsely populated, inhospitable. In this way, for example, the occurrence of hurricanes can also be predicted early.


Since the 1990s, an upward trend in solar radiation at the surface has been observed, which is reflected in the models and is related to economic developments and regulations on air pollution. and their direct and cloud-induced effects on radiation.


This can have all kinds of influences. In Europe the transition from coal to gas and the policy to use less aerosol gases. The “indirect” effect due to the modification of clouds remains one of the most important uncertainties in climate research

Information from the, by Marieke, suggested sources:

Global warming is observable in the Netherlands. Between 1901 and 2013, the average temperature in De Bilt (measurement center) rose by 1.8 degrees Celsius. That also applies to neighboring countries. Most of this increase, 1.4 degrees, was between 1951 and 2013 (Figure 1). Since 1951, the increase has been about twice the global increase in the mean temperature over the land and sea surface. In general, the land is warming faster than the ocean. Winters (December, January and February) were milder because the wind came more often from the west. Summers (June, July and August) were extra warm due to an increase in solar radiation. This is mainly the result of the decreased air pollution


Warm air can contain more moisture with the result that it will rain more. Observations show that the number of extreme showers will increase by 12 percent per degree of warming.


Since the 1950s, the amount of cloud cover in the Netherlands has not changed significantly, yet the amount of solar radiation has increased from the 1980s, by 9 percent between 1981 and 2013


One reason for this change is that the air has become cleaner and therefore more transparent. The sun can shine through the clouds better.

The potential evaporation (evaporation that occurs as long as the soil contains sufficient water) increased by 12%, possibly resulting in an increase in drought..


Marieke is now going to participate in a study of the air currents above the oceans. She tells me that the patterns of the wind change the shape of the clouds; flow models.



Clouds are a masterly manifestation of complex turbulent behavior in the world in which we live.

Pier Siebesma


In 2007 Pier Siebesma talk on clouds*


Our climate system is a dynamic system. Clouds help determine the temperature. They are very good reflectors for sunlight, which we know because it is colder on days when the sun does not shine through the clouds and it is therefore colder. It is mainly the low-hanging clouds such as cumulus and stratocumulus that contribute to this cooling effect. Clouds are also good at retaining heat that would otherwise radiate into the room. High cirrus clouds in particular are responsible for this warming effect.


The formation of low clouds in particular is strongly related to small-scale, local turbulence. This usually happens due to heating of the earth's surface, causing warm thermal bubbles that are lighter than the environment. As a result, they rise and cool down when they come higher. If there is enough moisture in such a thermal bubble, the moisture will condense, forming a cloud. Heat is released during this condensation of water vapor, allowing the cumulus cloud to rise further. How high such a cumulus cloud can rise depends on the amount of moisture available in the air and the stability of the atmosphere. The exact dynamics of the clouds thus strongly depend on the environmental conditions of the atmosphere, are driven by local turbulence on scales from 1 to 10 km and are further enhanced by thermodynamic phase transitions.


The central question is: how well are these processes represented in global climate models?


Climate models are grid-point models where the dynamics of the atmosphere, oceans, ice and soil are calculated and integrated over time. They can do this on surfaces of 100 km, but they are not good at representing phenomena and processes on scales smaller than 100 km. However, that is the area where cloud dynamics takes place. The consequence of this is that cloud processes are an uncertain factor in climate models.


There are also models that work on a smaller scale, such as a technique called Large Eddy Simulation (LES). This works at a resolution between 10 ~ 100 meters. These work on a theoretical basis and knowledge and increasingly serve as a virtual laboratory for clouds and turbulence and are used to develop and test parameterisations in climate models.


If these two models are viewed together, they will influence each other, you will get better insights.


Observations and new techniques (in 2007)


A new generation of satellites, A-train, can use active sensors such as cloud radar and Lidar systems to see through the clouds and build a complete 3-dimensional image of cloud climatology (Stephens et al., 2002).


We can also build such an image from the ground (Cabauw). By combining lidar and cloud radar measurements, it is now possible, for example, to routinely determine the coverage fraction of clouds at any height. For the first time, we are able to critically evaluate climate models with regard to their vertical structure. This allows us to determine how good or bad the models are. With the help of local measurements that are taken worldwide, we can find out what causes possible errors.


*This article is very outdated given the speed of cloud science, but it does perfectly explain the theory of how clouds work and what the problem with cloud models is, and that hasn't changed.




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© 2019 by Hilde Maassen 

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