Clouds - more than meets the eye
Talking about definitions
Clouds - those white fluffy-cauliflower looking water in the sky. Ohh, yeah, they can also be smooth looking like a good vanilla-flavor icing on a delicious cake, or intimidating giants spawning tornadoes and golf-size hail.
How well does this lumpy description serve as definition of a cloud?
Well, according to the official definition the answer is: not so bad. According to the American Meteorological Society (AMS) glossary, a cloud is "A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the Earth's surface". Following this definition, clouds are being classified into different types based on their shape, composition, and (visible) cloud base altitude (Cumulonimbus, Altostratus, Cirrus, etc.).
So we can conclude this post, right? Clouds are just water in liquid or ice phase, which are being sustained in the sky by some force, and are visible (to our eyes). Ummm, no, not exactly. With all due respect to our eyes (which are an amazing piece of equipment), they do not see everything, and by writing everything, I mean that they are sensitive only to the visible light frequency range of the electromagnetic spectrum (all the colors from violet through green up to red) and that they are limited in detecting particles (which emit light, e.g., flashlight, or scatter light, e.g., a tennis ball) depending on their size, distance, and contrast with respect to their surrounding. Other instruments which are man-made, such as radars (that work at frequencies relatively close to our microwave or cell phone's frequencies of operation) can succeed where the naked eye fails. That is because these instruments are much more sensitive to water particles (in both phases) than our eyes, and can therefore detect even the slightest concentrations of water particles in a certain air volume. The concentration and size of the particles can be so small that we would just observe clear skies, while the radar will detect a clear formidable picture of a cloud just lying there in mid-air. Yup, that might also be a cloud. Already here we can see that the very basic definition of a cloud can be quite vague.
Cloud physics research
Not less elusive is the physics of clouds, the processes that are taking place inside and around clouds; how are the winds (which also endorse the condensation of the water particles and keep them sustained in the air) affect the lifetime of a cloud? How do different water particles interact with each other? What is the structure of different types of clouds? How does the process of cloud electrification (which eventually results in lightning discharges) occur? What would be the shape of water ice particles in a cloud (as a function of water saturation, temperature, winds, etc.)? At what temperatures would we still be able to find liquid water in mid-air (theoretically, down to -38°C)?
These questions, which are connected to each other and to research questions in other atmospheric science fields, arise and should be answered not just for the gratification of our pure interest (although that is a major factor in answering them), but also because they are crucial for the understanding of the interaction of clouds with the environment through their impact on the hydrologic cycle and their radiative forcing on the atmosphere and the Earth's surface. The latter is very relevant these days, due to its link to the climate change we are experiencing worldwide. That is because the relative short lifetime of clouds is the source of highest uncertainty in climate models, which are trying to predict how would the atmosphere behave in the near-future, that is, the coming years and decades.
Ice, ice, warming
When I am thinking about climate models, there is a big irony regarding their current (climate change) purpose, as they perform poorly above the polar regions where in some sense, their accuracy is the most important. The poor performance is caused as a result of several factors; lack of observations is in many aspects one of the primary reasons.
The importance of understanding what exactly is going on in the polar regions with regards to climate change is due to a few main reasons. When we picture the poles, we most likely see some cute penguins or (cute from a distance) polar bears, but they are hanging around ice, and plenty of it. This white ice acts as an excellent reflector of solar radiation, thereby cooling the atmosphere (try entering a black car exposed to direct sunlight during a hot summer day; then feel the relative cool breeze upon entering a white car experiencing identical environmental conditions). The intriguing thing about our atmosphere's radiative equilibrium is its complexity. To me, it often reminds a Rube-Goldberg machine which restarts itself at the end of each run with a bit different features (or starting points). One example for this way of thinking is that warming of the atmosphere and ocean melts the polar ice a bit, which in turn allows more solar radiation to join the party, causing stronger warming, which forces a stronger melting, and so on. As one may understand, this can be one brutal chain reaction. Melting of some or all of this ice (the ice laying on the ground and not the ice-shelves or sea-ice) can generate an intense sea level rise, which can be critical to coastal cities (where most humans live).
Polar clouds
This is where clouds enter the picture again (they always add something unique to pictures, aren't they?). If you remember, there is a large uncertainty regarding their influence on the surface radiative budget; They can cool the surface by reflecting plenty of the incoming solar radiation back to space, but may also warm the surface by absorbing the heat release from the surface and radiating some of it back downwards (similar to a high quality coat, which keeps us warm during a cold winter day). The water particles size and concentration, their shape, and their phase (liquid/ice), affect these radiative properties, and generate altogether a large number of interactions between the particles and themselves or the environment, resulting in net cooling or warming of the surface.
The poles make this entire cloud-surface relationship even more complex. The polar regions (mainly Antarctica) are relatively clean of natural and anthropogenic (man-made) pollution. Many of the pollution particles are key ingredients in the determination of the cloud characteristics mentioned above. Therefore, clouds in polar regions behave differently from clouds in other parts of the world. Unfortunately, there are not many polar cloud observations, and as I mentioned before, this strongly affects climate models' accuracy. Satellite instruments provide plenty of valuable information about clouds. However, they have many limitations; their polar orbiting nature inhibits high observational temporal resolution, which is critical for the understanding of the processes happening within the clouds they observe (processes which are generally termed cloud microphysics), while geostationary satellites (the ones that observe a specific region of the globe) orbit above the equator, and are therefore obstructed from gathering high quality data about polar region clouds. This is where ground-based instruments enter the picture. Ground-based campaigns are capable of broadening the current polar cloud knowledge, as the instruments used in these campaigns provide a comprehensive picture of clouds and their environment, offering high temporal resolution even in the microscopic scales needed for the understanding of cloud microphysics.
Antarctic clouds research
One of these recent campaigns is the ARM West Antarctic Radiation Experiment (AWARE). This campaign takes place near the West Antarctic Ice Sheet, one of the fastest warming regions on the globe, where no substantial field campaign has taken place for more than five decades. This might sound surprising in research perspective as these observations are needed so desperately, but performing such an intense field campaigns demands plenty of budget, preparation, manpower, and infrastructure, in order to support and maintain the instruments and their operation.
As part of the AWARE campaign, radars are constantly observing the skies detecting clouds and gathering information about their composition. Lidars are used as well, in order to give a more thorough picture of the Antartctic clouds (unlike radars, lidars are based on lasers, and are commonly operating at frequencies within the visible spectrum, making them sensitive to the smallest cloud particles, droplets or ice, at a much higher level than radars). Together, these instruments can give information about the population of the water particles within the clouds at different altitudes, their relative velocity, their phase, and their shape. My job in the force (as a post-doctoral fellow at Penn State University) is to combine these observations and broaden (even by a tiny bit) the current understanding of the Antarctic cloud microphysics, which is so greatly needed for us to accurately estimate our near-future Earth.
Undoubtedly, clouds have much more than meets the eye, but wait a minute, remind me that definition of a cloud again?