As I’ve already mentioned on this blog, clouds are really important. But how do we measure them? The answer is that there are many methods, each with their strengths and weaknesses, that are typically used to measure cloud properties. A few of these are given below, but this list is not exhaustive.
1. From the ground
The first one is kind of obvious – you look at them! There’s not that many physical observers out there any more, and the number of manned meteorological stations is diminishing (or minimal to start with, in the case of Antarctica)
- cloud base height
- cloud amount
- cloud type
+ People have been making observations for a long time, so there is a long record
– it’s a subjective art – different people can record very different things
– stations are sparsely distributed, and most in Antarctica are located on the coast. Some have also moved, which can introduce bias.
– it is hard to see clouds at night! This can lead to under-reporting: in Antarctic winter, cloud coverage is reportedly 20% lower
From N A I T via Flickr
b. Ground-based infrared measurements
Instruments like pyrogeometers can be used to measure the radiative properties of clouds and cloud cover by measuring the amount of longwave and shortwave radiation that is transmitted to the surface from clouds, as well as the amount that is going from the surface up.
+ Can measure clouds just as well during night and day, because it doesn’t use visible light
+ Can reveal the radiative effects of clouds
– calibration required to deduce cloud cover from radiation measurements
– observations are few and far between because there aren’t that many stations
2. From the air
Being as clouds are in the air, it makes sense to take the instruments to them, and measure them directly, i.e. take in situ measurements. Aircraft can carry a huge number of different instruments, and the range of properties they can measure therefore varies depending on the complement of instruments on board (NASA has a huge number of instruments in its database, for instance).
In the UK, the Facility for Airborne Atmospheric Measurements (FAAM) takes measurements of clouds on its flights, and one of the British Antarctic Survey’s Twin Otter aircraft is equipped to record information about cloud parameters on its flights.
Some typical instruments for measuring cloud microphysics include:
- Cloud Imaging Probe (CIP) which takes pictures of particles as they enter its 2D field of view and can measure liquid water content, the size of particles, the number of particles of different sizes (the particle size distribution), the number concentration
- Cloud Droplet Probe (CDP) which measures small particles – these are almost always liquid droplets. The CDP can measure average drop diameter, particle concentration and size distribution.
- 2D-S which measures particle size distributions and number concentrations (this has been upgraded to a CIP on the FAAM, but it produces similar data).
- Cloud and Aerosol Spectrometer (CAS) measures particles across different size ranges, and is often included as part of the combined CAPS probe (below).
- Grimm aerosol spectrometer measures very small particles. This is used to measure the concentration of aerosols that can act as ice nuclei or cloud condensation nuclei.
- Hotwire liquid and total water content probes e.g. Nezorov or Johnson Williams. Both work by measuring the resistance across a heated wire.
- Cloud, Aerosol and Precipitation Spectrometer (CAPS) is a combined instrument which includes several instruments (CAS, CIP and hotwire LWC sensor) in the same canister. It measures the size and number of particles in various size ranges, their shape, and their optical properties, as well as liquid water content and atmospheric properties like temperature.
+ great source of high-res, in situ data
+ can amend flight tracks on-the-fly (yes, pun intended) to look at features of interest
+ vast number of properties that can be measured – and this can be adapted to changing needs
– only able to look at specific tracks and clouds, therefore the spatial and temporal coverage is minimal
– sampling bias (e.g. Field & Furtado, 2016) caused because of choices made en route, for instance because of dangerous weather or visibility (avoiding clouds during take-off/landing etc.), or because of scientific aims (consistently sampling larger clouds, for instance). Because of the difficulties of flying in Antarctica, cloud flights are limited to summer in Antarctica
– expensive, therefore not that much data (for example, the flight campaigns on the Antarctic Peninsula in 2010/11 were the first cloud flights since the 1980s there – that’s the kind of data gap we’re talking about)
Sondes are balloons equipped with a payload of instruments that measure atmospheric properties as they ascend through the atmosphere. They provide a high temporal resolution vertical profile of the atmosphere that can be used to infer cloud properties.
Sondes measure things like air temperature, dewpoint temperature (the temperature the air would have if it was cooled to the point where all the water vapour was condensed out), position, wind speed, wind direction, humidity and pressure. This small set of variables can be used to diagnose where clouds are likely to be present using a method such as that used in Zhang et al. (2012).
- cloud profile
+ get launched regularly from many locations
+ high temporal resolution
+ cheap to launch
– don’t directly measure clouds
– can’t get any information about cloud microphysics
– only launched at specific locations (weather stations), at prescribed times of day
3. From space
Remote sensing is increasingly used, and space-based satellites are one of the largest sources of data on clouds. They can measure an ever-growing number of properties which are of use for measuring clouds.
Passive satellite instruments measure signals that naturally come off the Earth: they are passive because they simply receive information that is transmitted from the surface. Active instruments on the other hand beam out their own signals so that they can measure them when they come back. The degree to which that signal is altered on its return can tell the instrument something about the properties of the thing that it is trying to measure.
Many different types of instrument can be placed on a satellite, so the list of properties that they can measure is a long one.
- cloud top temperature
- cloud top height
- cloud amount (coverage)
- cloud optical depth
- water/ice content
- thermodynamic phase (ice/water/mixed cloud)
- radiative properties
- anything else that the sensors can measure
+ global coverage
+ LOTS of data
+ regular passes, meaning fairly good temporal resolution with some instruments
– some instruments can only see the top layer of cloud – if the cloud deck has multiple layers to it, some satellites will not be able to see them
– not always so good for looking at one particular region if that region doesn’t get many satellite passes over it – Larsen C for instance
– passive instruments find it hard to distinguish between clouds and ice surface because the temperature and radiative properties are similar
4. From a combination of these
Light detection and ranging, or LIDAR, is a versatile technology that can be used to measure a variety of cloud properties. It works by firing light signals at clouds and measuring them as they come back. LIDAR instruments can measure from the ground, or be mounted on aircraft or satellites like NASA’s ICESat or the CALIOP instrument onboard CALIPSO. degree of polarisation can be used to determine phase
LIDAR can be used to measure:
- cloud profile, including cloud base/top height
- phase (whether ice or liquid)
- optical depth
- particle size distribution
+ high resolution data
– can’t measure particle size
– generates lots of data, which can be hard to process
– relatively expensive
– multi-layer clouds can be hard to measure
Like LIDAR, RADAR instruments can be used from the ground, mounted on satellites like CloudSat, or on aircraft, as NASA’s Airborne Cloud Radar. Radar instruments measure the reflectivity of objects in the same way as LIDAR. The main difference between them is the wavelength of the signals they send out: whereas LIDAR uses lasers in the visible and near-infrared, radar uses longer wavelength signals.
In practice, LIDAR and RADAR are often used together: LIDAR is often thought to be better for measuring cloud top, while RADAR is considered best for observing clouds from the bottom up. The combination of the two can often provide a more reliable picture of how extensive a cloud layer is in the vertical than either alone.
RADAR can be used to measure:
- cloud phase
- cloud profile, i.e. cloud base/top height
- liquid/ice water content
+ can be used day or night
+ reliable, high quality data
– difficulty in measuring small cloud drops when it is raining
– longer wavelength, therefore lower resolution than LIDAR