Clouds play a vital role in our climate by regulatingthe amount of solar energy that reaches the surface and the amount of theEarth's energy that is radiated back into space. The more energy that is trapped by the planet, the warmer our climate will grow.If less energy is collected, the climate will become cooler.Understanding this energy balance is fundamental to answering any of the questions posed by climate change.
The Earth's atmosphere hasseen large variations in it's history, but only recently haveclimatologists been finding signs that human activity on the planetis causing climate change. To forecast the impact and severity of these changes,climatologists run computer simulations based on our understanding of thephysical dynamics of the environment. These models must include all aspects of the atmosphere, surface andoceans which effect this balance in order to improve the accuracy of theirpredictions. Clouds are not well represented in these models due to a limitedunderstanding of their energy characteristics (how well they absorb or reflect energy) and distribution (where and how many clouds there are).
What questions do climate researchers ask about clouds?
- How many layers of clouds are there?
- Are the clouds made of ice or water?
- At what altitude is the cloud base? Cloud top?
- How much does it rain? Snow?
- How large are the water dropplets or ice crystals in the cloud?
- What fraction of the sky is cloudy?
 Figure 1: Energy can be absorbed, reflected or transmitted by clouds. |
 Figure 2: Radar image showing different phases of water in the arctic atmosphere: Ice clouds, water clouds and snow. |
 Figure 3: Lidar image showing the mixed phase of arctic clouds, containing both water and ice |
Cloud Characteristics
Depending on their altitude, structure and composition (ice or water) cloudswill regulate energy differently. One cloud may trap heat by reflecting energy back to the surface. Another may reflect sunlightand cause the surface to cool.You may have noticed that a cloudless night can be much colder than a cloudy night. Without the heating of the sun and a layerof clouds to insulate us, the surface radiates more heat into space on cloudless nights,making them colder.
To study these characteristics, researchers combine data from a variety of instruments to form a complete understanding of cloud radiative properties. Instrumentsused to study clouds include:
MilliMeter Cloud Radar (MMCR) developed at PSL uses electromagnetic waves todetermine the characteristics of cloud particles from radar reflectivity whichis the strength of the signal reflected from the cloud particles. The MMCR can also measureparticle velocity, which is determined from the Doppler shift that the particle motion adds tothe returned signal. The MMCR was specially designed to detect non-precipitating and weaklyprecipitating clouds, which have a major impact on climate. This makes it very different fromthe longer wavelength, weather radars you might see on television which are designed to detectrain, snow and hail.
Depolarization And Backscatter Unattended Lidar (DABUL) developed at PSL uses a laser to measure very weak clouds as well as aerosol particles. DABUL can also distinguish thephase of a cloud, in other words, whether the clouds is made of water dropplets or ice crystals.
Figure 2 and 3 are data taken in the Arctic demonstrates how complex clouds can be with multiple layers of ice and water regions. To distinguish between an ice cloud and a water cloud, scientists combine data from the MMCRand DABUL to determine the cloud boundaries, phase and particledistribution. The DABUL's laser becomes completely absorbedby the water in the clouds, which appear as bright orange bars.
How many clouds are there?
The distribution of clouds also plays an important role in regulatingour climate. By knowing how much of the Earth is covered with clouds,climatologists can improve their models and watch for signs of climatechange. Increases or decreases in the number and type of clouds couldindicate that climate feedback loops are at work cooling or warmingthe planet.
Counting clouds however is not easily done. From space, satllitesare limited to seeing the tops of clouds and many clouds are smaller than the satllite footprint. (A footprint isthe horizontal dimentions of a satllite measurement, which can betens of kilometers along a side.)
From the surface, cloud observations have traditionally been madeby a human observer, which are often very inacurate, especiallyat night. Radars and lidars are much more accurate. However, becausethe technology is new and expensive there are only a handful of theseinstruments operating. Surface observations will always be limitedin harsh, sparsely inhabited environments. This problem is beingaddressed by speciallized field programs which study clouds in theseenvironments.
Clouds in the Arctic
Fine ice crystals in the cold, dry air of the Arctic refract light into an icebow.
One such program, the Surface Heat Budget of the Arctic Ocean (SHEBA), at the University of Washington was designed to address the scarcity of climate data in the Arctic.The Arctic is a critical climate zone due the extremes of solar energyit receives and the effects it can have on the surrounding oceans. During SHEBA, researchers spent a year on the iceof the Arctic Ocean, measuring the energy exchanges between the ocean,the ice and the atmosphere. A critical component of the atmosphericmeasurements were the cloud observations made with the MMCR and DABUL.
Cumulus in the tropical western Pacific.
Clouds in the Tropics
In another sparsely inhabited climate zone, the Tropical Western Pacific (TWP),the PSL MMCR and DABUL will make identical measurements of tropical clouds.Deployed on the deck of the NOAA Ronald H. Brown, clouds measurements will be taken aspart of the JASMINE and Nauru99 programsThe TWP is considered the "furnace" of the planet, absorbingsolar energy and converting it into atmospheric water vapor (clouds). Climatechange in the TWP impacts world wideweather as has been dramatically demonstrated by El Niño.
