Cirrus clouds and climate 1 lated to convection. Ice crystal data in arctic cirrus Cirrus clouds and climate have also been collected which show their shapes Cirrus are thin, wispy clouds that appear at high to be a combination of pristine and irregular types altitude and consist of ice crystals. At midlatitudes, with sizes appearing to be larger than about 40 µm. clouds with base heights above about 6 km In the Antarctic, the extensive collection of ice (20,000 ft) are designated as high clouds, a cate- particles at a surface station reveals the prevalence gory that includes cirrus (Ci), cirrostratus (Cs), and of long, needle-shaped ice crystals. cirrocumulus (Cc). Cirrus clouds are globally dis- Ice crystals vary substantially in size and shape tributed at all latitudes over land or sea at any sea- from the tropics, to midlatitudes, to the polar re- son of the year. They undergo continuous changes in gions. In addition to the variety of intricate shapes area coverage, thickness, texture, and position. The and a large range of crystal sizes, the horizontal orien- most striking cirriform cloud features are produced tation of some cirrus columnar and plate crystals has by weather disturbances in midlatitudes. In the trop- been observed from a number of lidar backscattering ics, cirrus clouds are related to outflows from tower depolarization measurements, as well as limited po- cumulus associated with the convective activity over larization observations from satellites. Also, the fact the oceans. The global cirrus cover has been esti- that numerous halos (sundogs) and bright arcs sur- mated to be about 20–25%, but recent analysis using rounding the Sun have been observed demonstrates the satellite infrared channels at the 15-micrometer that a specific orientation of the ice particles must carbon dioxide (CO2) band has shown that their exist in some cirrus. An understanding of the climatic occurrence is more than 70% over the tropics. effect of cirrus clouds must begin with a comprehen- Cirrus composition. Cirrus clouds usually reside in sive understanding of their microscopic composition the upper troposphere, where temperatures are gen- and associated radiative properties. ◦ ◦ erally colder than −20 to −30 C(−4to−22 F). Be- Cirrus radiative forcing. The amount of sunlight cause of their high location, direct observation of that cirrus clouds reflect, absorb, and transmit de- the composition and structure of cirrus clouds is dif- pends on their coverage, position, thickness, and ice ficult and requires a high-flying aircraft platform. In crystal size and shape distributions. Cirrus clouds the 1980s, comprehensive information about cirrus can also reflect and transmit the thermal infrared composition became available because of the devel- emitted from the surface and the atmosphere and, opment of several airborne instruments to sample at the same time, emit infrared radiation according their particle-size distribution with optical imaging to the temperature structure within them. The ice probes using a laser beam, high-resolution micropho- crystal size and shape distributions and cloud thick- tography, and replicators, which preserve cloud par- ness are fundamental cirrus parameters that deter- ticles in chemical solutions. mine the relative strength of the solar-albedo (reflect- Ice crystal growth has been the subject of con- ing of sunlight) and infrared-greenhouse (trapping of tinuous laboratory, field, and theoretical research in thermal radiation) effects, which are essential com- the atmospheric sciences discipline over the past ponents of the discussion of cirrus clouds and cli- 50 years. It is the general understanding now that the mate. These radiative effects are determined by the shape and size of an ice crystal in cirrus is primarily basic scattering and absorption properties of the ice controlled by the temperature and relative humidity inside the cloud. If ice crystals undergo collision and 14 coalescence due to gravitational pulling and turbu- lence, more complicated shapes can result. In mid- 13 −70 latitudes, where most of the observations have been 12 made, cirrus clouds have been found to be composed −60 of primarily nonspherical ice crystals with shapes 11 C −50 ° ranging from solid and hollow columns to plates, 10 bullet rosettes, and aggregates, with sizes spanning from about ten to thousands of micrometers. Ob- 9 −40 servations in midlatitudes also revealed that at cloud height, km 8 −30 tops pristine small columns and plates are pre- temperature, dominant, whereas at the lower part of the cloud bul- 7 −20 let rosettes and aggregates are most common 6 (see illusration). −10 Limited measurements from high-flying aircraft in 5 200 µm tropical cirrus clouds, which extend as high as 15– 4 0 18 km (8–11mi), show that their ice crystal sizes 0 10 20 30 40 50 60 70 80 90 100 range from about 10–2000 µm with four predomi- relative humidity, % nant shapes—bullet rosettes, aggregates, hollow Ice crystal size and shape as a function of height, temperature, and relative humidity columns, and plates—similar to those occurring in captured by a replicator balloon sounding system in Marshall, Colorado, on November 10, midlatitudes. In the tropics, observations reveal that 1994. The broken and solid lines denote the relative humidity measured by cryogenic hygrometers and Vaisala RS80 instruments, respectively. (Graphic by Andrew Heymsfield, large ice crystal sizes are associated with warmer National Center for Atmospheric Research. data from K. N. Liou, An Introduction to temperatures or the development stage of clouds re- Atmospheric Radiation, 2d ed., Academic Press, 2002) 2 Cirrus clouds and climate crystals. Unlike the scattering of light by spherical position based on physical principals is a difficult water droplets (which can be solved by Lorenz-Mie task, and successful prediction using climate mod- theory), an exact solution for the scattering of light els has been limited. This difficulty is also associated by nonspherical ice crystals, covering all sizes and with the uncertainties and limitations of inferring cir- shapes that occur in the Earth’s atmosphere, does rus cloud cover and position from current satellite not exist in practical terms. Recent advances in this radiometers. In fact, there is not sufficient cirrus area have demonstrated that the scattering and ab- cloud data to correlate with the greenhouse warming sorption properties of ice crystals of all sizes and that has occurred so far. shapes, which commonly occur in the atmosphere, Another issue that determines the role that cir- can be calculated with high precision by a unified rus play in climate and greenhouse warming is re- theory for light scattering. This theory combines the lated to the variation of ice water content and crystal geometric optics approach for large particles and the size in these clouds. Based on aircraft observations, finite-difference time-domain numerical method for some evidence suggests that there is a distinct cor- small particles. Results of this theory have been used relation between temperature and ice water content to assist in the remote-sensing and climate-modeling and crystal size. An increase in temperature leads programs involving cirrus clouds. to an increase in ice water content. Ice crystals are To comprehend the impact of cirrus clouds on smaller at colder temperatures and larger at warmer the radiation field of the Earth and the atmosphere temperatures. The implication of these microphys- and thus climate, the term “cloud radiative forcing” ical relationships for climate is significant. For high is used to quantify the relative significance of the cirrus containing primarily nonspherical ice crystals, solar-albedo and infrared-greenhouse effects. Cloud climate model results suggest that the balance of radiative forcing is the difference between the ra- solar-albedo versus infrared-greenhouse effects, that diative fluxes at the top of the atmosphere in clear is, positive or negative feedback, depends not only and cloudy conditions. The addition of a cloud layer on ice water content but also on ice crystal size. in a clear sky would lead to more sunlight reflected This competing effect differs from low clouds con- back to space, reducing the amount of solar energy taining purely water droplets in which a temperature available to the atmosphere and the surface. In con- increase in the region of these clouds would result trast, the trapping of atmospheric thermal emission in greater liquid water content and reflect more sun- by nonblack (-body) cirrus clouds enhances the radia- light, leading to a negative feedback. tive energy, or heat, available in the atmosphere and Contrail cirrus. In addition to naturally occurring the surface. Based on theoretical calculations, it has cirrus, the upper-level ice crystal clouds produced been shown that the infrared greenhouse effect for by high-flying aircraft, known as contrails, or con- cirrus clouds generally outweighs their solar albedo densation trails, have also been frequently observed. counterpart, except when the clouds contain very Contrails are generated behind aircraft flying in suffi- small ice crystals on the order of a few micrometers, ciently cold air, where water droplets can form on the which exert a strong solar-albedo effect. The rela- soot and sulfuric acid particles emitted from aircraft tive significance of the solar-albedo versus infrared- or on background particles and then freeze to be- greenhouse effects is clearly dependent on the ice come ice particles. Based on a number of recent field crystal size and the amount of ice in the cloud. Be- experiments, contrails were found to predominantly cause of the complexity of sorting cirrus signatures consist of bullet rosettes, columns, and plates, with from satellite observations, actual data to calculate sizes ranging from about 1 to 100 µm. Persistent con- the global cirrus cloud radiative forcing is not yet trails often develop into more extensive contrails in available. which the ice supersaturation is generally too low to Cirrus and greenhouse warming.