1 SATELLITE INVESTIGATION OF TROPICAL CIRRUS LIFECYCLE AND RELATION TO WATER VAPOR AND DEEP CONVECTION Zhengzhao Luo Cooperative Institute for Research in the Atmosphere Colorado State University Fort Collins, Colorado William B. Rossow NASA Goddard Institute for Space Studies New York, New York Abstract conditions in which they occur. Some studies suggest that the response of tropical cirrus to external climate forcing might have a controlling effect on global Tropical cirrus lifecycle and relation to deep climate sensitivity (CRYASTAL Research Plan 2000). convection are examined by analyzing satellite Usually, tropical cirrus are thought to originate in derived cloud data and NCEP/NCAR reanalysis wind outflow from deep convective systems, producing a field. Also discussed is the relationship between wide range of high-level clouds from thick anvils to tropical cirrus and upper tropospheric water vapor thin cirrus that are sometimes all called cirrus, but (UTWV). The analysis in this study is conducted in a other dynamic processes, such as the mesoscale Lagrangian framework, which assumes cirrus, like circulations of larger storm systems, large-scale lifting other tracers, drift with the upper tropospheric wind, or gravity waves, can also produce cirrus. Beyond while at the same time, going through their lifecycle this, very little is known, at present, about the from formation to maturation to decay. The distribution of tropical cirrus, their variability and Lagrangian trajectory analysis shows that the decay formation-decay processes. of deep convection is immediately followed by the Tropical cirrus have been studied in a number of growth of cirrostratus and then the decay of field campaigns. The Tropical Ocean Global cirrostratus is followed by the growth of cirrus. Cirrus Atmosphere Coupled Ocean-Atmosphere Regional properties continuously evolve along the trajectories Experiment (TOGA COARE), although focusing as they gradually thin out and move to the lower mostly on sea-air interaction and deep convective levels. Typical tropical cirrus systems last for 19 - 30 clouds, obtained some observations that have been hours, which is much longer than cirrus particle used to study cirrus anvils in the tropical west Pacific lifetimes. Consequently, tropical cirrus systems can (Ye 1999). The Central Equatorial Pacific Experiment advect over large distances, about 600 - 1000 km, (CEPEX) also made measurements of the during their lifetimes. Based on their relationship to microphysical properties of tropical cirrus anvils convective systems, detrainment cirrus are (McFarquhar and Heymsfield 1996), which clarified distinguished from in situ cirrus. It is found that more the association between the cirrus microphysical than half of the tropical cirrus are formed in situ well properties and their relative high albedo (Heymsfield away from convection. The interaction between and McFarquhar 1996). A recent scientific program cirrus and UTWV is explored by comparing the more dedicated to tropical cirrus, Cirrus Regional evolution of UTWV along composite clear trajectories Study of Tropical Anvils and Cirrus Layers – Florida and trajectories with cirrus. Cirrus are found to be Area Cirrus Experiment (CRYSTAL-FACE), was associated with an upward transport of water vapor carried out in July 2002 to investigate tropical cirrus that moistens the upper troposphere. The cloud physical properties and formation processes. maintenance of UTWV is discussed by using a simple, conceptual model. These field programs enhance our understanding of some aspects of tropical cirrus. The CRYSTAL-FACE, for example, measured cirrus anvil 1. Introduction properties through as much of the cloud lifecycle as possible to advance our knowledge of the evolution of Tropical cirrus have been difficult to study cirrus anvils. However, these field campaigns are all because of their great altitude and the extreme very limited in space and time, making it difficult to 1 2 study larger-scale cirrus processes and their the lifecycle of the cirrus anvils and could not seasonal/geographical variations and preventing us determine where to draw a line between cirrus formed from developing and generalizing an understanding of out of convection and cirrus formed elsewhere by how cirrus form, evolve, and dissipate. In contrast, other processes. Recently, Pfister et al. (2001) and satellite observations have provided a global overview Massie et al. (2002) studied the different origins for of cloud systems at mesoscale to synoptic scale for tropical tropopause subvisible cirrus using aircraft and more than three decades. To understand cirrus satellite occultation measurements coupled with a evolution processes globally over a long period of back trajectory method. Massie et al. (2002) found time, we still need to analyze these satellite data. that deep convection was encountered only 49% of the time along the 5-day back trajectories for the The last three decades have witnessed regions 60E -180E and 20S - 20N. A common numerous specialized cirrus retrieval algorithms from assumption, when discussing cirrus feedback, is to satellite measurements, where we use the word cirrus assume that all tropical cirrus are derived from or to refer exclusively to the optically thinner (transparent closely linked to convective systems, technically to infrared radiation) high altitude clouds (e.g. reducing the cirrus feedback to a “convection-SST” Chahine 1974; Szejwach 1982; Inoue 1985; Liou et relationship (e.g. Ramanathan and Collins 1991; al. 1990; Wylie et al. 1994; Stubenrauch et al 1999b). Chou and Neelin 1999; Lindzen et al. 2001). Nevertheless, very few of these analysis methods However, as shown in this study, this is not the case; have been applied to satellite data to produce globally more than half of tropical cirrus are only loosely, if at uniform, long-term cirrus datasets; the notable all, connected to convection and thus tend to have a exceptions are Wylie et al 1994, Wylie and Menzel dynamics of their own. Deep convection can 1999, and Stubenrauch et al. 1999a. The produce a wide range of high-level clouds evolving International Satellite Cloud Climatology Project from thick anvils to cirrus. Cirrus produced in-situ (ISCCP), since its establishment in 1982, has may have their own way of forming, evolving and produced an 18-year long (so far), 3-hourly/30-km decaying. However, there has been no global survey cloud climatology (Schiffer and Rossow 1983; of tropical cirrus lifecycles, no study of cirrus lifetime Rossow and Schiffer 1991, 1999). Sensitivity to duration, let alone the evolution of cirrus properties. cirrus has been increased and biases in cirrus optical To characterize cirrus lifecycle and evolution thickness are reduced in the second version (D- processes, we developed a Lagrangian trajectory series) of the ISCCP cloud product (Rossow and method that tracks tropical cirrus systems by following Schiffer 1999). ISCCP retrieves cloud properties upper tropospheric air masses using winds from the from infrared (≈ 11 µm) and visible (≈ 0.6 µm) NCEP/NCAR (National Centers for Environmental radiances measured by the imaging instruments on all Prediction/National Centers for Atmospheric the operational weather satellites, so specific cirrus Research) reanalysis. information is available only during daytime. Several studies have quantified the successes and limitations We complement the cloud measurements with of the ISCCP cirrus representation (Liao et al. 1995a, determinations of upper tropospheric water vapor b, Jin et al 1996, Jin and Rossow 1997, Stubenrauch (UTWV) to examine its interaction with cirrus. The et al. 1999, Luo et al. 2002). In our study of cirrus key to this study is the development of a technique for variability and evolution, we develop a refined cirrus determining UTWV in the presence of cirrus by a dataset derived from only infrared observations by combined analysis of satellite infrared and microwave building upon the existing ISCCP DX data and TOVS radiances. This allows us for the first time to follow (TIROS-N Operational Vertical Sounder) products. the evolution of both UTWV and cirrus. The This cirrus dataset provides continuous coverage mesoscale convective systems transport a lot of water throughout each day by using two radiances from the upward and thus serve as a water vapor source for 1 IR window spectrum roughly at 11 µm and 12 µm. the vast tropical (30S – 30N ) upper troposphere, but Comparison with the ISCCP cirrus products will be the area coverage of these convective systems is shown. very small: about 0.03 of the ± 300 zone according to the ISCCP. The rest of the tropical upper troposphere We make use of the cirrus dataset to examine is either covered with cirrus (0.14), cirrostratus (0.05) the variability of tropical cirrus and to study their according to ISCCP) or no clouds in the upper lifecycle and evolution. Specifically, the following troposphere (here taken to be about the 440 mb or 7 questions are explored: 1) What is the relative km level). The thinnest cirrus that ISCCP misses but importance of deep convection and other dynamical other satellite instruments (e.g. solar occultation processes in producing cirrus? 2) How long do measurements) detect can cover another 0.05 – 0.10 tropical cirrus last and how do cirrus properties evolve of the tropics (Liao et al. 1995; Jin et al. 1996; during their lifecycle? It is not clear over the whole Stubenrauch et al. 1999). In addition, subvisual tropics how many cirrus are derived from