ARTICLES CLOUDSAT Adding a New Dimension to a Classical View of Extratropical Cyclones BY DEREK J. POSSELT, GRAEME L. STEPHENS, AND MARTIN MILLER Cloudsat probes the internal structure of frontal clouds and precipitation, and provides new perspective on a classical conceptual model. FIG. I. Illustration of frontal clouds and precipitation in the Norwegian Cyclone Model, as conceptualized in Bjerknes and Solberg (1922). xtratropical cyclones constitute some of the midlatitude low pressure systems provide much of largest and most complex weather systems on the precipitation received in the planet's temperate Eour planet and play a key role in regulating zones, and hence have an important effect on the Earth's energy balance, both through the meridi- availability of freshwater. Indeed, the earliest con- onal transport of energy from the equator to the ceptual models of extratropical cyclone structure poles, as well as through the effects of the clouds and development arose out of the need to under- they produce on the climate system. In addition, stand patterns of rainfall, and over the past two AMERICAN METEOROLOGICAL SOCIETY MAY 2008 BAF15* I 599 Unauthenticated | Downloaded 10/11/21 01:51 PM UTC centuries, research on extratropical cyclones has of atmospheric processes and the development of highlighted the continued need for understanding the telegraph network, culminated in the Norwegian the distribution of clouds and precipitation associ- Cyclone Model (NCM) in the early 1920s (Bjerknes ated with these storms. and Solberg 1922; Fig. I).1 This was the first conceptual During the mid- to late 1800s, an increased under- model to document cyclone evolution from inception standing of the thermodynamics of the atmosphere to decay. In addition, the NCM provided a remark- and, in particular, the energy released through the ably accurate depiction of the three-dimensional process of the condensation of water vapor, paved structure of fronts, describing the near-discontinuous the way for one of the first conceptual models of the demarcation between different air masses, and giving development of extratropical cyclones. In what has a reasonable, albeit qualitative, explanation for the since been referred to as the thermal theory of cyclones observed distribution of clouds and precipitation in (Kutzbach 1979), it was speculated that diabatic heating midlatitude storms. Though attention quickly shifted associated with widespread precipitation produced from the analysis of rainfall to attempts to understand troposphere-deep ascending motion, leading to the the origin and nature of the circulations observed low-level convergence and upper-level divergence that in cyclones, a key component of the observational characterize midlatitude low pressure systems. By support for the NCM was the distribution of clouds the turn of the century, an increasingly widespread and precipitation associated with cold, warm, and observing network demonstrated the asymmetric occluded fronts. nature of clouds, precipitation, and low-level circula- The rise of numerical models as tools for analysis tion in low pressure systems, and conceptual models and forecasting provided renewed impetus for the were modified accordingly. Though the understanding study of clouds and precipitation in extratropical of the processes that drive extratropical cyclone devel- cyclones in the middle of the twentieth century. opment has continued to evolve, observations of the These models allowed for the examination of internal structure of frontal clouds and precipitation the role of various physical processes in cyclone have been few and far between. With the addition development, in particular, latent heat release and of Cloudsat (Stephens et al. 2002) to the National cloud effects on radiation. Multiple studies noted Aeronautics and Space Administration's (NASA's) that latent heat release not only contributed to the A-Train in April 2006, new and truly global observa- intensity of low pressure systems and the rapidity of tions of the internal structure of clouds and precipita- cyclogenesis (Smagorinski 1956; Krishnamurti 1968; tion are now available. A few of the new insights that Tracton 1973; Chang et al. 1982; Reed et al. 1988; Kuo these observations provide on the structure of clouds in et al. 1991; Davis 1992; Whittaker and Davis 1994; extratropical cyclones are highlighted in this paper. Stoelinga 1996), but also acted to change the spatial patterns of surface pressure and precipitation, as well A BRIEF HISTORY OF EXTRATROPICAL as the structure and evolution of fronts (Baldwin et al. CYCLONE RESEARCH FROM THE 1984; Dudhia 1993; Posselt and Martin 2004; Reeves NORWEGIAN CYCLONE MODEL TO THE and Lackmann 2004). It was also found that changes PRESENT. The advent of vertical soundings, in the cloud effect on radiative fluxes could have a coupled with the increased physical understanding significant effect on the development of baroclinic waves. In particular, Simmons (1999) showed that an overproduction of upper-tropospheric cloud- AFFILIATIONS: POSSELT—Department of Atmospheric, Oceanic induced cooling led to the anomalous enhancement and Space Sciences, University of Michigan, Ann Arbor, Michigan; of a simulated upper-level wave and the prediction STEPHENS—Department of Atmospheric Science, Colorado State of a strong cut-off low pressure system that was not University, Fort Collins, Colorado; MILLER—European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom 1 CORRESPONDING AUTHOR: Dr. Derek J. Posselt, 2455 Although Bjerknes and Solberg (1922) set out to describe Hay ward Street, University of Michigan, Ann Arbor, Ml 48109-2143 the evolution of cyclones from inception to decay, the clouds E-mail: [email protected] and precipitation associated with fronts in the NCM are best The abstract for this article can be found in this issue, following the shown in Fig. 1. Because the NCM assumed that the structure table of contents. of the fronts did not change significantly during the cyclone DOklO.I I75/BAMS-89-5-599 life cycle, we will compare the clouds and precipitation in this figure with Cloudsat observations, but the reader should In final form 11 October 2007 ©2008 American Meteorological Society keep in mind the fact that extratropical cyclones and their associated fronts are evolving dynamical systems. 600 I BAflS* MAY 2008 Unauthenticated | Downloaded 10/11/21 01:51 PM UTC subsequently observed. After decades of analysis, it (downward)-pointing radar that operates at a wave- is now well known that clouds, precipitation, and the length of approximately 3 mm (94-GHz frequency). associated latent heat release and effect on visible and The relatively short wavelength (as compared with infrared fluxes play a key role in cyclone formation the 10 cm used for NEXRAD) and large range of and evolution. However, the specifics of the effect sensitivity (from -30 to +50 dBZ)2 is designed to of clouds on extratropical cyclone dynamics is still allow the CPR to observe a wide range of cloud not completely understood. This is, in part, due to types and thicknesses. The radar has a footprint that the nonlinear feedback between latent heat release, measures 1.4 km across track by 3.5 km along track radiation, and circulation, and also the fact that the at the surface, with a pulse width of 3.3 ^s (pulse- fidelity of the numerical representations of clouds and repetition frequency of4,300 Hz) and vertical range precipitation critically depends on the model's physi- bins of 250 m. Data are averaged every 0.16 s along cal parameterizations (Simmons 1999; Lackmann track, are quality controlled, and can be obtained et al. 2002; Mahoney and Lackmann 2006). online from the Cloudsat data processing center. Along with numerical models, observational Temperature, water vapor, and ozone profiles from studies have played a key role in understanding the the European Centre for Medium-Range Weather distribution of clouds and precipitation in extra- Forecasts (ECMWF) operational analysis are time tropical cyclones. Field experiments [e.g., the Fronts and space interpolated to the Cloudsat track, and and Atlantic Storm Track Experiment (FASTEX); top-of-the-atmosphere visible and infrared radi- Joly et al. 1997,1999] have revealed the tremendous ances from channels 20 and 27-36 on the Aqua range of cyclone features that exist in nature, as well Moderate-Resolution Imaging Spectroradiometer as the high degree of spatial and temporal variability (MODIS) instrument are matched to Cloudsat CPR of clouds and precipitation. Conversely, satellite data. In addition to radar reflectivity, a growing composites have been effectively used to aggregate number of retrieved cloud parameters are computed observed cyclones into various categories, and have and archived as part of the Cloudsat dataset; these aided in the development of modern conceptual include cloud classification, cloud liquid and ice models (Lau and Crane 1995,1997; Browning 1999; water content, cloud optical depth, and long- and Tselioudis et al. 2000; Tselioudis and Rossow 2006; shortwave radiative fluxes and heating rates. [For Field and Wood 2007). Though current observations a detailed description of each dataset available in can provide information on the extent of clouds for the Cloudsat data stream, the reader is referred to a range of cyclone types, to effectively assess the the online Cloudsat Data Processing Center (www. effects of clouds on cyclone evolution, it is essential cloudsat.cira.colostate.edu/index.php).] to