Geography Compass 2 (2008): 10.1111/j.1749-8198.2008.00166.x An Observational History of Small-Scale Katabatic Winds in Mid-Latitudes Greg Poulos1 and Shiyuan (Sharon) Zhong2* 1Clipper Wind Power Development Inc 2Department of Geography, Michigan State University. Abstract Katabatic winds have been the subject of investigation since about the 1840s. These winds, which flow down the topographic gradient as a result of surface cooling, provide a major transport and dispersion mechanism in mountainous regions and affect the energy exchange between the earth’s surface and the atmosphere. Various theories of their structure, evolution, and fundamental dynamics have been proposed. Initial interest in katabatic winds, which was prompted by field observations, has been followed by a long history of observational studies. This article reviews observational work undertaken on small-scale katabatic winds in mid-latitudes, with an emphasis on the historical background, and recent work on the causes of their variation. 1 Introduction From its Greek origins the term katabatic, where ‘kata’ means downward and ‘batos’ means moving beyond, can be interpreted to refer to essentially any flow that moves downward. This broad interpretation is not exactly consistent with the general use of the word, however, which typically refers to winds that flow down the topographic gradient or out of a valley due to surface cooling that gives this air a greater density than the free atmospheric air. This cooling of slope surfaces, which is due primarily to a net negative surface radiative balance, produces a temperature difference between the air adjacent to the slope and the ambient air away from the slope. Winds then accelerate from the slope toward the ambient air, where gravity forces the dense flow to follow the sloping surface (Figures 1 and 2). In a valley terrain configuration, the katabatic flows on the slopes of the valley sidewalls coagulate in the valley base where they will continue their course out valley (or down valley if the valley is sloped; Figure 1). This katabatic flow out of a valley is often called mountain wind or mountain breeze or down-valley flow (with sloped valley floor). Numerous authors have offered their opinions on which terminology is best used in © 2008 The Authors Journal Compilation © 2008 Blackwell Publishing Ltd 2 Observing mid-latitude katabatic flows Fig. 1. Illustration of slope and valley katabatic winds driven by the differential cooling and the resulting temperature differences between the air adjacent to the slopes and away from the slopes in the valley and between the air inside and outside the valley. various circumstances (Atkinson 1981; Barry 1992; Jeffreys 1922; Talman 1911; Whiteman 1990), but in the following the term katabatic flow or katabatic wind will be used to generally represent surface-based flow caused by surface cooling on either a slope or in a valley, where the context (valley or slope) will be obvious from the topic at hand. The terms ‘downslope wind’ and ‘mountain wind’ may also be used where the former refer to katabatic flow on a slope as opposed to the latter term which refers to katabatic flow out of a valley. Downslope wind in this context should not be confused with the same terminology used in the meteorological literature to describe wind storms in the lee of a mountain barrier. Although katabatic winds have been observed over sloping terrain of different scales all over the world, including Antarctica (Ball 1956, 1957; Bromwich and Parish 1998; Mawson 1915; Rees 1991; Renfrew and Anderson 2002), Greenland (Broeke et al. 1994; Heinemann 2002; Loewe 1935), Europe (Defant 1951; Ekhart 1934; Oerlemans et al. 1999; Smeets et al. 1998; Tollner 1931), North America (Buettner and Thyer 1965; Clements et al. 1989; Doran et al. 2002; Haiden and Whiteman 2005; Horst and Doran 1986; Moni et al. 2002; Princevac et al. 2005; Tower 1903), and the Mediterranean (Martinez et al. 2006), this review will focus only on small-scale katabatic winds that occur in mountainous regions in mid- latitudes. There is a large amount of literatures on large-scale katabatic winds over Polar ice sheets, which will not be reviewed here. Note also that this article will not describe anabatic flow, the closely related daytime analogue to katabatic flow, that moves upslope or up valley due to surface heating. The observations of small-scale katabatic flows and the development of theories describing these flows have a long history whose origins began in the early 1800s. In recent decades, numerical modeling studies of katabatic © 2008 The Authors Geography Compass 2 (2008): 10.1111/j.1749-8198.2008.00166.x Journal Compilation © 2008 Blackwell Publishing Ltd Observing mid-latitude katabatic flows 3 flows have proliferated, providing insights into the forcing mechanisms of the katabatic flows and their interactions with synoptic-scale winds and with the ambient environments. Due to the page limitation and the vast amount of literatures on this subject, this review article will concentrate primarily on the observational studies of the katabatic winds with an emphasis on an understanding of the historical work and the more recent work on causes of variations in such flows. Other reviews of downslope flows and mountain breezes can be found in Atkinson (1981), Barry (1992), Defant (1951), Geiger (1975), Mahrt (1982), Poulos (1996), Sturman (1987), Thyer (1966), Vergeiner and Dreiseitl (1987), and Whiteman (1990). The most thorough analysis and interpretation of the first-century (1800s through 1945) katabatic flow investigations can be found in Hawkes (1947). 2 Katabatic Flow Observations in Mountain Valleys Katabatic flows were first observed in valleys because of the natural tendency for mankind to dwell in the base of valleys where water sources are readily available. Although knowledge of the diurnal nature of valley winds had been known to the agricultural community for many years before, the first scientifically reported observations began in Europe near more populated valleys beginning with Fournet (1840) in the Savoie region of France and later in the Alps by Ekhart (1932a,b, 1934) and in England by Heywood (1933) among others. Tower (1903) was one of the first recorded observations of these mountain winds within several valleys in the northern Colorado Rockies of the United States. These observations made certain features of the katabatic winds clear: (i) they develop near sunset when the surface starts to cool, (ii) generally clear skies and quiescent synoptic conditions are most conducive to their development, (iii) they are from tens of meters to a few hundred meters deep, (iv) they form within a developing temperature inversion, (v) their wind speed generally increases from the ground to 2–6 m/sec at some fraction of the inversion height and decreases from this jet to the top of the inversion, and (vi) they are common and frequent around the world. Some of these observed general characteristics of katabatic flows are illustrated in Figure 2. Using katabatic valley flow observations taken during an ASCOT (Atmospheric Studies of Complex Terrain) field campaign in the Brush Creek Valley of Colorado, Clements et al. (1989) show an approximately 7 °C nocturnal temperature inversion in the lowest 300–400 m of the valley atmosphere with near isothermal structure above that to the ridgeline. They show that the depths of the katabatic valley flows correspond rather well with the depth of the temperature inversion. The katabatic flow jet of 6 m/sec lies at approximately 100 m above ground level (AGL) or 0.25 of inversion depth, agreeing well with the early theoretical predictions of Prandtl (1942, 1952). Davidson and Rao (1963) found this jet height to be 0.4–0.5 of inversion depth in a valley near Manchester, Vermont, and © 2008 The Authors Geography Compass 2 (2008): 10.1111/j.1749-8198.2008.00166.x Journal Compilation © 2008 Blackwell Publishing Ltd 4 Observing mid-latitude katabatic flows Downslope wind Fig. 2. Illustration of typical wind and temperature profiles in katabatic wind layer (adapted from Whiteman 2000). suggest that the ratio becomes smaller at steeper slope angles. The observations in the Brush Creek Valley exhibit some evidence of the return flow above the katabatic flow, the so-called ‘anti-wind’, compensation current, or return flow. This feature was routinely observed by Buettner and Thyer (1965) in a valley wind system near Mt. Rainier, Washington, over four consecutive summer seasons, which is consistent with the theoretical explanation of katabatic flow (Defant 1933) based on the circulation theorem (Bjerknes 1902; Kelvin 1866). They find that the ‘anti-wind’ layer has the same depth as the down valley katabatic flow but weaker. Anti-winds are not always found in katabatic flow observational studies (Reiter et al. 1983), and for that reason the theoretical necessity of their existence is questioned (Davidson and Rao 1963; Ekhart 1932a,b). Most likely is that an anti-wind is simply difficult to observe in many cases, because synoptic or regional-scale influences generally dominate above the strongly forced near-surface katabatic flow (Defant 1951). Anti-winds are also likely to be difficult to observe in sinuous portions of a canyon. The time evolution of valley katabatic flow has been well observed by a number of studies, but generalization is difficult due to differences in the valley configurations, observational locations, and measurement techniques. For instance, the measurements of Buettner and Thyer (1965) showed a katabatic flow peaking just before sunrise, whereas Neff and King (1987) reported on a valley configuration where pooling in the lower valley © 2008 The Authors Geography Compass 2 (2008): 10.1111/j.1749-8198.2008.00166.x Journal Compilation © 2008 Blackwell Publishing Ltd Observing mid-latitude katabatic flows 5 slowed katabatic flow much before sunrise. Clearly, valley katabatic flow can peak in velocity at a variety of times relative to sunrise and sunset, depending on when the along-valley pressure gradient is maximized at a particular location.