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Downloaded 10/05/21 03:15 PM UTC 20.2 METEOROLOGICAL MONOGRAPHS VOLUME 59 CHAPTER 20 S M I T H 20.1 Chapter 20 100 Years of Progress on Mountain Meteorology Research RONALD B. SMITH Yale University, New Haven, Connecticut ABSTRACT Mountains significantly influence weather and climate on Earth, including disturbed surface winds; altered distribution of precipitation; gravity waves reaching the upper atmosphere; and modified global patterns of storms, fronts, jet streams, and climate. All of these impacts arise because Earth’s mountains penetrate deeply into the atmosphere. This penetration can be quantified by comparing mountain heights to several atmo- spheric reference heights such as density scale height, water vapor scale height, airflow blocking height, and the height of natural atmospheric layers. The geometry of Earth’s terrain can be analyzed quantitatively using statistical, matrix, and spectral methods. In this review, we summarize how our understanding of orographic effects has progressed over 100 years using the equations for atmospheric dynamics and thermodynamics, numerical modeling, and many clever in situ and remote sensing methods. We explore how mountains disturb the surface winds on our planet, including mountaintop winds, severe downslope winds, barrier jets, gap jets, wakes, thermally generated winds, and cold pools. We consider the variety of physical mechanisms by which mountains modify precipitation patterns in different climate zones. We discuss the vertical propagation of mountain waves through the troposphere into the stratosphere, mesosphere, and thermosphere. Finally, we look at how mountains distort the global-scale westerly winds that circle the poles and how varying ice sheets and mountain uplift and erosion over geologic time may have contributed to climate change. 1. Introduction to the field of mountain The influence of mountains on weather has been dis- meteorology cussed for at least 2000 years. In about 340 BC, Aristotle speculated about the role of mountain heights in de- Mountain meteorology is the study of how mountains termining the altitude of clouds (Frisinger 1972). In influence the atmosphere. This subject has drawn curi- 1647, Blaise Pascal measured the decrease in atmo- ous investigators to it from the earliest days of the spheric pressure with altitude along a mountain slope to physical, geophysical, and geographical sciences. Some prove that air has weight. Even with this long history, the investigators are attracted to the subject by their love of observational tools of meteorology were poorly de- mountain adventure: skiing, hiking, and climbing. Some veloped in the early twentieth century. One hundred appreciate the beauty of mountain scenery and fast- years ago, we had only a few weather balloons, analog changing patterns of mountain clouds, winds, and recording instruments, and permanent mountaintop weather. Some are drawn by the excitement of flying weather stations (e.g., Mt. Washington, New Hamp- over mountains in gliders or aircraft. Many enjoy the shire, and Sonnblick, Austria). Now, as we begin the challenging physics and mathematics of the unsolved twenty-first century, many powerful observational tools mountain airflow problems. Most important are the are in use: instrumented aircraft, satellite passive visible practical applications of mountain meteorology, such as and infrared imagery, active lidar and radar remote predicting damaging winds; forest fires; clear air turbu- sensing, and even water isotope analysis. Accordingly, lence; air pollution; wind, solar, and hydropower; water our knowledge has increased manyfold. resources; and patterns of regional and global climate. In the last 60 years, many small and a few large co- ordinated field projects have provided a valuable ob- servational database for mountain meteorology. Some Corresponding author: Ronald B. Smith, [email protected] examples are given in Table 20-1. These national and DOI: 10.1175/AMSMONOGRAPHS-D-18-0022.1 Ó 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/05/21 03:15 PM UTC 20.2 METEOROLOGICAL MONOGRAPHS VOLUME 59 TABLE 20-1. Some mountain meteorology field projects. Project Acronym Year Location Reference Sierra Wave Project 1951/52/55 California Grubisic´ and Lewis (2004) Colorado Lee Wave Experiment 1970 Colorado Lilly and Kennedy (1973) Alpine Experiment ALPEX 1982 Alps Kuettner and O’Neill (1981) Hawaiian Rainband Project HaRP 1989 Hawaii Austin et al. (1996) Taiwan Mesoscale Experiment TAMEX 1989 Taiwan Kuo and Chen (1990) Pyrenees Experiment PYREX 1990 Pyrenees Bougeault et al. (1997) Southern Alps Experiment SALPEX 1993/95 New Zealand Wratt et al. (1996) California Landfalling Jets Experiment CALJET 1997/98 California Neiman et al. (2002) Mesoscale Alpine Project MAP 1999 Alps Bougeault et al. (2001) Improvement of Microphysical IMPRoVE 2001 Washington State Garvert et al. (2007) Parameterization Terrain-Induced Rotor Experiment T-REX 2006 California Grubisic´ et al. (2008) Cumulus Photogrammetric In Situ and CuPIDO 2006 Arizona Damiani et al. (2008) Doppler Observations Convective and Orographically Induced COPS 2007 France and Germany Wulfmeyer et al. (2011) Precipitation Study Persistent Cold-Air Pool Study PCAPS 2010/11 Utah Lareau et al. (2013) Dominica Experiment DOMEX 2011 Dominica Smith et al. (2012) Deep Gravity Wave Project DEEPWAVE 2014 New Zealand Fritts et al. (2016) Olympic Mountains Experiment OLYMPEX 2015/16 Washington State Houze et al. (2017) international projects broadened and connected the at- and fluid dynamics of how mountains influence weather mospheric science community. and climate. In section 4, the impact of mountains on Equally important to the history of mountain meteo- surface winds is reviewed, including mountaintop winds, rology was the rapid development of numerical simu- downslope winds, barrier jets, gap jets, wakes, thermally lations of geophysical fluid flows, including microscale, driven slope winds, and cold pools. In section 5, oro- mesoscale, and global weather and climate models. (e.g., graphic precipitation is summarized. In section 6,we Durran 2010). Starting in the 1970s, numerical models describe the generation of mountain waves that propa- with steadily increasing resolution and accuracy have gate deep into the upper atmosphere. In section 7,we provided a powerful tool for sensitivity and hypothesis consider the global effects of mountains on climate over testing regarding mountain influences. Earth’s history. Stimulated by these exciting field experiments and advances in numerical modeling, scientists in this field have met annually to discuss new discoveries. These TABLE 20-2. Book-length references on mountain meteorology. meetings were organized in alternate years by the Reference Title American Meteorological Society’s Committee on Mountain Meteorology and the European International Smith (1979) The influence of mountains on the atmosphere Conference on Alpine Meteorology (ICAM). Hide and White (1980) Orographic Effects in Planetary The literature on mountain meteorology is extensive. Flows A guide to the major publications is given in Table 20-2. Whiteman and Dreiseitl Alpine meteorology: Translations of In this list are only ‘‘book-length’’ publications with a (1984) classic contributions by A. broad scope. Other special review articles will be cited in Wagner, E. Ekhart, and F. Defant Price (1986) Mountains and Man: A Study of the individual sections. The books in Table 20-2 provide Process and Environment a deeper account of mountain meteorology than we can Blumen (1990) Atmospheric Processes over give in this short article. As they span nearly 40 years, Complex Terrain they also give a sense of how the subject has advanced Baines (1995) Topographic Effects in Stratified through time. Flows Ruddiman (1997) Tectonic Uplift and Climate Change The current article is divided into seven main sections. Whiteman (2000) Mountain Meteorology: The second section ‘‘How high is a mountain?’’ dis- Fundamentals and Applications cusses the physical properties of the atmosphere that Barry (2008) Mountain Weather and Climate control the influence of mountains. The third section Chow et al. (2013) Mountain Weather Research and summarizes the mathematical description of terrain Forecasting: Recent Progress and Current Challenges geometry. The following four sections treat the physics Unauthenticated | Downloaded 10/05/21 03:15 PM UTC CHAPTER 20 S M I T H 20.3 2. Atmospheric reference heights: How high is a TABLE 20-3. Atmospheric reference heights. mountain? Name Typical value (km) Variation Mountains are usually ranked by their peak heights. Earth radius 6371 Fixed Citizens take pride in their nation’s highest peaks. Climbers Density scale height 8.5 Nearly fixed rightly brag about the highest peak they have ‘‘bagged.’’ Flow lifting height 0.5–3 Widely variable This rank order of mountains, with Mt. Everest (8848 m) at Tropopause 8–15 Variable Trade wind inversion 2–4 Variable the top, dominates our thinking about mountains. Daytime inversion 1–4 Variable In this chapter, however, we judge mountain height Water vapor scale height 2–3 Variable differently: not by comparison with other mountains, LCL 0.5–4 Variable but with reference to various height scales in Earth’s Freezing level 0–5 Widely variable atmosphere (Table 20-3). This simple exercise in ‘‘rel- Equilibrium line 0–5 Widely variable Tree line 0–5 Widely variable ativism’’ will carry us a long way toward
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