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Maps of Snow-Cover Probability for the Northern Hemisphere

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Maps of Snow-Cover Probability for the Northern Hemisphere June 1967 R. R. Dickson and Julian Posey 347 MAPS OF SNOW-COVER PROBABILITY FOR THE NORTHERN HEMISPHERE R.R. DICKSON AND JULIAN POSEY Extended Forecast Division; NMC, Weather Bureau, ESSA, Washington, D.C. ABSTRACT Map analyses are provided depicting the probability of snow-cover 1 inch or more in depth at the end of each month from September through May for the Northern Hemisphere. 1. INTRODUCTION To supplement this primary data source, empirical snow-cover probabilities were computed for the 193 1-50 Recent work dealing with thermodynamic [l] and period for a network of 110 stations in the United States synoptic [8] aspects of long-range forecasting has em- from data included in US. Weather Bureau Station phasized the importance of considering the heat balance Record Books (available on microfilm from the Atmos- of the earth and the atmosphere when dealing with the pheric Sciences Library of ESSA) . Canadian snow-cover long-term evolution of the atmospheric circulation. data were extracted from a recent publication [lo]. This An important factor in such a heat balance is the albedo was augmented for May and September by data from of the earth’s surface and this in turn is critically depend- additional stations during 1940-64, obtained from monthly ent upon the snow-cover distribution. published records [5]. Thus a need exists for broadscale climatic analyses Analyses for China and Korea are based upon data depicting the areal extent of snow cover throughout the published by their respective meteorological offices [9] , [4]. year. While map analyses of average snow depth and Both sources give the a$erage number of days with snow average dates of first and last snowfall are readily available, cover for each month. These frequencies were assumed to data dealing with snow-cover probability remain some- apply at mid-month, allowing estimation of end-of-the- what fragmented and in graphic or tabular form. The month values by interpolation. Since periods of record present study attempts to assemble and present in map were short in many instances, erratic patterns not sup- form (figs. 1-9) all readily available data on the proba- ported by topography were smoothed. bility of snow cover 1 in. or more in depth at the end of Although no snow-cover frequencies were readily availa- each month from September through May. The critical ble for Japan, an atlas [3] provides normal depth of snow depth of 1 in. was arbitrarily selected as the level at which for each month. As a rough approximation, it was assumed snow cover begins to affect markedly the thermal prop- that a normal depth of 2 cm. approximated a 50 percent erties of the surface. Recent research [6] verifies that probability of 1-in. snow cover. Although such estimates with this snow depth the surface albedo is rapidly chang- are more representative of mid-month, no hterpolations ing from its minimum value with no snow to its maximum were made in view of the assumption made. possible value with snow. Snow-cover probabilities over the British Isles were estimated from US. Weather Bureau [14] monthly sum- 2. DATA AND ANALYSIS maries of the number of days with snowfall and days with snow lying (more than half of the surrounding country The most extensive summary of sno\i--cover data covered by snow). The latter statistic, taken to represent available to the authors was ti publication of the US. snow cover as considered by this report, was available for Army Engineers [2] which provides snow-cover probabili- only a few scattered locations. However, by establishing ties in graphical form for much of the Northern Hemi- the ratio of the two quantities where both were available, sphere. Data sources and periods of record used in the the number of days with snow lying was approximated at preparation of this publication vary widely; the interested neighboring stations from the observed number of days reader may obtain such details from the referenced source. with snowfall. Although such estimated frequencies are In this instance, as was the case in all other data sources most applicable to mid-month, no interpolations were considered, snow-cover probabilities are empirical in made to obtain end-of-the-month values, again because of nature. the type of data available and the assumptions employed. Unauthenticated | Downloaded 10/03/21 02:57 AM UTC \ 3 48 MONTHLY WEATHER REVIEW Vol. 95, No. 6 3 Unauthenticated | Downloaded 10/03/21 02:57 AM UTC June 1967 R. R. Dickson and Julian Posey 349 Unauthenticated | Downloaded 10/03/21 02:57 AM UTC 3 50 MONTHLY WEATHER REVIEW Vol. 95, No. 6 Unauthenticated | Downloaded 10/03/21 02:57 AM UTC 351 June 1967 R. R. Dickson and Julian Posey d 262-026 0 - 67 - 5 Unauthenticated | Downloaded 10/03/21 02:57 AM UTC 352 MONTHLY WEATHER REVIEW Vol. 95, No. 6 Unauthenticated | Downloaded 10/03/21 02:57 AM UTC June 1967 ‘ R. R. Dickson and Julian Posey 353 Over the Tibetan Plateau only a rough approximation requirement for final analyses. Furthermore, the prospects of snow extent was possible from some temperature- for successfully adjusting snow-cover probabilities to a snow-cover relations derived by Manley [7] for the standard period do not appear bright because of the British Isles and from unpublished hemispheric maps of joint dependence of snow cover upon temperature and normal surface air temperature prepared by R. Taubensee precipitation. Nonetheless, the present study represents of the Extended Forecast Division. Such an approach a first step in hemispheric analysis of snow-cover prob- was deemed justified since even a rough estimation was ability and should prove useful until conventional hemi- desired and a few snow-frequency observations were spheric data for a uniform period augmented by satellite available in the area to calibrate Manley’s relationship. coverage in remote areas becomes available. Unpublished maps based on satellite observations of snow-cover extent, prepared by the National Environ- mental Satellite Center since March 1966, have also been REFERENCES useful in locating the zero-probability snow line in this 1. J. Adem, “On the Physical Basis for the Numerical Prediction region of little or no conventional data. of Monthly ahd Seasonal Temperatures in the Troposphere- A similar procedure was followed in areas such as Ocean-Continent System,” Monthly Wealher Review, vol. 92, Turkey, the Alps, and Greenland where no data were No. 3, Mar. 1964, pp. 91-103. readily available. In all such instances analyses not 2. Arctic Construction and Frost Effects Laboratory, Corps of Engineers, Depth of Snow Cover in the Northern Hemisphere, supported by observed snow-cover frequencies are shown U.S. Army, Boston, 1954, 56 pp. as dashed lines. Dashed lines were also used where the 3. Central Meteorological Observatory of Japan, The Central zero snow-cover probability line has been estimated on Climatological Atlas of Japan, Tokyo, 1949, 54 pp. the basis of the occurrence or non-occurrence of snow as 4. Central Meteorological Office of Korea, Climatic Atlas of Korea, reported in the climatological atlases. Since depiction of 1931-1960, Seoul, 1962, 299 pp. 5. Dept. of Transport, Meteorological Branch, Monthly Record of the large-scale snow-cover distribution was the purpose Meteorological Observations in Canada, Toronto, 1940-1964. of this survey, no estimates have been provided for certain 6. F. C. Kung, R. A. Bryson, and D. H. Lenschow, “Study of a limited areas, such 8s Iceland, where data were not Continental Surface Albedo on the Basis of Flight Measure- readily available. In the case of Arctic regions, the ments and Structure of the Earth’s Surface Cover over North interested render is referred to atlases dealing wich sea-ice America,” Monthly Weaiher Review, vol. 92, No. 12, Dec. 1964, pp. 543-563. [13]). distribution (e.g., [12], Such data have been used 7. G. Manley, “On the Occurrence of Snow Cover in Great in the present survey to assist in locating the normal snov- Britain,” QUaTteTly Journal of the Royal Meteorological Society, line over northern lakes and seas. vol. 65, No. 1, Jan. 1939, pp. 2-27. The accompanying maps (figs. 1-9) have obvious 8. J. Namias, “Surface-Atmosphere Interactions as Fundamental limitations in mountainous areas. Although some attempt Causes of Drought and Other Climatic Fluctuations,” pp. has been made to account for gross mountain effects, 345-359 in Changes of Climate; Proceedings of the Rome Sym- posium organized by UNESCO and the World Health Organiza- analyses generally reflect conditions at primary weather tion, Paris, 1963. stations located at relatively low elevations. Thus our 9. Office of Climatological Data Rcsearch, Central Meteorological analyses undoubtedly underestimate snow-cover prob- Bureau, An Atlas of Chinese Climatology, Map Publishing abilities at high elevations. Weekly snou -cover prob- Company, Peiping, 1960, 293 pp. (English translation by Joint Publication Research Service, Washington, D.C., JPRS 16, abilities prepared for the United States by the Thorns 321, Nov. 1962.) [11] also suggest that this is so. Unfortunately, differ- 10. J. G. Potter, “Snow Cover,” Dcpt. of Transport, Meteorological ences in the definition of snuw cover, in period of record, Branch, Climatological Studies No. 3, Toronto, 1965, 69 pp. and also in results in non-mountainous areas preclude the 11. H. C. S. Thom and M. D. Thom, “Weekly Maps of Snow Cover use of these analyses in the present study. Probability,” pp. 476-492 in Synoptic Types Associated with As is apparent from source references, datit used in this Critical Fire Weather, Pacific Southwest Forest and liangc Experiment Station, Berkeley, 1964. survey were drawn from a variety of time intervals. These 12. U.S. Navy Hydrographic Office, Ice Atlas of the Northern vary from pre-1915 data in the case of most of the Soviet Hemisphere, H.
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