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A Comparison of Precipitation from Maritime and Continental Air

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A Comparison of Precipitation from Maritime and Continental Air 254 BULLETIN AMERICAN METEOROLOGICAL SOCIETY A Comparison of Precipitation from Maritime and Continental Air GEORGE S. BENTON 1 and ROBERT T. BLACKBURN 2 OR many decades, knowledge of atmospheric These 123 periods of precipitation for 1946 were F movements of water vapor has lagged behind grouped as indicated below. Classifications for other phases of meteorological information. In which precipitation was necessarily from maritime recent years, however, the accumulation of upper- air are marked with an asterisk; classifications for air data has stimulated hydrometeorological re- which precipitation was necessarily from contin- search. One interesting problem considered has ental air are marked with a double asterisk. For been the source of precipitation classified accord- all other classifications, precipitation may have ing to air mass. occurred either from maritime or continental air, In 1937 Holzman [2] advanced the hypothesis depending upon the individual circumstances. that the great majority of precipitation comes from maritime air masses. Although meteorologists I. Cold front have generally been willing to accept this hypothe- A. Pre-frontal precipitation sis on the basis of their qualitative familiarity with *1. MT present throughout tropo- atmospheric phenomena, little has been done to sphere determine quantitatively the percentage of pre- **2. cP present throughout tropo- cipitation which can actually be traced to maritime sphere air. Certainly the precipitation from continental 3. MT overriding cP in warm sec- air masses must be measurable and must vary in tor importance from region to region. B. Post-frontal precipitation In the course of an analysis of the role of the 1. MT overriding cP atmosphere in the hydrologic cycle [1], the au- *a. Clouds only in MT air thors had occasion to study the source of precipi- mass tation at Huntington, West Virginia, for the year b. Clouds in both air masses 1946. Huntington has the particular advantage **2. cP overriding cP of being situated away from the influence of the II. Warm front Great Lakes but far enough from the Gulf to ensure frequent occurrence of both MT and cP A. Pre-frontal precipitation air masses. To avoid bias in air mass classifica- 1. MT overriding cP tion and frontal analysis, published maps of the *a. Clouds only in MT air United States Weather Bureau were used. How- mass ever, it was necessary to supplement these maps b. Clouds in both air masses with frequent, detailed surface and upper-air charts **2. cP overriding cP in order to determine times of frontal passages B. Post-frontal precipitation and probable source of precipitation when more *1. MT present throughout tropo- than one air mass was present above the station. sphere Whenever necessary, hourly airway observations **2. cP present throughout tropo- were consulted and atmospheric cross-sections sphere were drawn. Hourly precipitation totals were 3. MT overriding cP in warm sec- obtained from the U. S. Weather Bureau. tor A total of 123 precipitation periods occurred III. Occluded front during the year. By "precipitation period" is A. Warm type meant an interval during which the synoptic situa- B. Cold type tion at Huntington remained unchanged by frontal passage. Thus a given storm might qualify as IV. Non-frontal precipitation one or more periods of precipitation depending *A. MT present throughout troposphere upon the number of frontal passages observed 1. No front in vicinity during the storm. 2. Cold front in vicinity, not pass- ing station 1 School of Engineering, Johns Hopkins Univ., Balti- **B. cP present throughout troposphere more, Md. 2 Dept. of Meteorology, University of Chicago. C. MT overriding cP Unauthenticated | Downloaded 10/04/21 04:39 AM UTC VOL. 31, No. 7, SEPTEMBER, 1950 255 It is to be noted that the only air masses men- of 0.78 inches (2% of the precipitation for the tioned above are maritime tropical and continental year). polar. The few cases for which maritime polar air Detailed analysis of the remaining 47 periods of were observed at Huntington are included with precipitation enabled tentative classification of the maritime tropical air; similarly, the few cases for source of precipitation on numerous additional oc- which continental arctic air were observed are casions. Of the 18 periods of precipitation after included with continental polar. the passage of a cold front when maritime air was TABLE I shows the number of occurrences and present above continental air and clouds were the amount of precipitation for each of the above present in both air masses (classification I-B- classifications. It is to be noted that in 47 of the 1-b), 11 were periods during which the only 123 periods, maritime air was present throughout clouds in the continental air were stratus or strato- the troposphere. These 47 periods account for cumulus incapable of producing the observed pre- 21.10 inches of precipitation (53% of the total for cipitation. In many instances, these clouds were the year). In 26 additional cases, representing apparently formed by evaporation of precipitation from aloft. A similar situation existed in 4 of TABLE I. CLASSIFICATION OF PRECIPITATION PERIODS the 12 analogous cases of precipitation in advance HUNTINGTON, WEST VIRGINIA, 1946 of a warm front (classification II-A-l-b). By including all instances for which the source of Inches Synoptic Sub- Occur- of Per- precipitation could be determined from synoptic Type situation type rences precipi- centage tation analyses, it was found that 32.09 inches (81% of the precipitation for the year) occurred from (A) 1 6 1.03 2.6 maritime air masses, and 2.89 inches (7%) from Pre- 2 0 0.00 0.0 continental air masses. The balance of 4.85 inches (I) Frontal 3 4 0.42 1.0 Cold (12% of the total for the year) occurred during Front (B) la 13 2.13 5.4 periods when the source could not be determined Post- lb 18 3.78 9.5 with any satisfactory degree of confidence. 0.78 2.0 Frontal 2 3 It may be interesting to examine the character (A) la 13 3.86 9.7 of these undetermined cases in a little more detail. Pre- lb 12 3.07 7.7 In 15 cases, representing 4.16 of the 4.85 inches, (ii) Frontal 2 0 0.00 0.0 precipitation occurred from a warm or cold front, Warm with precipitating clouds apparently existing both Front (B) 1 6 2.02 5.1 Post- 2 0 0.00 0.0 above and below the front. Situations of this sort Frontal 3 1 0.02 0.0 are often associated with rapid modification of the underlying continental air mass due to evaporation an) (A) Warm — 3 0.85 2.1 of falling precipitation. In such circumstances, Occlusion (B) Cold 4 2.70 6.8 the source of precipitation eventually becomes a 1 20 10.11 25.4 mixture of the two air masses and the front itself (A) mT air (IV) 2 15 7.94 19.9 is so diffuse that it is difficult to distinguish on Non- upper-air soundings. Although precipitation of Frontal (B) cP air 0 0.00 0.0 (C) mT/cP — 5 1.12 2.8 this sort has been classified as of undetermined origin, it is probably true that the original source Totals 123 39.83 100.0 of the great majority of the water reaching the surface of the earth was a maritime air mass. an additional 5.99 inches, maritime air was pres- In the light of the above data, it can be conser- ent above continental air but clouds were present vatively estimated that for the year 1946 at Hunt- only in the maritime air mass. Thus, in a total ington at least 85-90% of the precipitation could of 73 of the 123 periods it could be definitely be traced to maritime air masses, and not more established that the source of precipitation was than 10-15% to continental air masses. Compari- maritime air. Furthermore, the 73 periods in- son of air mass frequency, number of days of pre- cluded a total of 27.09 inches (68% of the pre- cipitation, and the total amount of precipitation cipitation for the year). In contrast, only 3 cases for 1946 with other years of record at Huntington were encountered for which it could be definitely indicates that this result is probably representative. established that the source of precipitation was It is interesting to consider the probable results continental air. These 3 periods included a total of an extension of this study to other areas in Unauthenticated | Downloaded 10/04/21 04:39 AM UTC 256 BULLETIN AMERICAN METEOROLOGICAL SOCIETY the eastern part of the United States. In the tively the hypothesis of a maritime air mass source vicinity of the Gulf of Mexico, 95% or more of of precipitation for the eastern United States. the precipitation must be from maritime air. In the north, this percentage will be greatly reduced. REFERENCES As a result of preliminary studies, however, the [1] Benton, G. S.; Blackburn, R. T.; and Snead, V. O., "The Role of the Atmosphere in the Hydrologic authors would estimate that even in the northern Cycle," Trans. American Geophys. Union, v. 31, states east of the Mississippi River the percentage No. 1 (Feb., 1950). of precipitation from maritime air does not fall [2] Holzman, B., Sources for Moisture for Precipitation in the United States, U. S. Dept. Agric., Tech. below 70%. The conclusions confirm quantita- Bulletin No. 589 (1937). Service and assigned to Hq., United States Air Force NEWS AND NOTES as Assistant Deputy Chief of Staff for Development. Col. W. O. Senter, since promoted to Brig. Gen., was appointed Chief of the Air Weather Service upon Gen.
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