The January Thaw in New England * (An Example of a Weather Singularity)

Transcription

1 380 BULLETIN AMERICAN METEOROLOGICAL SOCIETY The January Thaw in New England * (An Example of a Weather Singularity) EBERHARD W. WAHL Geophysics Research Directorate, Air Force Cambridge Research Center, Air Research and Development Command, Cambridge, Mass. ABSTRACT An investigation of temperature and additional records shows the existence of a pronounced January thaw in New England. This singularity can be connected to the existence of typical changes in the circulation over the United States. Further world-wide relations indicate important consequences valuable for further research on singularities. INTRODUCTION ALTHOUGH the study of singularities, i.e., recurrences of weather phenomena on specific key dates, has become a fairly well established part of meteorological research in Europe, notably in Germany [1], few investigations have been conducted in this field in this country. This is due, in part, to the difficulties in obtaining the necessary long-period records of daily values for different elements, and, also in part, to the controversial character of this problem in general. In spite of the long history in this type of research, no general agreement has been reached as to whether singularities are actually realities or only random effects. In the following discussion, some added evidence for the possible existence of singularities in the United States will be presented. Certain ideas about large-scale relationships, suggested by interconnections between singularities found here and in Europe will also be offered. THE JANUARY THAW In the folklore of New England we find the rather distinct conviction that a warm spell occurs in the middle of January, often leading to thawing weather, then followed by heavy frost. Definite evidence of the existence of such an anomaly during the period of January is brought out in a detailed study of temperature records which was made by G. Slocum [2] for Washington, D. C. Day-by-day unsmoothed temperature normals of other stations in the Eastern United States confirm these findings by showing similar temperature anomalies. FIGURE 1 depicts an example of such day-by-day normals for Boston, Massachusetts, computed * Revision of paper read at the New York Meeting, Jan. 28, from (max + min)/2, over the period 1873/1952. Each daily normal, is the result of averaging 80 values. The standard deviation of a single daily value in winter is about F, thus the standard deviation of a daily normal is about 1.1 F. The anomaly on January exceeds this standard deviation by a considerable amount, but this fact in itself is not sufficient proof of the reality of this anomaly. Further tests will be necessary to prove that it may have a real physical basis. STATISTICAL TESTS Because of the extreme difficulty encountered in proving statistically that a certain singularity is real, in the sense that random effects as generating processes can be ruled out with certainty, a proof of this particular "singularity of the January thaw" will not be attempted. However, we may determine the probability that such an occurrence is solely due to chance and by this indicate the more reasonable certainty of the existence of singularities in general. Several such tests can be applied. The f-test of Student [3] can be applied to two daily normals, provided that values are compared which do not FIG. 1. Temperature Normals for Boston, Massachusetts ( ). Unsmoothed daily values derived from the daily average temperature record (unpublished). The time of the January thaw is marked by an arrow.

2 VOL. 33, No. 9, NOVEMBER, show any marked auto-correlation. ' Applied to the normals for 17 and 22 January (to eliminate any influence of the day-by-day auto-correlation, which disappears for all practical purposes after three days) this test shows a probability of coexistence of such values in random series of only about seven per cent. Another test applicable to these data further substantiates the above finding. Assuming only random effects, it is equally probable for any one value to be either higher, in between, or lower than two other values: for example, some days earlier and later. The theoretical probability is for each of these cases. In a finite sample, the result of an actual count will fall with a given probability between the respective "confidence limits" [4]. This second test, as applied to the three values of 19, 22 and 25 January, sur- FIG. 2. Normals of other elements during January. Each thunderstorm symbol represents an occurrence at this date at Boston during the 61 years of this record, e.g., on 24 January there Were two thunderstorms recorded, and none on 11 January.

3 382 BULLETIN AMERICAN METEOROLOGICAL SOCIETY FIG. 3. Frequency of winds of a given direction at specified days compared to the monthly normal, Blue Hill Observatory, (unpublished). passes the 95% confidence limits (22 January > both 19 and 25 January : 48.7%, upper limit 45% ). The time difference of three days was chosen to eliminate, in this case also, the influence of any serial correlation. These results suggest the possibility that certain physical causes may be responsible for the appearance of such anomalies in the temperature records. Further investigations will proceed under this assumption. OTHER ELEMENTS The next step is to assemble further evidence for the occurrence of such a warm spell, showing especially that it is independent of the special location. FIGURE 2 gives the result of the evaluation of several other weather records. The dayby-day temperature normals for Washington, D. C.; New York, Boston, Blue Hill Observatory, and Columbia, Mo., are plotted against time (date). Furthermore, the snow occurrence and percentage of snow in the total precipitation at Washington, D. C., part of the sunshine record of Blue Hill Observatory, and the thunderstorm occurrence during 62 years of record at Boston are given. Added to these curves (which are computed from the individual daily records) are the average pressure gradient (gradient wind direction and amount of gradient per 20 lat. ) as derived from the 40-year Historical Weather Maps for the coordinate crossing 40 N, 85 W (from 5-day overlapping averages). It is clear at first glance that between 17 and 25 January some special phenomena do occur. The temperature curves of all Eastern stations show the same anomaly during January and it is rather obvious that a similar anomaly appears some days earlier at Columbia, Mo., farther to the West. The flow as indicated in the gradient wind direction, in early January, is from about , turning toward the southwest (260 ) during January. After this date a decisive turn appears toward the northwest to nearly 350. As this gradient applies to a point far west of New Eng- FIG. 4a. Normal sea-level pressure map for January (whole month).

4 VOL. 33, No. 9, NOVEMBER, land, the wind shift will occur in New England some days later. FIGURE 3 shows the more southerly component (compared to the monthly average) in the wind at Blue Hill ( ) still existing (and strongest) during January. Here too, a change takes place, and on January the northerly components are strongly increased. It might be mentioned that the two wind records (FIG. 2 and FIG. 3) are only partly overlapping. The complete picture can be seen from the synoptic point of view in the maps of FIGURE 4. There, the mean maps for 20 and 27 January are given, derived as average maps of all 20 and 27 January dates through the 40 years of the Historical Maps, together with the normal January pressure map. Compared to the average pressure FIG. 4C. Normal sea-level pressure map for 27 January. FIG. 4b. Normal sea-level pressure map for 20 January. map for January (FIG. 4a), the maps of 20 and 27 January exhibit distinct differences. While in the normals the main troughs are just off the coasts, and in the Midwest there exists a ridge of comparatively higher pressure, this picture is reversed on the map of 20 January (FIG. 4b). A trough has now formed within the United States and the Atlantic Coast trough has moved far out to sea. Thus a rather strong southwesterly flow results along the Eastern Seaboard, and the synoptic picture of a typical warm spell is apparent. Some days later, this type of circulation changes completely. The trough in the Midwest is pushed east, and the trough line off the East Coast is regenerated. The high-pressure cell in Northwest Canada extends far to the south and creates a strong high-pressure area (cold high) over the Midwestern States. The northwesterly flow in the East thus brings colder air down toward the East Coast. This type of circulation is generated very rapidly. Three days after 20 January this thrust of cold air to the south is already quite obvious, and until the end of January, this type then remains the governing feature of these average maps. The changes which take place from 20 January to this different type of map (for which 27 January is just an example) are tremendous. It might be mentioned that for a coordinate crossing in the center of these changes (which are of the order of ±4 mb), the standard deviation of the average pressure value over 41 years is only of the order of ± 1 mb. It seems quite natural that such changes will have a widespread effect on the weather picture all along the Atlantic Coast, especially in temperature and sunshine duration. This, therefore, accounts for the different effects which we saw in FIGURES 2 and 3. FREQUENCY ISOPLETHS OF TEMPERATURE As noted above, about 49% of all years had a higher temperature on 22 January than on 19 and 25 January. This implies that the January thaw does not appear (during this particular day at least) in every year. By averaging over all years, therefore, we will find only a residual effect in these temperature normals. Actually, a pronounced January thaw may be a more frequent phenomenon but may be shifted some days forward or back. Due to such shifts in time, simple averaging does not clearly bring out the regularity of occurrence.

5 384 FIG. 5. BULLETIN Frequency distribution of daily average temperature at Boston for 4 selected days. Another way to gain more insight into the mechanism of natural periods, which would primarily take into account the probability of occurrence of such a rise in temperature or prevailing of higher values, must be found. This can be done by plotting frequency distributions of temperatures on a given date through the years; for example, the percentage frequency in a 2-degree interval (number of values inside such 2-degree interval divided by total number of cases). Such distributions are well known as are their usual characteristics. For the most part they are rather close approximations to Gaussian curves, and sometimes slight deviations on the outer ends exist (in winter too many low temperatures, often resulting in a slight skewness). But one fact is clear: there normally exists only one definite peak or frequency maximum coresponding to the most likely temperature, while all other temperature values are less probable. In F I G U R E 5, such frequency distributions are given for four dates, 13, 22, 29 January and 5 February. It is quite obvious that the curves of both 22 and 29 January are "not normal," showing definite indications of a secondary maximum of frequency. On 22 January, this secondary maximum is about displaced from the usual one toward higher temperatures. On 29 January this higher maximum has completely AMERICAN METEOROLOGICAL SOCIETY disappeared. The usual maximum is also rather weak and the maximum to the left, which is 10 lower than normal, is very strong. The meaning of this fact is quite clear. On 22 January, it is nearly as probable to have temperatures around 38 F as to have "normal values" around 30. On 29 January, on the other hand, temperatures around 38 are rare, but values near 20 F are much more common. It should be possible to associate such variations with definite changes in the weather patterns. One would assume that around 22 January, weather situations bringing higher temperatures to Boston should be far more abundant than either before or especially after that time. The frequency distribution therefore reflects the probability of weather types during a given time of the year. It seems logical to use such diagrams to find out more about changes in the probability of warm or cold weather situations. A convenient way to follow the behavior of such distributions is the plotting of isopleths of these frequencies as computed day by day from the temperature record. F I G U R E 6 shows such a chart for Boston daily temperature averages. This graph reveals several facts: a) The change in temperature behavior at Boston after 24 January is very decisive. b ) The January thaw is only part of this greater change-over from the probability of having high temperatures to the alternate probability of very low values. c ) The entire period can be divided into three clearly different parts (which might be called "natural periods of the weather"), namely, up to January a normal seasonal temperature, then through January a warm spell and finally until 4-5 February an extremely cold period. Before 10 January one might add another period, including the slight secondary peak around 5-7 January (another January thaw!) but the limits there can only be fixed with aid of December graphs. The same applies to the periods after 4-5 February. These periods pertain to the Boston region only, and are tentative pending further confirmation from other locations and/or elements. It is to be expected that there will be a shift of the occurrence time in other regions (e.g., occur earlier in the Midwest, etc.). WORLD-WIDE CONNECTIONS The maps in F I G U R E 4 indicate that the "January thaw" at Boston is only part of a much larger phenomenon, namely, of a change-over from a

6 VOL. 33, No. 9, NOVEMBER, FIG. 6. Isopleths of occurrence frequency. Daily average temperature at Boston. westerly circulation pattern to a NW flow pattern. The usually assumed picture of winter circulation over the United States, a trough off the East Coast and high pressure over the Midwest, is first disturbed by the northward movement of the Bermuda High and formation of a trough in the Midwest, which moves during January toward the east and then restores and intensifies the normal picture. This strongly suggests the association of the "singularity" with other changes in the weather in other regions, and it might very well be that the whole hemispheric behavior of the general circulation shows some definite change during this time. Namias [5] already has reported that in February a disturbance in the normal pattern of the zonal westerlies index occurs. The onset of this particular index cycle seems to be related to the beginning of the warming of the continents after the extreme midwinter. It is not the same as the phenomenon discussed here, which just initiates the midwinter. But his findings show that such disturbances of a world-wide nature do occur. Further investigations of such changes in the pattern of the general circulation could be made by plotting hemispheric maps of the same kind as given in FIGURE 4. Such maps will be ready soon. Until they are produced we can only suggest that strong evidence can be found for such large-scale interconnections in the results of workers in singularity research in Europe. In FIGURE 7 some of these results are reproduced. As can be seen, the pressure in Central FIG. 7. Selected European daily normals for January: (a) No. of anticyclonic days over England, 52 years (from [6]). (b) Station pressure over Germany, 44 resp. 59 years (from [1]). (c) Sea-level pressure at 40 N, 75 W, 41 years (from Historical Weather Map series).

7 386 Europe, according to Flohn [1], is highest during the period around January. Parallel to this, C. E. P. Brooks [6] finds a marked increase of anticyclonic and decrease of cyclonic days over England. This, and some preliminary studies on the position of the Azores High during years when the January thaw at Boston was most pronounced, suggests that at the same time the Bermuda High moves north, the Azores High acts similarly and extends into Central Europe, while a trough develops in the Central Atlantic. Incidentally, this very often results in the coldest time of the winter in Europe. Flohn [1] denotes this singularity as WM (midwinter). This illustrates the fact that the timing of the actual singularities is coupled with changes of the general circulation, while the effect of these changes may be quite different in different parts of the world, depending on the position of the area under investigation with respect to the cells of major change. Therefore, it is not too surprising that it is very difficult to prove statistically the existence of singularities in temperature records. There exists a large random effect due to the slight variations in the positions of the principal action centers, which might be quite insignificant for the large-scale situation, but has great effects on the temperature behavior of a specific area. One other fact seems to offer some evidence for the existence of such singularities. If a singularity is coupled with the change of the general circulation at a rather fixed time, then this change must happen according to certain antecedent conditions. This means that the occurrence or nonoccurrence of a singularity in a given year may be most important in the course of investigation. Ultimately it may even be possible to attempt to BULLETIN AMERICAN METEOROLOGICAL SOCIETY forecast the development of a given singularity. The results as described above offer some additional evidence (to the hypothesis) of the probable existence of singularities. It may not be possible to use singularities directly in extended forecasting practice, but they may be helpful in our attempts to penetrate farther into the problems of the behavior of the general circulation. ACKNOWLEDGMENTS The author gratefully acknowledges the help of Mr. Henry A. Salmela during the compilation of the data and numerous computations. The Regional Office of the U. S. Weather Bureau in Boston, Mass., has kindly furnished the basic daily data (not published), as did the Blue Hill Observatory of Harvard University. This investigation would have been impossible without this valuable help which is profoundly appreciated. REFERENCES [1] Flohn, H.: Witterung und Klima in Deutschland. Verlag S. Hirzel, Leipzig (1942), pp. 145 ff. Fiat Review of German Science, Vol. 19: Meteorology and Physics of the Atmosphere. Wiesbaden, Meteorological Abstracts and Bibliography (AMS), Headings: "Singularities, Extended Forecasts." [2] Slocum, G.: "The Annual March of Temperature at Washington, D. C.," Bull. Amer. Met. Soc., Vol. 22, pp (1941). [3] For Student's f-test see: Hoel, Paul G.: Introduction to Mathematical Statistics. John Wiley & Sons, Inc., New York (1947). [4] Tables of confidence limits in: Snedecor, George W.: Statistical Methods. The Iowa State College Press (1946). [5] Namias, J.: "The Index Cycle and its Role in the General Circulation," Journ. Met., Vol. 7(2) ; pp (1950). [6] Brooks, C. E. P.: "Annual Recurrences of Weather: Singularities"; Weather, Vol. 1, pp , (1946).