JOURNAL OF CLIMATOLOGY, VOL. 6, 593-603 (1986) 551.555.3(712.3)

CHINOOKS IN SOUTHERN ALBERTA: SOME DISTINGUISHING NOCTURNAL FEATURES

LAWRENCE C. NKEMDIRIM Department of Geography, The University of Calgary, Calgary, Alberta, T2N IN4

Received 4 November 1985 Revised 14 April 1986

ABSTRACT Various attempts to define chinooks from commonly observed surface weather phenomena have met with limited success. It is now apparent that many of the features normally considered unique to chinooks in southern Alberta are shared with other weather types. A new study found that there were statistically significant differences between both the means and variances of weather variables measured in chinooks and non-chinooks in only one of eight variables considered. The means differed significantly in three parameters whereas the variances differed in four. But the significance attained by the difference in variance was always higher than attained in the mean, indicating that discrimination between the two weather types was more strongly expressed in the range of conditions experienced than in the mean condition. Collectively the variables were able to separate chinooks from non-chinooks in all but one of the events studied.

KEY WORDS Chinooks Southern Alberta Foehn Chinook arches Winds Discriminant functions Statistical summary

1. INTRODUCTION

This paper examines some of the characteristics of the chinook in southern Alberta, Canada, and suggests a method for separating chinooks from non-chinook events using values of six variables measured near the ground.

In southern Alberta, a chinook is loosely defined as a strong, dry and frequently gusty wind blowing from the Rockies. It is the North American equivalent of the European foehn. Although chinooks occur in summer, their impact on the weather and the environment is more clearly defined in the winter. In winter, chinooks are the most striking feature of the weather, resulting in dramatic changes from bone-chilling temperatures to spring-like warmth within hours. A temperature rise of up to 22°C in one hour during a chinook is not unusual (Meteorological Branch, Canada Ministry of Transport, 1971). The mean winter temperature and climate in Calgary, for example, are determined to a large degree by chinook frequency. Here, the mean temperature is -8-2"C. In a chinook-rich winter it could exceed 0°C (1930131). The converse is also true. In 1949-1950, a chinook-poor season, the average temperature dropped to a frigid -15.5"C.

The chinook belt extends unevenly to an approximate depth of 300 km east of the Rockies from Central Alberta in Canada to New Mexico (Glenn, 1961). The western limit is normally taken as the Continental Divide. In Canada, chinooks may be felt as far east as central Saskatchewan. In general, their intensities decrease eastwards.

The belt as a whole is rarely active simultaneously. The east-west extent of an individual chinook is generally less than the width of the belt, especially during the early stages of development (Danielew- icz, 1977). Ashwell (1971) believed that this occurs because of mesoscale waves in the airflow interacting with advective warming during the passage of the Arctic front.

The wind has a far reaching impact on the environment, resources and life in the chinook belt. From the time of the first European settlers to the present there is ample evidence to demonstrate that the

0 1986 by the Royal Meteorological Society 0196- 1748/86/060593- 11 $05.50

594 L. C. NKEMDIRIM

fate of agriculture and related resources in southern Alberta is in part determined by the chinook. The word itself is of Indian origin, meaning ‘snow eater’ because of the swift erosion of snowpacks which almost invariably follows its appearance. Outdoor ranching in the winter was promoted by Chinooks, and ranch failure was not unusual in chinook-free winters (Ashwell, 1971). Vegetation kill can be widespread, caused by the reaction of plants to the false spring created by the chinook and a subsequent and sudden return to winter conditions (Marsh, 1965). In years when chinooks are numerous, southern Alberta could be water deficient (Holmes and Hage, 1971). Chinooks also encourage soil erosion.

Although the warmth of the chinook is generally welcomed by most residents of the province, the foehn type wind has long been associated with what Sulman (1976) described as foehn disease. Similar effects, which include headaches, irritability, depression and general debility, have been reported for chinooks (Anderson, 1982; Nkemdirim, 1984). Nkemdirim and Leggat (1978) and Mathews and Hicks (1979) have shown that air pollution levels in Calgary are higher than normal during chinooks.

Taken together it would appear that the adverse effects of the chinook outweigh its benefits. It is partly for this reason that more attention to chinooks and the processes with which they are associated is needed if strategies are to be developed to combat their negative effects and harness the positive ones. But first, chinooks must be properly identified and separated from other warm winter weather events which have been mistakenly described as Chinooks.

2. THE PROBLEM OF DEFINITION

The search for a comprehensive definition of chinooks in southern Alberta is a continuing if frustrating activity. The problem stems in part from the difficulty encountered in finding an objective and precise way of separating chinooks from other westerly winds. Indeed the dominant wind in the winter is westerly, occurring on the average in 48.5 per cent of the season (Brinkmann, 1969; Danielewicz, 1977). Based on Longley’s definition, chinooks occur only an average of 25 to 30 days per winter in the Calgary area (Longley, 1967).

Longley (1967) determined the frequency of chinooks in southern Alberta by defining a chinook day as a winter day with a maximum temperature exceeding 40°F ( 4 4 C ) . Brinkmann (1970) searched for a geographical definition for the wind which is at the same time dynamically correct. She compared selected characteristics of the weather associated with winds from WNW to WSW in which the dynamic criteria required in a chinook were observed with similar characteristics in other westerly winds. The results were mixed. Brinkmann concluded that an all-encompassing geographical definition was not possible and that definitions should be based on the use intended.

Lester (1976a) sought a more flexible definition. He defined a chinook day as one in which the maximum temperature exceeded the normal for the month, with wind speed in excess of 4-5 m s-’ from a direction between SSW and WNW for at least one hour. Danielewicz (1977) observed that the definition could include about 48 per cent of the days between December and February.

Danielewicz (1977) attempted a definition based on the onset of increasing temperature on the mesoscale as indicated by the upward revision of the distribution of maximum temperature in southern Alberta from the previous day. This definition did not exclude other synoptic activities such as a passage of a warm front through the region in which the dynamic criteria necessary for the creation of a chinook might be present.

In spite of the apparent lack of agreement among these investigations, it is clear from their collective experience that any event which includes all of the following: a strong westerly wind, preferably a flow normal to the mountains (Brinkmann, 1969); dry adiabatic lapse rate; leewaves (Beran, 1966); warm temperatures (Longley, 1967; Lester, 1976b), including an abrupt rise and an equally abrupt termina- tion of unseasonably warm temperatures (Nkemdirim, 1970; Danielewicz, 1977); a low relative humidity (Brinkmann, 1969); a chinook arch (Lester, 1976b) and gusty winds (Ives, 1950) must be a chinook. The task at hand is to specify a minimum mix of variables which collectively identify true chinooks at ground level with a high degree of accuracy.

CHINOOKS IN SOUTHERN ALBERTA 595

3. METHOD OF INVESTIGATION

This study is based on data obtained during the winter of 1984-1985. The data themselves were restricted to night-time events, thereby excluding the impact of solar irradiance on the energy and water balance of the lower atmosphere. In addition, by confining the data only to night-time (18.00-07.00) the impact of the diurnal cycle of temperature which enforces or reduces the effect of advective warming by a chinook is reduced.

The sample data consisted of weather events involving the replacement of the Arctic front by the Polar front, with wind prevailing from any direction between WNW and SSW for at least seven hours (half the length of the nocturnal period as defined).

The synoptic settings in which chinooks occur vary in detail, but there are a number of essential factors which must be present to ensure a true chinook. A requirement for the flow of maritime air into the interior plain is the establishment of a strong pressure gradient towards the Hudson Bay. This is normally provided by the presence of a ridge of high pressure to the west of the mountains, preferably centred over the north-west U.S.A. and southern British Columbia and connected to a low centred over northern Alberta and the Hudson Bay. Figures 1 and 2 are examples of the distribution of pressure at sea level and 500 mb found in chinook-producing weather. However, similar synoptic settings have not always ensured a chinook. Three subsets of the sample were identified from data obtained from synoptic maps for the surface and 500 mb, and temperature, humidity and wind collected by the Atmospheric Environment Service. Group 1 events included all those deemed to be ‘true’ chinooks. They possessed all the dynamic and surface criteria described in the last paragraph of section 2. Group 2, described hereafter as ‘non-Chinooks’ comprised other west winds which resulted in improving and improved temperature following the replacement of the Arctic airmass by the Polar

Figure 1. Distribution of sea level pressure during a chinook. Sea level pressure map showing the presence of a warm front to the east of the study area. The dominant features are the two lows over the west and the north, and the ridge over the south-west (NW U.S.A.). Counterclockwise flow around the Pacific based low sustained a westerly wind into the study area. Subsidence first over the Coastal mountains and subsequently over the Rockies and rain out on the windward side of the mountains produced a

chinook in the target area. Warm temperatures were experienced in a broad zone from Red Deer to Lethbridge (L) .

596 L. C. NKEMDIRIM

Figure 2. Distribution of pressure at 500 mb in a chinook dominated weather. Map showing the corresponding 500 mb surface. Heights are in metres. Note the direction of streamlines as they approach the study area.

airmass. It is distinguished from group 1 by a lack of evidence of lee and hydrostatic waves in the area. Temperature was deemed improved if the minimum exceeded the long-term mean by more than 1°C in a broad belt extending from Red Deer in the north to Lethbridge in the south-east. Group 3 included all other events in which the requisite airmass replacement occurred but where leewave activity was ill-defined or where data were incomplete.

Following this initial classification answers were sought to two questions, namely (1) how do selected weather variables in the first two groups differ from each other near the ground, and how can the differences be used to tell the two groups apart in the absence of upper air data? (2) To what group do the events in group I11 belong? Do they form a separate third group? The variables used to answer these questions are listed in Table I. They include some of those whose behaviour under chinook conditions has been studied in the past and others whose features were identified by the author as being sufficiently aberrant during a chinook to justify their use in the present study (Nkemdirim, in submission).

The data used to answer these questions were obtained from a meterological tower located in an open grassed field at the University of Calgary’s Weather Research Station.

Table I. Variables measured at the University of Calgary Weather Research Station ~

Height at which measurement

Variable name Instrument used was made, m

Windspeed and variability Contact cup anemometer 10 Station pressure and pressure variability Microbarograph 2 Temperature and temperature change Copper constantan thermopiles 2, 5, 10 Humidity Capacitor sensor 2, 5 Radiative cooling Pyrradiometers (Swissteco) 2, 10

CHINOOKS IN SOUTHERN ALBERTA 597

4. COMPARISON OF SURFACE FEATURES

Nineteen events were classified under group 1, 13 were in Group 2 and 10 events comprised group 3. A statistical summary of the results of the comparison between variables in groups 1 and 2 is presented in Table 11. Two statistical tests were undertaken on each variable. In the first test the difference between the variance of the two groups was tested €or significance. If the test showed that the two variances were equal then in the second test, in which the means were compared, the pooled t test was used. If they were unequal the ‘separate’ t was used. All tests were at LY = 0.05.

Table 11. Comparison of means and variances of variables in chinooks (1) and non-chinooks (2)

Standard Means deviations t-statistic F for equal Variance Means

Variable 1 2 1 2 Separate Pooled variance equal? equal?

Pressure (mb) Pressure variability (mb/hr) Wind (m/s) Wind variability (m/s) Specific Humidity Wkg) Temperature change (“C/h) Temperature change variability W h ) Ratio of radiative to actual cooling rate

861 858 35.4 28.1

8.8 5.5 4.8 4.5

0-28 0.28 1.58

2.2 2-0 1.11

6.3 5-1 2.4 0-81 0.42 0.38 0.13 0.11

1.4 3.2 0.62 0.55

-0-29 -0.57 0.63 0.27

1.56 0.63 0.69 0.28

10.4 0.73 25.4 26,4

1.98 1-6 9-08 0.79 0.76 1.31

8.7 8.5 1 a27

1.74 1.52 5.23

5.2 4-5 5.98

1.65 1.36 92.054

Yes Yes

Yes No

No Yes Yes No

Yes No

No Yes

No No

No Yes

Pressure and pressure change

The mean pressure during chinooks was higher than in non-Chinooks. This is in general agreement with the synoptic setting found in most chinooks (Danielewicz, 1977). However, both the mean and variance of pressure in chinooks were not statistically significantly different from those encountered in non-Chinooks. Consequently, pressure by itself could not be used to identify Chinooks. However, although the variances in pressure change were not statistically different from non-Chinooks, the means were generally higher in chinooks than in non-Chinooks. Richner (1979) showed that rapid fluctuation of pressure is a major feature of foehns. He found waves with a significant frequency between 4,and 6 minutes in the time series of pressure during foehns. Mathews (1982) appeared to have observed similar frequencies in Chinooks.

Many of the features associated with Chinooks, such as the gustiness of winds (Ives, 1950), rapid fluctuation of temperature and humidity (Holmes and Hage, 1971; Lester, 1975) and pressure (Richner, 1979) are believed to be related to the fluctuation of pressure found in chinook systems.

598 L. C. NKEMDIRIM

Wind speed and wind variability

The mean wind speed in a chinook is higher than its non-chinook equivalent but not statistically significantly so. This is in agreement with results of other studies. Thus the view that chinooks are strong winds (Ives, 1950) is only valid if it is recognized that all strong westerly winds in southern Alberta are not necessarily chinooks. What is enlightening, however, is the high significance attained in the variance test. This suggests that the range of wind speed found in chinooks is larger than those experienced in non-Chinooks. Although mean variability in wind speed is higher in chinooks the difference is not statistically meaningful. The variance in mean variability is even less significant than the mean wind speed, which appears to contradict the generally held view that the chinook is more gusty than other equally strong westerly winds in southern Alberta.

Temperature change and its variability

The mean cooling rate in chinooks is about half of its non-chinook equivalent (Figure 3) but the difference is not statistically significant. However, the difference in the variances of the two samples was highly statistically significant, suggesting that the range of cooling rates experienced in chinooks was larger than in non-Chinooks. This is reflected in the fact that both the mean and the variance of the variability factor were significantly larger in chinooks than in non-chinooks (Figure 4).

C o o l i n g r a t e s ( 'C lhr ) '1

_ - - N o n Ch inooks L' -2

18 20 22 00 02 0 4 06 08 Time ( H o u r s )

Figure 3 . Cooling rates in chinook and non-chinook weather

s] Var iab i l i ty of coo l ing r a t e s ( 'C /hr )

Figure 4. The variability of cooling rates in chinooks and non-chinooks

CHINOOKS IN SOUTHERN ALBERTA 599

Table 111. Correlation matrix

Wind Pressure Temperature Specific Temperature Wind Radiative/actual Pressure speed change change humidity variability variability cooling rate

Chinooks Pressure 1 Wind speed -0.568 1 Pressure change -0.037 -0.222 1 Temperature -0.062 0.093 0.534 1

Specific humidity -0.212 0.457 -0.315 -0.234 1 Temperature -0.46 0.064 0.622 0.695 -0.228 1

change

variability Wind variability 0.013 -0.174 0.479 0.303 -0.26 0.391 Radiative/actual 0.491 0.378 0,194 0.187 -0.097 0.069

cooling rate Non-chinooks

1 0.599 1

Pressure Wind speed Pressure change Temperature

change Specific humidity Temperature

variability Wind variability Radiative/actual

cooling rate

1 -0.345 1 -0.141 -0.105 1 -0.265 0.476 -0.05 1

0.285 -0.181 -0.413 0.223 1 0.234 -0.533 0.251 -0.662 -0.17

-0-023 -0.269 0.602 -0-257 -0.347 -0.253 0.443 -0,216 0.399 0.149

1

0.68 1 -0.73 -0.71 1

The larger variability in cooling rates in chinooks is perhaps a reflection of the enhanced fluctuation of pressure to which it is correlated (Table 111). It might also reflect the pulsating motion of the mesoscale waves found in chinook-producing systems (Lester, 1975).

Humidiv

The flow of maritime air across and over the mountains may be accompanied by dewatering. Air currents originating from the Pacific hold significant quantities of heat in latent form due to their high moisture supply. Lifting on the windward side of, first, the coastal range and secondly, the Rockies, results in cooling initially at the dry adiabatic rate up to dew point and thereafter at the wet adiabatic rate approximately half the initial cooling rate. If precipitation occurs on the upwind side, and if the air subsequently descends to lower ground on the lee of the mountain, it will gain temperature at the dry adiabatic rate. Thus for two positions on the same elevation temperature will be higher on the lee than on the upwind side. Ives (1950) believed that the release of latent heat due to condensation and precipitation following forced lifting of the air over the mountains is a prerequisite for chinooks.

For a long time this process was held to be essential for the warmth of the chinook in southern Alberta; a view which was encouraged by the observation that some of the more intense chinooks experienced there followed heavy precipitation in the interior of British Columbia. Recently, Richner (personal communication) asserted that at least 80 per cent of foehn events are associated with the release of latent heat.

However, precipitation on the sea-side of the Rockies has not been observed in many chinook events (Danielewicz, 1977) and rain out, when it occurred, was not always sufficient to account for all the warming found in chinooks (Riehl, 1965; Barry and Chorley, 1968).

But then the question arises as to why the temperature on the lee side must exceed its upwind equivalent at the same elevation before a west wind qualifies as a chinook. Even if the two values were

600 L. C. NKEMDIRIM

- z z .-

E

s -5- m

- 1 0

equal, thereby removing the requirement for the release of latent heat, the ensuing temperature on the lee must represent an improvement over the temperature which prevailed in the now-displaced Arctic airmass. But this condition may not qualify as a chinook. The result of this study appears to support this view.

The specific humidity of the atmosphere near the ground was significantly lower in chinooks than in non-chinooks even when the data were adjusted to reflect differences in mean monthly values. This result was unexpected in the light of conclusions reached in previous investigations (Brinkmann, 1969, for example). It would appear, therefore, that the low relative humidity experienced in chinooks is the result of both a low absolute humidity and higher temperatures. It also appears to lend support to those who like Ives (1950) and Richner (personal communication) believed that true chinooks require the release of latent heat on the sea-side of the mountains.

The inequality observed in means did not extend to the variance, indicating that the ranges of humidity experienced under the two weather conditions are not dissimilar.

Radiative to actual cooling rates (RIA) Elliott (1964) showed that the ratio between the radiative temperature tendency and the actual

cooling rate in the atmospheric layer between 2 and 10 m above a grassed surface was about 2.5. Under normal weather the two indicators of cooling are in phase (Figure 5) and in agreement with results obtained by Funk (1960) and Nkemdirim (1978). In chinooks the two indicators were frequently out of phase, on 11 February 1985 for example (Figure 6).

qad ia t i on coo l i ng ra re 57-,

\ I \ I \l

I I I I I I 1

b)

0 01 r r 9

20 2 2 00 0 2 0 4 06 o a 2 0 1 I 2 / 8 4 2 I I 1 2 / 8 4

Figure 5. Comparison of radiative and actual cooling rates in ‘normal’ (non-chinook) weather. The radiative cooling rate dR/d t was calculated from the divergence of net longwave radiation ( d L * / a z ) between Z, = 2 m and Z, = 10 m. aR/a t = pcp dL*/az.

The actual cooling rate S T / & is the temperature change measured at 5 m

Rad ia t i on c o o l i n g r a t e O l

0 - 0

; 0 . 0 0,

_- Actua l coo l i ng r a t e

\ ---------.-----

/ /

/

/’

CHINOOKS IN SOUTHERN ALBERTA 601

The average R/A is higher in chinooks than in non-Chinooks, but the difference was not statistically However, the difference in the variance was highly significant, reflecting the wide range of

values significani fo nd in chinooks. Nkemdirim (in submission) has suggested that the discrepancy between the statistics in the two types of weather might reflect the stronger influence of advection on the relationship between radiative and actual cooling rates.

5. DISCRIMINANT ANALYSIS

The correlation matrices (Table 111) for the variables in groups 1 and 2 highlight two themes. 1. There appears to be a broader and somewhat stronger relationship between pressure and pressure

change and the remaining variables in chinooks than in non-Chinooks. This is particularly true among those variables which measure change and the rate of change, underscoring the view that chinooks are characterized by intense fluctuations of most atmospheric variables (Holmes and Hage, 1971; Lester, 1975) and that rapid change in pressure is the major forcing variable in foehns (Richner , 1979).

2. The correlation coefficients range from weak to moderate. As a result no one variable is sufficiently strong to adequately express the strength of other variables or groups of variables in defining a chinook or in discriminating between chinooks and non-Chinooks.

Comparison of statistical measures on individual variables showed that, with the exception of cooling rate variability, no single variable among those studied was sufficiently differentiated in both its mean and variance to form a basis for separating the two types of weather. This is believed to be a major reason for the difficulty encountered by previous studies in defining chinooks on the basis of the mean values of a small number of variables. However, there was enough separation either in terms of the mean or the variance, though rarely in both, to suggest that a collective use of the variables in a discriminant function could provide an adequate framework for classification.

The formulation of the discriminant function was done stepwise, starting with pressure. The proportion of misclassified cases was gradually reduced by sequentially adding new variables to the function (Table IV). This procedure was repeated ten times using different orders to input the variables. In the end only one item was placed in a group to which it did not belong. The coefficients of the discriminant functions of the best combination of variables for the discriminant function are listed in Table V.

Station pressure, pressure change, wind speed, temperature change, specific humidity and tempera- ture variability jointly placed 96 percent of the events in their proper group with an average probability of 99 percent. The addition of new variables to the group of six did not lead to a further alteration in the number of cases misclassified. The F statistic showed that the two groups differed significantly at the

Table IV. Variables entered into discriminant function and percentage correctly predicted

Proportion correctly Average

Variables entered predicted* probability

Station pressure - - Pressure variability 0.5 0.99 Wind speed 0.78 0.99 Temperature change 0 4 1 0.99 Specific humidity 0.86 0.99 Temperature variability 0.96 0-99 Ratio of radiative to 0.96 0.99

Wind variability 0.96 0.99 actual cooling rates

* Values given are cumulative.

602 L. C. NKEMDIRIM

Table V. Coefficients for variables in discriminant functions

Group variables Chinooks Non-chinooks

Intercept -764.912 -727.215 Station pressure 1.574 1.523 Wind speed 13.95 12.257 Pressure change 0.6 0.601

Specific humidity 1 -279 8.344 Temperature change -21.279 -17.4

Temperature 39.441 33.744 variability

six variable level (a = 0-05). The statistic continued to increase significantly with the addition of new variables, but that did not lead to further reclassification. This suggests that the first six variables represent the minimum number of predictors necessary to maximize the sensitivity of the classification based on the sample.

Upon examination of the data it was found that the one case which was shown as misclassified was a doubtful chinook. Thus the discriminant analysis provided a tool for revising the allocation of individual cases on the basis of the objective criteria established by the analytical tool.

Four events in group 3 were grouped into 1 whereas 6 were included in 2. The probability associated with this reclassification averaged 0-83.

A replication of the procedure on a new set of sample data confirmed the stability of the method used.

6. CONCLUSION

Chinooks are a major factor in the environment of southern Alberta. Previous attempts to provide a definition for the wind using ground level features have met with limited success, because almost all the characteristics loosely considered as unique to chinooks in southern Alberta are also present in other types of west-wind-dominated weather experienced in that area. Chinooks represent several features and derive their uniqueness, to the extent that the description is appropriate, from the collective expression of a group of variables on the weather. Unseasonable warmth is only one aspect of the chinook. The nature of the warming and the process through which it occurs are equally important in giving the warmth associated with the wind a definition.

When station pressure, pressure change, wind speed, temperature change, specific humidity and temperature variability are combined in a discriminant function, they appear to provide a stable basis for discriminating between chinooks and non-chinooks in a lower atmosphere dominated by a westerly wind in southern Alberta.

ACKNOWLEDGEMENT

This research was supported with funds from the National Science and Engineering Research Council of Canada. The assistance of Edward Rhodes, Lynn Huntley and Debbie Snow is gratefully acknowledged.

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