extra urban and intra urban rainfall enhancement by a medium sized city

VOL. 17, NO. 5 WATER RESOURCES BULLETIN

AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 1981

EXTRA URBAN AND INTRA URBAN RAINFALL ENHANCEMENT BY A MEDIUM SIZED CITY’

Lawrence C. Nkemdirim’

ABSTRACT: An experiment on urban effects of warm season rainfall of a moderately sized city and its downwind towns found no evidence of global enhancement. However, there are grounds for believing that the intra urban distribution of precipitation is influenced by urban variables among which air pollution and urban roughness are considered primary. The presence of an urban heat island did not appear to improve rainfall. On the contrary, it appears to be a factor in the relative aridity of the downtown sector of the city. (KEY TERMS: urban rainfa, enhancement; air pollution; heat islands.)

INTRODUCTION

Urban influence on climate has been reported for over a century. Since Howard (1 833) made his study of the climate of London, several reports and monographs have been pub- lished on that subject (Peterson, 1969). If, as the literature suggests, a city can enhance precipitation within itself (Sander- son and Gorski, 1978) or downwind (Changnon, 1968) or both (Changnon, 1979), the recurrence interval for floods of all sizes in the area may be shortened. And since the construction of most engineering structures are based on expected flood frequencies, their capacity to fulfill design goals may be compromised by urban induced rainfall enhancement. Leo- pold (1968) discussed the consequences of urban landuse on stream flow on the premise that the change in land use is not paralleled by one in climate. But if the city does induce a change in the local rainfall regime, realistic models in urban hydrology should incorporate the impact of the change. How- ever, that effort will require the creation of a larger data base on the scope and nature of urban induced rainfall enhancement than what currently exists. It is with that in mind that the experiment on rainfall distribution in Calgary was initiated.

A survey of warm season rainfall in Calgary (pop. ca 5 10,000) was undertaken from 1978- 1980 as part of a climato- logical study of the city and its surrounding country. Among the objectives of the study were to determine (a) the extent to which the city has affected precipitation in the city and its downwind towns, (b) if there are significant intra urban differences in rainfall which result from urban influence, and

(c) to provide base data and analysis for the assessment of the impact of future growth on the city’s rainfall pattern.

RAIN GAGE NETWORK Twenty rain gages were used in the study. Of these, 15

were standard Canadian copper gages, 4 were tipping bucket recording gages, and 1 was a recording weighing type. In addition to providing information on the duration and intensity of rainfall the two sets of automatic gages were calibrated against a standard gage and were consequently used as part of the standard gage network.

Decision on the location and allocation of gages was governed by the desire to achieve a uniform coverage while reflecting the topographical diversity of the city. This was done by dividing the study area into twenty 20-square kilo- meter rectangular grids. The area was then broken down into elevation districts using 38 m intervals. The percentage of land within each district was determined, and the gages were located in the grids in a manner that took account of the elevation breakdown. Fortuitously a good representation of other relief features such as aspect and slope was also achieved.

The gages were sited in accordance with exposure specifica- tion of the WMO (1971). However, in a small number of cases, standard exposure was compromised by factors of safety and/ or accessibility. But such departures did not destroy the design objectives of the network. Nearest neighbor statistic was calculated as a means of testing the form of the distribution. The statistic confirmed that the gages were uniformly dis- tributed over the study area.

During the study, the standard error the mean rainfall based on the network averaged less than 5 percent. For or- ganized storms such as cyclones with heavy rainfall (> 10 mm/event) the error was about 2 percent but was as high as I1 percent for convectional storms of lower intensity (< 2 mm/ event). The most widely dispersed storm occurred on July 4, 1978, when the error was about 30 percent. These figures suggest that the network was adequate for the assessment of the areal rainfall in the area. Indeed a ‘control’ experiment

‘Paper No. 80105 of the Water Resources Bulletin. Discussions are open until June 1, 1982. Department of Geography, The University of Calgary, Calgary, Alta. T2N 1N4, Canada. 2

753 WATER RESOURCES BULLETIN

Nkemdirim

involving a similar gage network outside the University Weather Research station suggest that in the worst exposure situation the network was at least 75 percent accurate in its assessment of “true” rainfall.

There was considerable areal variability as indicated by the number of days when rain, though occurring at several stations within the city, did not occur in at least one site. The minimum number of days in which such an event was experienced was nine (1978) at Station 7. The maximum was 21 (1980) at Station 15.

Figure 1. Calgary: Topography, Land Use, and Rain Gage Stations.

STUDY PERIOD AND DATA SUMMARY

The study was conducted over three years from 1978-1 980. It includes all rainfall events between May and September. In 1978 and 1980, the mean warm season rainfall was above average for the time of the year; but 1979 was relatively dry (Table 1). Taken together, the mean of the three years is very close to the normal rainfall expected at that time of the year.

TABLE 1. Summary of Rainfall Data. ~~ ~ ~~

1978 1979 1980 Mean

Total Rainfall (mm) 330 164 320 271 Percent of Mean for May-September 125 62 121 103 Number of Rain Days 74 59 75 69 Number of Events* 51 43 57 50.3

*The number of events is less than the number of days of rainfall because some events extended beyond one day.

754

RESULTS

Potential Inmence of Calgary on Downwind Rainfall Since the Changnon’s (1968) La Porte, Indiana, study we

have become aware of the potential of atmospheric pollutants from urban sources to significantly enhance rainfall and related weather downwind. Although not generally a serious environ- mental problem in Calgary, there has been anoticeable increase in the level of air pollution in the past few years (Nkemdirim, et al., 1975) and under certain types of weather, pollution has reached levels that can be considered hazardous (Nkemdirim and Leggat, 1978).

Winds prevail from the northwest in Calgary (Brinkman, 1969), but during storms they are equally likely to come from the southeast (Audrey, 1979). During the study period 61 percent of the storms were accompanied by northwesterlies or southeasterlies demonstrating the dominance of these directions in the rainfall process. Consequently, if there is a downwind effect, it should be experienced in Gleichen and High River to the south and east and at Cochrane and Crossfield to the northwest (Figure 2). Two tests were conducted to verify possible enhancement. In the first test, trend line were fitted to May- September data (1962-1980) for Calgary airport and for downwind sites. The rainfall regimes in these places (Figure 3) was not different from those for other areas in the Prairies (Long- ley, 1972) nor was the declining trends for each of the stations statistically significant (0.05 level). Moreover, there was no statistically significant difference in the slope of the trend line among the stations. In the second test, downwind sites were paired against nearby nondownwind stations, and the ratio of the annual warm season rainfall (1962-1980) between the pairs computed. The pairs used were Crossfield against Madden, Gleichen against strathmore, Cochrane against Madden, and High River against Blackie. There was neither a net increase in the size of the ratios over the period in question nor were any trends observed in the time series. In addition, Calgary airport whose rainfall is lower than the regional average did not ex- perience any net gain in relation to Gleichen, High River, Crossfield, and Madden over that period. These results suggest that there has not been any rainfall enhancement downwind of Calgary as yet and that Calgary itself has not received more warm season rainfall as a result of urbanization. The results support Changnon (1 976) who found no eivdence of anomalies in smaller cities. Perhaps they suggest that the city has not grown either in size or in pollution status to influence the climate of the area around it. However, the results do not ex- clude the possibility of an internal distribution of rainfall in- side the city which may be traced to urban influence.

WATER RESOURCES BULLETIN

Extra Urban and Intra Urban Rainfall Enhancement by a Medium Sized City

- \ MacLeod

-? -. 7-

Figure 2. Distribution of Mean Warm Season Rainfall in Calgary and Surrounding Towns (adjusted for elevation

difference) May-September 197 8-1 980.

CALGARY , GLEICHEN

*- HIGH RIVER

250 250

1962 1978 1962 1978

Figure 3. Trends in Warm Season Rainfallin Calgary and Three Downwind Towns.

Intra Urban Rainfall Dism'bution The data from all 20 stations were tabulated according to

stations and date (event). A two-way ANOVA test showed that the differences in stations and the distribution of storm

rainfall were statistically significant at 0.05 level. The conclusion was therefore drawn that the variable spatial distribution of the warm season rainfall (Figure 4) was real. The south- west received about 35 percent less rainfall than the airport while an axis running northwest-southeast through city center equaled or exceeded the airport value. This axis henceforth referred to as the 'corridor' includes the communities of Silver- springs and Ranchlands to the North and Ogden (the industrial center) to the south. Higher rainfall was also found to the south of the Glenmore Reservoir, an open lake in the south- west corner of the city. Audrey (1979) successfully linked its higher rainfall to downwind influence of the lake.

Figure 4. Distribution of Mean Rainfall in Calgary (May-September 1978-1980).

The higher rainfall in the corridor coincides with the dominant wind direction during storms. Forty percent of the events were accompanied by northwesterly airflow, 2 1 percent by southeasterlies, and the remainder shared winds from all the other directions. Audrey (1979) noted that one of the three major storm tracks that affect the city passed through the corridor.

In the following subsections the possible causes of Calgary's urban rainfall 'pattern' are explored.

Elevation. The considerable differences in elevation within the city suggest that altitude could be a factor in the spatial pattern. It was with that possibility in mind that the network was designed to reflect topographical differences. However,


Nkemdirim

the correlation between total rainfall at the stations and station altitude was very poor; less than 5 percent of the variance in the rainfall was accounted for by elevation. Moreover, the correlation was a negative one, a direction which runs against the accepted positive correlation between the two variables. Nkemdirim (1968) showed that up to 40 percent of the rainfall variance in upland areas may be attributed to elevation. Because high density urban development occurs in the lower grounds, the negative correlation between the variables suggests that the distribution of warm season rainfall in Calgary is biased in favor of the builtup area. In fact one of the zones of maximum rainfall occupy the lowest lands. The Industrial Park (Ogden) in the southeast with its manufacturing plants and extensive residential development is the lowest part of the city.

Wind Direction. The effect of wind direction on the distribution of rainfall may be inferred from the response of the corridor to storms from the two dominant directions. With northwesterly winds, the highest rainfall occurred on the upwind side of downtown (Figure Sa). The minimum occurred over downtown (CBD) followed by a secondary maximum in Ogden. With easterlies and southeasterlies maximum rainfall was received in Ogden on the upwind side. Rainfall in the CBD is again the lowest and a secondary maximum is found downwind in Silverspring and Ranchlands. These results are in agreement with Sanderson and Corski (1978) who found a similar distribution in Detroit-Windsor. They argued that mechanical destabilization of rainstorms by the urban fabric upwind of the city was responsible for the enhanced rainfall over that edge of the city. They attributed the lower rainfall of the CBD to 'overseeding' by airborne nuclei.

O W ' Silver Springs C B D Ogden Ranchlands (Downtown) (Industrial ) -

0 1 2.5 5 K m

Figure 5.. Cross Section Showing the Distribution of Total Rainfall During the Study Period: (a) With Northwesterly Winds; (b) With Southeasterlies.

Whereas mechanical destabilization provides an adequate explanation for the upwind maximum, it is doubtful that the relative dryness of the downtown is caused by overseeding given that the particulate count in the CBD is among the lowest in the city (Figure 6).

There are two major factors which could provide a reason- able explanation for a drier downtown. The first of these is

the lower moisture content of the atmosphere over city center. Nkemdirim, et at! (1 978), found that absolute humidity in the summer was lower at city center than anywhere else in the city. Apart from a reduced capacity to resupply a storm al- ready dehydrated by the heavier rainfall upwind, the lower moisture status in that part of the city is unlikely to promote localized convective showers.

UlLLS

RANCULANDS

SILVER SPRINGS

Figure 6. Average Dust Fall (tonneslmonth) in Calgary.

The second factor is urban temperature related. In the summer, the average temperature in the CBD is about 4' higher than the surrounding countryside (Nkemdirim and Truch, 1978). This additional heat load should improve the capacity of the atmosphere to hold moisture in the form of vapor, and consequently weaken the precipitation process. Braham and Dungey (1978) found that cloud base was higher in St. Louis, Missouri, than in the county because of the thermodynamic effect of the higher urban temperature.

The two factors are perhaps aided by the virtual absence of such versatile seeding agent as sulphur dioxide from the downtown core (Nkemdirim, et at!, 1975).

The secondary maximum found in Ogden with northwesterly airflow is perhaps the result of a more effective cloud seeding by effluents from manufacturing plants (this is discussed in the next subsection) while the equivalent phenomenon with southerly air flow is perhaps due to upslope motion.



?he Role of Air Pollution. Evidence gathered during the study strongly suggests that atmospheric pollution is a factor in the intra urban distribution of rainfall in Calgary. City- wide, dustfall correlates moderately but significantly with rainfall (r = 0.44). In the corridor, however, the correlation coefficient is even stronger (r = 0.56). The discrepancy between these two sets of figures prompted the speculation that the inclusion in the city-wide analysis of pockets of heavy dustfall in areas of new road and residential construction may have weakened the correlation between the two variables since a high proportion of dust from construction may be inactive in the condensation process in contrast with the effluents of manufacturing and processing plants located in the corridor.

In light of these results, there can be little doubt that the heavier air pollution of Ogden (Figure 6 and Nkemdirim, et al., 1975) is one of the factors which has contributed to making it the wettest part of the city (Figure 4). Ogden also receives the highest frequency of heavy rainfall (> 10 mmlevent) in the city (Figure 7) and the largest amount of localized thunder- storms (Figure 8).

The possible impact of air pollution on the intra urban distribution of rainfall in the city was further emphasized by an analysis of localized convective storms. The amount and frequency of these storms at Ogden were compared in turns against those of six randomly selected control stations using a student ‘t’-test. The Ogden regime was significantly different from those of the control stations. In contrast, a two-way

ANOVA did not reveal any statistically significant difference among the control stations. This led to the conclusion that Ogden had a regime which is unique in the city and that its most obvious attribute, air pollution, was an important factor in fostering that regime.

Is the City as a Whole Wetter Because of Urban Infkcence? Weekendmeekday Distribution. Lawrence (1973) sug-

gested that an urban climate could induce more precipitation. He produced empirical evidence to support a claim that rainfall in urban areas occurred more frequently on weekdays that weekends. Because weekdays are associated with a higher level of industrial activity and stronger heat islands, it was believed that the observation was evidence of urban enhancement of precipitation.

There was no evidence that storms occurred more frequently on weekdays than weekends in Calgary during the study. If rainfall frequency is unbiased in favor of any day of the week approximately 14 percent of the events should occur on each of the seven days. In the study, 27 percent of the events occurred on Saturdays and Sundays while 73 percent fell on the remaining five days. A chi-square test showed that there was no statistically significant difference between the expected frequency from an unbiased day of the week distribution and the observed frequency. It follows therefore that the day of the week was not a factor in determining rainfall

Figure 7. Mean Frequency of Heavy Rainfall (> 10 mm/ event) in Calgary During the Study Period.

757

I I I

RANCHLANDS

SILVER W I N G S / I I

Figure 8. Distribution of Mean Rainfall From Localized Convective Storms.

WATER RESOURCES BU LLETl N

frequency. Similar results were also found at Cochrane. For- tuitously, air quality as measured by hydrocarbon in the atmosphere was not significantly lower over the weekend than any other day.

However, there is some evidence of day of the week effect on the intra urban distribution of rainfall. A comparison of the total study period rainfall at the four automatic stations showed that rainfall was more strongly biased in favor of the weekday at Ogden than at the other three stations. The weekend/weekday ratio at Ogden was 0.34, 26 percent less that at the other stations whose ratios ranged from 0.44 to 0.48. This is in agreement with Braham and Dungey (1978) who reported similar results from Metromex and suggest possible air pollution induced enhancement of rainfall within the city.

A Heat Island Effect. Parry (1956) suggested that a 1’ differential between urban and rural temperature is sufficient to enhance precipitation in an urban area. Support for the view include Atkinson (1971) who deduced that thundr- storms in London could be initiated by that city’s heat island. Changnon (1 979) thought that the heat advantage produced convergence zones within the city which in turn enhanced its precipitation. Further, Changnon, ef aL (1 976), observed that the heat island destabilized the city’s atmosphere sufficiently to add to its rainfall.

Calgary’s well defined heat island has two peaks, a primary maximum at mid-morning and a late evening secondary one (Nkemdirim and Truch, 1978). If the strength of the heat island is a forcing function in the enhancement of rainfall then one should expect a correlation between the timing and duration of rainfall and maximum heat island strength. No such similarity was found between the two variables (Figure 9). Huff (1975) found that local storms in St. Louis occurred most frequently in the late afternoon and early evening, a pattern similar to Calgary’s. He attributed the timing to the combined effect of solar heating and the presence of an urban heat island. But since a similar pattern was found in Cochrane and since the urban-rural temperature differential in Calgary is a minimum over those hours, it is doubtful that the timing of these storms is related to the heat island.

An Air Pollution Factor? Earlier it was demonstrated that urban air pollution could be a factor in the internal distribution of rainfall intensity and frequency within the city. It has yet to be shown that the air pollution level within the city is sufficient to initiate as opposed to redistribute rainfall. There is a weak correlation between the diurnal pattern of rainfall frequency and the combined air quality (r = 0.25). The correlation which is due primarily to the occurrence of a secondary peak in air pollution around 1600 (Figure 9) is not statistically significant and may well be fortuitous. It is therefore doubtful that air pollution is a significant factor in urban rainfall in all parts of the city.

DISCUSSION AND CONCLUSION There is not evidence at this point in time that Calgary and

its downwind towns have become wetter because of urban influence. At the current rate of growth the city’s population

is expected to reach the million mark by the turn of the century when according to Changnon (1 979) rainfall enhancement may be expected to occur. Should enhancement occur at that stage, air pollution may well prove to be the most important factor in that process. Although Calgary’s air pollution is pre- dominantly traffic related (Alberta Environment, 1973) there is an expanding manufacturing industry especially petrochemi- cals.

10 I* ! ’.

3 - x 2 - st

-10 .E 8 8 3

- 20

1 -

L

Nkemdirim


, , , , I I I I ! I , I

0 02 04 06 08 10 12 1d 16 18 20 22

Time ( i ) Air quality (in) Heat island intensity ( i i i ) Rainfall frequency

Figure 9. Mean Diurnal Distribution of Rainfall, Air Quality, and Urban Heat Island Intensity.

The intra urban distribution of the available rainfall suggests a very strong urban influence. Among the most important factors in promoting that pattern, are atmospheric pollution, wind direction, and urban fabric. These variables com- bine to produce convincing evidence of rainfall enhancement in the industrial sector of the city perhaps at the expense of other areas such as downtown Calgary which is considerably drier.

LITERATURE CITED

Alberta Environment, 1972. Inventory of Air Pollution Sources and Emission in the City of Calgary, 1971.

Atkinson, B. W., 1971. The Effect of an Urban Area on the Recipita- tion From a Moving Thunderstorm. Journal of Applied Meteoro-

Audrey, J. K., 1979. A Spatial Study of Precipitation in Calgary 1978.

Braham, R. R., Jr. and M. Dungy, 1978. Effect of St. Louis on Con- Journal of Applied Meteorology 17:

Brinkman, W. A., 1969. Definition of the Chinook in the Calgary Area. Unpublished M.Sc. Thesis, University of Calgary.

Changnon, S. A., Jr., 1968. The La Porte Weather Anomaly - Fact or Fiction? Bull. h e r . Meteor. SOC. 49:4-11.

Changnon, S. A., Jr., 1976. Effects of Urban Areas and Echmerging on Radar Echo Behaviour. Journal of Applied Meteorology 15:

logy 10~47-55.

Unpublished M.Sc. Thesis, University of Calgary.

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Changnon, S. A, Jr., 1979. What to Do About Urban Generated Wea- ther and Climate Changes. J. Amer. Plan. Assoc. 45:3647.

Changnon, S. A., Jr., R. Semonin, and F. Huff, 1976. A Hypothesis for Urban Rainfall Anomalies. Journal of Applied Meteorology 15 :

Howard, L., 1883. The Climate of London Deduced From Meteorolo- gical Observations Made in the Metropolis at Various Places Around It. 2nd Ed., London, J. & A. Arch, CombiU, Longman & Co.

Huff, F. A., 1975. Urban Effects on the Distribution of Heavy Con- vective Rainfall. Wat. Res. Res. 11:889-896.

Lawrence, E. N., 1973. High Values of Daily Area Rainfall Over Eng- land and Wales and Synoptic Patterns. Meteor. Mag. 102:361-366.

Leopold, L. B., 1968. The Hydrologic Effects of Urban Landuse in Hydrology for Urban Land Management. U.S. Geological Survey, Circ. 554,18 pp.

Longley, R. W., 1972. The Climate of the Prairie Provinces. Climato- logical Studies 13,79 pp.

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Nkemdirim, L. C., G. Lunn, and'R. D. Rowe, 1975. Pollution Concen- tration and Stratification in an Urban Heat Island. Wat. Air, Soil

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extra urban and intra urban rainfall enhancement by a medium sized city

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