Atmospheric Environment Vol. 11. pp. 597-604. Pergamon Press 1977. Printed in Great Britain.
URBAN-RURAL
WIND
VELOCITY
ROBERT D. BORNSTEIN and
Department
of Meteorology,
DIFFERENCES*
DOUGLAS SCOTT JOHNSON
San Jose State University,
San Jose, California
95192
(Received 2 August 1976, in final form 9 December 1976)
Abstract-Wind
speeds along a streamflow line through New York City are found to be decreased
below (increased above) those at sites outside of the city during periods with regional wind speeds
above (below) about 4 m/s. The decrease is attributed to increased values of the surface roughness
parameter in the city, as compared to values in nearby non-urban regions. The increase is associated
with accelerations produced by a well-developed urban heat island.
Cyclonic turning of the flow along the streamflow line through the city during the high wind speed
periods results from decreases in the value of the Coriolis force associated with the increased urban
frictional drag. During periods of acceleration over the city, either cyclonic or anticyclonic turning
was found, due to a combination of two effects. The origin of the flow was found to influence the
direction of the turning over the city. during night-time heat island hours. For easterly flows the
origin is a highly stable rural region, while for westerly flows it is most likely one of the less stable,
but aerodynamically smooth, water bodies around New York City.
INTRODUCTION
(
turn in air passing over a city, thus increasing the
speed of the flow.
Urban areas affect the wind flow pattern and hence - The speed of the undisturbed flow is also important
the transport of contaminants in the atmosphere. Few in determining the magnitude of the urban effect, as
air pollution studies have had sufficient wind observa- a heat island tends to be weak with high wind speeds
tions to establish the time and space variation of the (Bornstein; 1968). For example, Georgii (1968)
transport velocity in and around an urban area.
reported that during clear, calm nights in Frankfurt,
Most investigations into the effects of urban areas urban-induced circulations reached a speed of from
on regional flow patterns have used observations 2-4 mls only when the prevailing regional speed was
from two towers or two'W eather Service stationsless than 3-4 m/s. Chandler (1965) compared wind
one within the city and one at an "airport" site out- speeds from one upwind rural- station and one Lonside of the city. Such studies, e.g., Landsberg (1956), don Ul'ban station, and found that the' nighttime
Frederick (1964), and Graham (1968), for New York urban wind speed was greater than the rural speed
City, Nashville, and Fort Wayne, respectively, have only when the regional speed was less than 5 m/s.
generally shown wind speeds over the urban area to
.This paper investigates the effect of regional wind
be less than those -outside of it. This decrease has
speed, time of day, and regional wind direction on
been a.ttributed to the greater aerodynamic surface changes of wind direction, as well as speed, caused
roughness of a city compared to its surrounding rural by the roughness and warmth of New York City.
environs.
These changes are evaluated at various distances
Chandler (1965), however, demonstrated that the upwind and downwind of the city, using data from
distribution of surface wind speeds in and around a denser network of anemometers than previously
London, England, depends on such factors as upwind available. Some of these results have been previously
rural wind speed, season, and time of day. During presented by Bornstein et al. (1972) and by Johnson
periods with a well-developed "urban heat island" he and Bornstein (1974).
.
found that the surface wind speed over London was
METHOD OF ANALYSIS
actually greater than at nearby rural sites.
There are two ways in which the urban heat island
A mesoscale network of anemometers was estabaffects the wind flow over a city. First, the urban tem- lished in and around New York City as part of the
perature excess creates a convergent "sea breeze" type NYUfNYC Air Pollution Dynamics Project of
circulation cell, with flow near the surface directed
1964-68. This network, described by Davidson (1967),
into the warm city, and with the cell superimposed
Druyan (1968), and Bornstein et al. (1976), consisted
on any existing mean flow. This effect has been
of 97 sites located in a rectangle centered on the west
observed by Okita (1960,1965) and Pooler (1963), in side of mid-town Manhattan. The rectangle was
Asahikawa (Japan) and Louisville, respectively. 220 km long in the E-W direction and 110 km long
Second, the reduced nighttime stability of the urban
in the N-S direction. Data from the network were
atmosphere increases the downward flux of momenused to study the effects of synoptic features on
mesoscale flows (Scudder, 1965) and mesoscale trajec* This work is part of the M.S. Thesis of the second
author.
tories (Druyan, 1968).
597
598
ROBERT D. BORNSTEIN and DOUGLAS SCOTT JOHNSON
The anemometer network consisted of 14 airport
stations (National Weather Service and Federal Aviation Administration); 4 military bases (Air Weather
Service and Naval Weather Service); 10 Coast Guard
bases; 15 utility companies; 14 industrial sites; 29
public agencies and institutions (Public Health, sanitation, schools, etc.); and 11 sites established by New
York University.
Wind data were collected from each of the sites
during twenty observational periods, each three to
five days in duration and randomly selected during
the years from 1964-1967. The data were averaged
over an hour, centered on the hour, except for those
from the airport, military, and Coast Guard stations,
which were standard hourly synoptic observations.
During a shakedown period, wind speeds were corrected to a height of 30.5m by use of power law profiles. However, since the magnitude of the cOITections
was generally less than 1 mps, the procedure was discontinued.
The hourly-averaged wind speed and direction data
were then plotted onto maps, and streamflow and isotach analyses were made (see Fig. 1 for one such
analysis). A streamflow line is here defined to be
everywhere parallel to the flow; however, the spacing
between adjacent lines is not proportional to flow
speed as it is with stream lines. When constructing
these maps over the complex urban teITain, it was
necessary to ensure that the wind at each site was
representative of the flow in its vicinity.
In addition to missing data (about 25% for a given
map), various problems were encountered during
these analyses, e.g.,instrument orientation problems,
local channeling effects, and inCOITectlyplotted directions. A summary of the various problems encoun74"45'
30'
15'
74°00'
tered at each site is given in Bornstein et al. (1976).
Most of the problems were transitory in nature, and
the vast majority of the data could be included in
the analyses, even under low wind speed conditions.
The influence of time of day on the urban-rural
wind velocity was determined by separately considering nighttime (midnight and 2 a.m.) and daytime
(noon and 2 p.m.) observations. At each of these
times, on every day in the study, the streamflow line
passing closest to the U.S. Weather Service station
at Central Park (CP) in Manhattan was determined.
The wind speed and direction at the Park were compared with those at distances of ten and twenty miles
(16 and 32 km) upwind and downwind from the Park
along the streamflow line, if an observation was available within one mile of the specified distance.
There were three daytime and four nighttime convergent wind direction cases, in which no single
streamflow line passed through New York City, as
illustrated by Fig. 1. In these cases the rural wind
speed was always nearly calm, the main circulation
being induced by the urban heat island. Because of
the problem in finding a single streamflow line passing through the city in these cases, they were not
included in the analyses.
URBAN-RURAL
WIND SPEED DIFFERENCES
The Central Park station was initially taken to be
the "most urban" location, and deviations between
the wind speed at the Park and at each of the other
locations along the streamflow line were determined.
The deviations were averaged for the day and night
cases separately, and these
values were
used to com.',
;
45'
30'
. 15'
73°00'
4,°15'
41°15'
4'°00'
41°00'
40°45'
40030'
40°45'
40" 30'
c~\...~
,
"--2.5
AUGUST
0600
40°15'
74°45'
30
15'
74000'
45
30'
15'
7,1964
E.S.T.
40°15'
73°00'
Fig. 1. Analyzed streamflow pattern showing a convergent heat island induced circulation southwest
of Manhattan (shaded area).
Urban-rural
.
10
-------.
DAY
--- ---
J:
a..
::2E
B
~
6
.:::-./"/"\.
20UP
10UP
DISTANCE
FROM
.
CP
10 DOWN
20 DOWN
CENTRAL
PARK (MILES)
Fig. 2. Day and night wind speeds at various distances
from Central Park.
pute the average values for the non-Central Park
locations,
Overall
599
wind velocity differences
Speed Differences
The average distribution of the daytime and nighttime wind speeds across New York City is shown
in Fig. 2. During the day, the average speed decreases
as the wind approaches the city, as might be expected
with increasing surface roughness. The surface roughness parameter increases from as little as 0.01 em over
mudflats to several meters over built up urban areas
(Oke, 1974). However, as the surface roughness is
again reduced downwind of the city, the wind speed
should return to approximately its upstream rural
value. The fact that it does not is due to the afternoon
(easterly) sea breeze, which is opposed to the direction
of the regional flow on about two-thirds of the study
days. Statistical summaries of all of the results in the
current study are given in Johnson (1975).
The peak in the nighttime curve of Fig. 2 is caused
by the nocturnal Urban heat island, the effect of which
can overshadow the effect of the increased aerodynamic roughness of the city. The maximum speed
occurs at ten miles (16 km) downwind, rather than
at Central Park, due to a combination of the large
size of the city and the frequent westerly flow direction. As will be explained below, the maximum speed
should occur near the downwind edge of an urban
area.
Eff~ct of Wind Speed
Landsberg (private communication, 1971) suggested
that there is a critical prevailing regional wind speed
below which wind speed is a maximum over an urban
area, and above which wind speed is minimum over
a city. To test this hypothesis, the magnitude of the
upwind rural speed at which the average deviation
changed sign was determined. This value was
8-9 mph (about 4 m/s) for the nighttime cases and
7-8 mph (about 3 m/s) for the daytime cases. Thus,
the data were separated into high- and low-speed
categories, as shown in Fig. 3, where missing points
contained fewer than five observations.
The high-speed daytime curve parallels the daytime
overall curve because. of the frequency of high daytime wind speeds. Conversely, the daytime wind speed
maximum oyer the city in the low-speed case is due
to accelerations produced by the heat island. In general, the daytime heat island is weak, but it is strongest when the prevailing regional speed is low.
The low-speed nighttime curve of Fig. 3 parallels
the nighttime overall curve because of the high frequency of low nighttime wind speeds. The wind speed
maximum for the nighttime low-speed cases occurs
at the same location as in the low-speed daytime case.
The' high-speed nighttime decrease in speeds over
the city is also similar to that found in the corresponding daytime cases. However, at night this decrease is followed by a sharp increase ten miles
(16 km) downwind of the Park. This difference
between the night and day high-speed cases arises
because the daytime sea breeze is far stronger than
the nighttime land breeze.
The daytime and nighttime low-speed cases were
further divided into two subgroups (0 to 3 and
~ 4 mph, or 0 to 1.4 and ~ 1.8m/s), but only the
nighttime subgroups had enough da,ta to be analysed.
The maximum speed occurs over the city for all three
night low-speed categories (Fig. 4), but the percent
increase at Central Park is greatest for the 0-3 mph
DAY
12
'~.,
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10
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a.
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::>6
4
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::~"
--
.
./...---.
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14
.~"
NIGHT.
12
10
~"--./
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a.
::EB
::>
6
4
:~:?:?:~"
Fig. 3. Day and night wind speeds at various distances
from Central Park, grouped according to upwind rural
wind speed.
600
ROBERT D. BORNSTEIN and DOUGLAS SCOTT JOHNSON
NIGHT
8
/'~
4-8
6
I
0..
:::;:
::>41
2
./'
~~'
?~.
20UP
DISTANCE
.~.'
/
~'~'V'
10UP
\
CP
10 DOWN
20 DOWN
FROM CENTRAL PARK (MilES)
Fig. 4. Night wind speed at various distances from Central
Park during low-speed conditions, grouped according to
upwind rural wind speed.
rural speed: 212% at 20 miles (32 km) upwind and
112% at both 10miles upwind 20 miles downwind.
The corresponding values for the 4-8 mph (1.8 to
3.6 m/s) cases are only 19, 5, and - 3%, respectively,
demonstrating that the urban heat island effect is
most intense when the rural wind speed is very small.
Effect of Wind Direction
tude for the sea breeze. The pronounced maximum'
speed at 10 miles (16 km) downwind for the daytime
low-speed, west wind cases (Fig. 5) is, of course, due
to the daytime urban heat island, while the sea breeze
is responsible for the quick recovery to the upstream
value at 20 miles downwind.
The nighttime east and west overall and low-speed
curves of Fig. 6 show that the maximum speed is
always found at the downwind edge of the city, the
location of which changes with flow direction. The
greater smoothness of the curves of Fig. 6, compared
to those of Fig. 4, is due to the separation of the
data into east and west wind flow cases in the former
figure. The large magnitude of the nighttime urban
maxima, as compared to the daytime urban maxima
of Fig. 5, is due to the stronger nighttime urban heat
island.
The night-time cases do not show any effect of a
land breeze (which would act to cause the upwind
rural speed to be larger than the downwind rural
speed), because the land breeze is far weaker than
the sea breeze. The percent increase at Central Park
for the 0-3 mph (0-1.4 m/s) west wind case is again
extremely large, i.e., 350% at 20 miles (32 km) upwind,
200% at 10 miles upwind, and 99% at 20 miles downwind. The corresponding values for the 4-8 mph
With easterly flow, Central Park is near the downwind edge of the city, but with westerly flow; the edge
"
" "->8 MPH' WEST
12
of the city is about ten miles (16 km) downwind of
';.
Central Park, i.e., in Brooklyn or Queens. The effect
of wind urban-rural wind speed differences has been
;",.,
Ii >.~.,. --.
,. :--',0.'
---.......
evaluated by dividing the wind data into westerly and
10
......
.,.< ,.
.
'.
easterly flow cases:Westerly.cases are those for which
the wind direction at the first upwind rural station
~,,"
.":; .~ .,:",
:t:
was between 181 and 360°, and easterly cases correa. 8
.:
::;:
~
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spond to directions between 1 and 180°. These data
::>
were used to construct the curves in Fig. 5 and 6,
'
in which categories having fewer than five compari/"
"
6
sons for two or more locations are again omitted.
/'
"
/
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The daytime east and west wind curves of Fig. 5
/
'\
differ markedly, because the sea breeze tends to re//\
4
inforce easterly flows. Thus, the daytime east wind
0-7
./
cases include mostly highspeed cases. The wind speed
decreases over the city in the two daytime easterly
EAST
13 . - -,,>8 MPH
flow curves because of the increased roughness over
the city, and the lack of penetration of the sea breeze
flow. Since the sea breeze rarely reaches areas down:t:
wind of the city at these times, the flow does not
a.
:::;:
completely recover. to the large upwind rural values.
:5
\
/
However, with a strong westerly flow, the opposing
sea breeze prevents the speed at 20 miles (32 km)
9
downwind from returning to the upwind rural value.
In fact, the speed at 20 miles downwind during these
periods is actually slightly less than at 10 miles downwind.
The effect of the sea breeze is also demonstrated
by the fact that the speed decrease during high speeds Fig. 5. Day wind speed at various distances from Central
for easterly winds is double that for westerly winds. Park for west and east wind cases, grouped according to
upwind rural wind speed.
The difference implies a 2-3 mph (about 1 m/s) magni-
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,
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.----
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"
.~.
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~'\
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:\,
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~:V
Urban-rural
WEST
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0
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0"'"
0-8 MPH
54
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6 °
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.---
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ALL
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"o~
0-;,0
/,"&-8 MPH
0"
20 UP
10UP
"'<~:
CP
10DOWN 20 DOWN
DISTANCE FROM CENTRAL
PARK (MilES)
Fig. 6. Night wind speed at various distances
Park for west and east wind cases, grouped
upwind rural wind speed.
from Central
according to
(1.8-3.6 m/s) case are only 20, 11, and 4%, respectively.
URBAN-RURAL
WIND DIRECTION
to increase with increasing drag (Blackadar, 1962).
The southeast to northwest directed sea breeze, found
downwind of the city (for the westerly flows making
up two-thirds of the present cases), would explain the
increased cyclonic turning seen in the figure at that
location. In contrast, Angell et ai. (1972), found that
downstream of Los Angeles, where the sea breeze
effect is generally in the same, direction as the mean
flow, daytime flow directions quickly readjust.
Effect of Wind Speed
\
\
0/
601
wind velocity differences
DIFFERENCE
The deviation of wind direction' 8.Iong each available streamflow line was determined relative to the
wind direction at the first upwind rural site (either
10 or 20 miles (16 or 32 km) upwind). A positive deviation indicates anticyclonic turning, while a negative
value indicates cyclonic turning. The average deviation at each point downwind (of 20 miles upwind
from Central Park) was computed separately for day
and night.
The average direction deviations were corrected for
those cases in which the first upwind rural observation was at 10miles (16 km) upwind instead of
20 miles upwind. The average turning between these
two distances was computed (if five or more comparisons were available), and then used to recompute new
values of the average deviations along those streamflow lines not originating at 20 miles upwind.
Direction changes for the high- and low-speed
upwind rural subgroups are plotted in Fig. 8. Similar
amounts of frictionally induced cyclonic turning are
shown for the first 40 miles of the low- and high-speed
daytime cases. However, while the high-speed case
shows significant additional cyclonic turning at
20 miles (32km) downwind, the low speed curve does
not. For the predominantly westerly flow cases, this
turning is due to a passage through the sea breeze
front (into the marine air). Why this pattern does not
appear in the low-speed cases is unclear, except that
perhaps the value at 20 miles do~wind is based on
,
only five comparisons.
The low night-time speed curve of Fig. 8 shows
that the air turns anticyclonically as it passes from
the very stable rural regions into the less stable urban
atmosphere. This is consistent with the idea that
cross-isobaric flow angles are reduced as stability is
decreased(Yamamoto et ai., 1968):In the high-speed
case, the increased friction again causes a turning
toward lower 'pressure over the city. However, since
the turning at 20 miles (321m) downwirid is less cyclonic than that at 10 miles downwind, recovery to the
upwind flow direction has l;Iegun.
,'U
Effect 'of Wind Direction
The high- and low-speed wind data were further
separated into cases with easterly or westerly flow,
as was done for the wind speed data. The daytime
east and west wind direction curves (Fig. 9) are similar to the curve for the overall daytime data (Fig.
24
20
/
16
"
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~ 12
oS
~
z
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t;;
4
, Fig.7, Corn" ve"io~
p'p",on","od
wm"" ve"ioo,
..
Overall Direction Differences
The overall direction differences are summarized in
Fig. 7 (in which the wind direction is that relative
to 20 miles (32 km) upwind). There is almost no turning over the city for the average night-time case,
although there is a small amount of anticyclonic turning downwind of the city. The cyclonic turning over
the city for the daytime cases is due to the increased
frictional drag associated with the increased urban
surface roughness, as cross-isobar flow angle tends
15
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.. 0
.. origi",1
/
........
----
,""'-
NIGHT
~
DAY
/
I
/
,,'
""
,/'
//
/
ffi
4
....
..
...J 0
"4
~
--- -----
20 UP
10 UP
CP
10 DOWN 20 DOWN
DISTANCE FROM CENTRAL PARK (mile.)
Fig. 7. Day and night streamflow lines through New York
City.
602
ROBERT D. BORNSTEIN and DOUGLAS SCOTT JOHNSON
28
DAY
:;
I
24
I
,,
20
16
12
/
8
'E
~0
/
/
Z
.J
NIGHT
,
---
--
-----
~
LOW
'
,
,,~
,
:.
,
-,-,
.
20 UP
10 UP
CP
fO DOWN 20 DOWN
DISTANCE FROM CENTRAL PARK (miles)
.J
--""---'-
20 UP 10 UP
CP
10DOWN 20 DOWN
DISTANCEFROMCENTRAL PARK (miles)
Fig. 8. Day and night streamflow lines through New York
City, grouped according to upwind rural wind speed.
7), as both showing cyclonic turning. The sea breeze
penetration is seen as a 40° cyclonic shift at 1Omiles
(16 km) upwind in the east wind data, while in the
west wind case it appears as a 30° cyclonic shift at
10 miles downwind.
For the night-time flows of Fig. 9, only the west
wind case shows the expected anticyclonic turning
over the city consistent with a decrease in atmospheric stability. The east wind case of Fig. 9 shows
cyclonic turning consistent with an increase in surface
roughness. The difference between the two curves is
due to dissimilarities in the air entering New York
City from east and west directions. In the former,
the air is highly stable, as it is coming from a rural
DAY
///"
24
_20
';16
]12
UJ
/
,//
//
/
Fig. 10. Night streamflow lines for westerly and easterly
flows grouped according to upwind rural wind speed.
region. In the latter, it is most likely to be less stable
than that from the east, as it originates over one of
the water bodies around New York City. Hence, for
westerly flow the stability difference between the origin and the urban sites is great, while for easterly
flow the roughness difference between the origin and
the urban sites is great. The near,.zero average turning
of the night-time curve of Fig. 7 resulted from averaging the anticyclonic turning of th'e west wind cases
and the cyclonic turning of the east wind cases.
'The east and west wind direction data were separated into high- and low-speed wind subgroups. The
night-time west.wind, high-speed curve jnFig. 10
shows the expect~d frictionallyjnduced cyclonic turning, while the. single easterly, high-speed. qase is not
shown. The night-time west' wind, ,low-speed curve
shows the expected stability induced anticyclonic
turning over the city, and the corresponding east wind
curve shows the expected roughness induced cyclonic
turning. The magnitude of the night-time west wind,
low-speed anticyclonic turning is greater than that for
the overall west wind curve in Fig. 9, as the highspeed cases with their cyclonic turning have been
/
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20 UP
DISTANCE
,
,
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NIGHT """""'---
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20 UP 10 UP
CP
10DOWN 20 DOWN
DISTANCEFROMCENTRALPARK(miles)
15
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Iii
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HIGH
g 0'--
//-.,;/
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WEST
8
4
4
0
,
10 DOWN 20 DOWN
FROMCENTRAL PARK (miles)
Fig. 9. Day and night streamflow lines through New York
City during east and west wind cases.
/
/
//
HIGH
.-/ ---
---~--
--
LOW
---~
20 UP
I
I
I
-........
10UP
CP
10DOWN 20 DOWN
DISTANCEFROM CENTRALPARK (miles)
Fig. 11. Day streamflow lines for westerly flow, grouped
according to upwind rural wind speed.
Urban-rural
,603
wind velocity differences
removed. In addition, the magnitude of the night-time
east wind, low-speed cyclonic turning is less than the
cyclonic turning of its overall curve (Fig. 9), as its
high-speed case had extremely large cyclonic turning.
The daytime east wind cases did not have enough
data to subdivide into high- and low-speed cases, but
the daytime west wind, high-speed curve of Fig. 11
shows the expected roughness induced cyclonic turning over the city, followed by extreme cyclonic turning as the flow encounters the sea breeze at a point
10 miles (16 km) east of Central Park. The daytime
west wind, low-speed curve turns cyclonically over
the city, as have all previous daytime curves (Figs.
8 and 9).
SUMMARY
Analyses of surface wind speed data obtained from
the New York University mesoscale anemometer
network for the metropolitan New York City area
showed that urban-rural surface wind velocity differences between the city and surrounding non-urban
areas depend on time of day, wind direction, and
upwind rural wind speed. During both daytime and
night-time hours there exists a critical upwind rural
wind speed below (above) which air is accelerated
(decelerated) as it flows over the rough, warm city.
The acceleration is due to horizontal pressure gradients directed inward to the center of a city, and
to the decreased stability of the urban atmosphere.
Both of these effects are associated with a well-developed urban heat island. The deceleration, on the
other hand, is due to the large value of the urban
surface roughness parameter, as compared to that of
the surrounding non-urban regions.
The critical speed was found to be 8 mph (3.6m/s)
for daytime and 9 mph (4.1m/s) for night-time, in
agreement with results obtained by Chandler (1965)
for London. The results also agree with those predicted by the numerical heat island model of Bornstein (1975) in that the fastest winds over New York
City were found at its downwind edge. The effect of
the daytime easterly sea breeze was demonstrated by
the low-speed winds downwind of New York City
with prevailing westerly flow, and by the high-speed
winds upwind of the city with prevailing easterly flow.
Wind direction is also affected by the city. When
air decelerated as it flowed over the city, it generally
turned cyclonically. The wind direction is also
affected by the sea breeze, which caused daytime cyclonic turning downwind of the city with westerly
flows and upwind of the city with easterly flows.
During periods of acceleration over the city, either
cyclonic or anticyclonic turning was found, due to
a combination of .two effects. The origin of the flow
was found to influence the direction of the turning
over the city during night-time heat island hours. For
easterly flows the origin is a highly stable rural region,
while for westerly flows it is most likely one of the
less stable, but generally very aerodynamically
smooth water bodies around New York City.
For westerly flows the stability difference between
the origin of the flow and the city was great, and
the flow turned anticyclonically over the city, i.e., the
cross-isobaric flow angles decreased in the less stable
air. While for the easterly flows, the roughness difference between the origin and the urban site was
greater, and the flow turned cyclonically over the city.
However, for daytime periods' of acceleration the flow
only turned cyclonically, as the thermal stability-difference effects is small during such periods.
It should be noted that the stability-difference argument presented above is only completely valid for an
infinitely-wide city, with flow perpendicular to its
boundaries. This model corresponds to the solution
obtained from a two-dimensional, vertical plane,
urban boundary layer model, such as that of Born. stein (1975). For a city of finite size, the surface crossisobaric flow angle would also be affected by convergence into the warm city. This pattern is consistent
with that of three-dimensional urban heat island, as
illustrated by the wind tunnel measurements of SethuRaman and Cermak (1974). Depending on the wind
direction, and on whether the flow originates above
or below the center of the heat island, either cyclonic
or anticyclonic turning may occur over the city.
The present study is based on changes along a
single streamflow line, so nothing can be said about
the effect of stream line convergence, even though the
results of SethuRaman and Cermak (1974) show that
this effect should be present downwind of the urban
center and not over its center, due to the effects of
advection. Additional observational studies are
needed to determine the conditions under which convergence becomes significant. It is interesting to note,
however, that the region of convergence in Fig. 1 is
found downwind of the warmest section of New York
City, i.e., downwind of Manhattan Island.
The predicted trajectory and size of a puff of pollution passing over New York City were found by
Johnson and Bornstein (1974) to be greatly affected
by the varying wind speed, direction, and stability
induced by the city. Thus, an urban air pollution
transport and diffusion model which attempts to compute concentration distribution, based solely on the
standard meteorological observations taken as a
single U.S. Weather Service "airport" station located
outside of an urban area, will produce erroneous
results.
.
Acknowledgements~ This work was supported by NSF
Grant GA-41886. The preliminary draft was typed by
Susan Johnson, while the final draft was typed by Linda
F. LaDuca. Professor Al Miller reviewed preliminary versions of the final manuscript. Final preparation' of the
figures used in this report was carried out at Brookhaven
National Laboratory.
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