Download Case Study of Urban Interactions with a Synoptic Scale Cold Front

Meteorology,
Meteorol. Atmos. Phys. 38, 185-194 (1988)
and Atmospheric
Physics
cg by Springer-Verlag 1988
551.588.7:
551.515.1
Department of Meteorology, San Jose State University, San Jose, California, U.S.A.
Case Study of Urban Interactions with a Synoptic Scale Cold Front
D. Gaffen* and R. D. Bornstein
With 7 Figures
Received January 20, 1987
Summary
A two-day test period (10-11 March 1966) during which a
slow moving synoptic cold front passed over New York City
has been analyzed. As the front passed over the city it was
destroyed near the surface by the urban roughness effect. The.
upper segment of the front passed over the city and rejoined
the rest of the front downwind of the city. Evidence for the
above phenomenon was seen in both the pibal wind and
helicopter (temperature and SOz) soundings.
Distortion of the front forced polluted surface urban air
upwards, as within the warm sector of an occluding synoptic
frontal system. The outward spreading of this air aloft produced increasing concentrations over the city at heights above
200m.
1. Introduction
The research of Helmut Landsberg provides major
contributions in many areas of climatology; one
of the most important is in the field of urban
climate. Most of the urban climate studies discussed in his comprehensive book The Urban Climate (Landsberg, 1981) were comparisons of concurrent climatic elements at sites within a city with
those at locations in its surrounding environs.
The problem with such comparisons is that it
is hard to estimate the magnitude of the microclimatic differences that would exist between the
two sites in the absence of the urban area. Prof.
Landsberg was able to avoid this problem in his
* Current Affiliation: National Oceanic and Atmospheric
Administration, Rockville, MD 20852, U.S.A.
unique study of the climate effects of urbanization
resulting from construction (from scratch) of the
planned community of Columbia, Maryland
(Landsberg, 1979).
The work reported in this paper discusses the
retarding effects on the movement of a synoptic
scale cold front caused by the increased surface
roughness of New York City (NYC). When it was
suggested to Prof. Landsberg that this was the
first time that such an effect had been observed,
he thought for a very short time before remembering a Ph.D. Thesis written some 50 years before, when he was a student in Germany. That
study had used data from a precipitation network
to plot isochrones of precipitation initiation during a synoptic frontal passage over an urban area.
When the results showed a later starting time over
the city center, the student concluded that the increased urban surface roughness had retarded the
frontal movement over the city. The conclusion
from this is that, at least when dealing with Prof.
Landsberg, there are very few new and original
research ideas.
2. Background
Surface urban heat islands induce updrafts over
a city, which in otherwise calm conditions produce
convergent country winds. Observations of complete circulation cells (including diverging winds
186
D. Gaffen and R. D. Bornstein
aloft and downward motion outside the city) are
rare, because such a circulation may resemble "the
motions of an amorphous, slowly-pulsating jellyfish" (Munn, 1970).
With higher general wind speeds, heat island
formation is inhibited, and heat island induced
flows less likely to be observed (Bornstein and
Johnson, 1977). However, cities can also modify
mesoscale and synoptic scale flow patterns, e.g.,
Bornstein and Johnson showed surface winds decelerated during non-heat island periods in NYC
due (to the large urban surface roughness) and
accelerated during low speed heat island periods.
Sea breeze fronts have also been observed to
decelerate at the surface as they move over urban
areas, e.g., in Boston (Barbato, 1978) and NYC
(Bornstein etaI., 1988). The NYC deceleration
produced a steepening of frontal slopes over the
city.
Likewise, Loose and Bornstein (1977) found
surface synoptic frontal movement over NYC retarded due to friction during non-heat island periods. However, during heat island periods frontal
movement was retarded over the upwind half of
the city and accelerated by the heat island over its
downwind half.
Lower pollutant concentrations generally accompany onshore sea breeze winds (Harrison and
McCartney, 1980), but sea breezes can increase
concentrations when they return pollutants carried offshore by the previous nocturnal land breeze
(Shair et aI., 1982). The NYC sea breeze brings
decreasing surface sulfur dioxide concentrations
to coastal stations in NYC and increasing concentrations to those further inland (Bornstein and
Thompson, 1981). The decreasing concentrations
result from initial onshore advection by the sea
breeze. When the front stalls over the city due to
its increased surface friction, the marine air becomes polluted. Its eventual advection inland
causes increasing concentrations. These changes
were found to extend several hundred meters into
the urban boundary layer by Gaffen and Bornstein
(1988).
Synoptic frontal passages also influence urban
pollutant concentration patterns, e.g., low wind
speeds and shallow mixing depths associated with
a quasi-stationary front caused high pollutant
concentrations in NYC in January 1971 (Nudelman and Frizzola, 1974). Concentrations rapidly
decreased when the front finally passed through
the area. In general, cold frontal passages through
NYC are associated with decreases in surface concentrations in Manhattan and increases to the east
and west (Bornstein and Thompson, 1981). However, the one highly distorted front in the study
caused increased surface concentrations in Manhattan, suggesting that it never truly passed
through the city at the surface.
Previous studies have either investigated surface sea breeze or synoptic frontal movement over
NYC, or have analyzed the response of the polluted PBL over NYC to sea breeze frontal passage.
The current study will analyze the response of the
polluted PBL over NYC to a slowmoving synoptic
cold front passage.
3. Data Base and Analysis
The New York University (NYU) Urban Air Pollution Dynamics Project of 1964-68 (Davidson,
1967; Bomstein et aI., 1977a, b) collected surface
and boundary layer wind, temperature, and SO2
concentration data in the NYC area. The data
have been used in previous observational and
modeling studies of various aspects of the polluted
urban coastal boundary layer over NYC, e.g., heat
islands (Bomstein, 1968), a two-dimensional planetary boundary layer (PBL) model (Bprnstein,
1975), surface winds (Bornstein and Johnson,
1977), surface synoptic fronts (Loose and Bomstein, 1977), surface SO2 concentrations (Bomstein and Thompson, 1981), atmospheric moisture
(Clark et aI., 1985), three-dimensional polluted
boundary layer model (Bomstein et aI., 1985a, b;
Pechinger et aI., 1985), sea breeze frontal movement (Bornstein et aI., 1988), and vertical structure
of a polluted PBL during a sea breeze frontal
passage (Gaffen and Bomstein, 1988). The current
study uses the data set to investigate effects of the
heat island and a synoptic cold front on the structure of the polluted coastal urban boundary layer
over NYC during 10-11 March 1966.
The period was selected for analysis because of
its interesting meteorological conditions and high
data density. Mesoscale streamline analysis were
constructed for the area around NYC using hourly
average wind data from a 97-site anemometer network. The synoptic cold front, observed on the
11th, was identified as a wind shift area in the
analyses; frontal zones were depicted by lines representing the best estimate of their leading edge.
187
Case Study of Urban Interactions with a Synoptic Scale Cold Front
74° 15'
74°00'
73°45'
41°00'
NEW
JERSEY
40°45 '
40°45 '
QUEENS
40° 30'
..
0
Helicopter
Pibal
74°15'
Frontal advance speeds were computed from measured distances travelled by the front at various
locations during consecutive hourly periods. Details of the data and analysis can be found in Loose
and Bornstein (1977).
About 80 single theodolite piballaunches were
made during 10 and 11 March at two sites, i.e.,
Research Building 4 (RB 4) and NYC (Fig. 1).
Wind speed and direction values were obtained at
37.5 m intervals in the lowest 1000 m of the atmosphere (assuming a constant rate of balloon
rise of 150 ill min ~ 1). Values were smoothed using
a 3-point equally weighted running mean.
Speed and direction height-time sections were
constructed for both days at both pibal launch
sites. Unreasonably high wind speed values
(> 25 m s I) were assumed associated with erroneous observations. Isopleth smoothing removed
high frequency disturbances, i.e., those with time
~
Soundings
Sites
NWS Sites
79th
Fig. 1. New York City area and data
collection sites
Launch
Street
74°00 '
73°45
'
scales less than 2 hr and height scales less than
200 m. Fronts were identified as regions of maximum directional shear and minimum wind
speeds. Details of this analysis can be found in
Gaffen (1984).
Temperature and SO2concentrations from near
the surface to about 1000 m were obtained from
about 50 urban helicopter soundings during lOII March (Fig. 1 and Table 1). Climbing traverses
and spiral descents (at approximately 35m s - I)
were made at sunrise, sunset, and/or midday. The
semiconductor temperature sensor had a 0.2°C
relative accuracy, 0.5°C absolute accuracy, and
0.2 s time constant. The pressure-height unit, two
aneroid cells driving a potentiometer, had a time
constant less than 0.05 ms and an accuracy of
about 10m. The electro conductivity SO2unit, calibrated to negate interference from abient CO2,
had a 30 s time constant and probable accuracy
188
D. Gaffen and R. D. Bornstein
Table 1. Data Sites Shown in Fig. 1
Name
Indicator
National Weather Service Sites
Ambrose Lightship
Central Park
AL
CP
Pibal Launch Sites
NYU Tech.
Research Building 4
NYU
RB4
Helicopter Sounding Sites
Battery Park
Cypress Hills Reservoir
Cloisters
Eastchester Bay
Ellis Island
East River Park
Fort Washington Park
Governor's Island
Gravesend Bay
Narrows
Narrows Bridge
Newton Creek
Orchard Beach
Statue of Liberty
Van Cortlandt Park
Williams berg Bridge
40th Street and New Jersey
40th Street
155th Street
BAT
CHR
CLO
ECB
ElL
ERP*
FWP
Gm
GVB
NAW
NBG
NCK
OBC
SOL
VCP
WBB*
40NJ
40th
155
* Almost coincident.
of 10-30%. Criteria for smoothing sounding data
are described in Bornstein et al. (1977 a).
Surface SO2 concentration were measured by
electro conductivity and wet chemistry devices at
fixed sites and mobile laboratories. Effects offrontal passage on PBL concentration profiles were
determined bvy computing concentration difference profiles, L1S( z), using pre- and post-frontal
helicopter soundings at the same sites. Frontal
passage time at an individual sounding site was
determined from sea breeze surface frontal positions.
Vertically averaged concentration changes were
obtained by height averaging each L1S(z) curve.
Spatial distributions of these vertically averaged
values were then compared to similar distributions
of surface values for approximately the same locations and time periods obtained by Bornstein
and Thompson (1981).
Frontal slopes over NYC were calculated (using
helicopter temperature soundings, pibal winds,
and/or previously analyzed surface frontal positions) by three techniques. In the two techniques
involving helicopter sounding data, fronts were
defined by an elevated inversion base and/or a
sharp change in SO2concentration. The SO2 sensor has a greater time constant than the temperature sensor, so temperature took precedence
when the two positions did not coincide.
The first of the two helicopter-sounding based
techniques for determining frontal slope used
frontal height aloft values versus perpendicular
distance from the helicopter sounding site to the
interpolated surface frontal position. The slope of
the best fit (linear regression) line through all the
height aloft data points was taken as the frontal
slope. Thus this first technique produces one
space-averaged frontal slope value from a series
of single frontal height values at various sites. With
two inversions at a single site, an average value
was used.
The second of the two methods for computing
frontal slope using helicopter sounding data involves two concurrent frontal positions, one aloft
and one at the surface (an interpolated position
from the frontal isochrone analyses). The slope
was then defined as the ratio of the frontal height
aloft to the perpendicular horizontal distance between the site and the surface frontal position.
Thus this technique yields one slope estimate per
site at a given sounding time.
The third (and final) technique determined
frontal slope at each of the two sites for which
pibal wind time sections were constructed. Frontal
position at various times (and therefore at different heights at the same site) were assumed to correspond to areas of wind speed minima and directional shear maxima. Slopes (dz/ds) were computed by dividing the slope of the line connecting
the frontal positions at the site (dz/dt) at different
times by the average wind speed at the front (ds/
dt), assuming equality of the frontal advance and
atmospheric wind speeds. Because of this assumption, frontal slope estimates obtained at the
two pibal sites by this technique are the least accurate.
4. Results
Following three days of fair weather, a weak cold
front entered the test area on 11 March 1966
(Fig. 2). Mesoscale surface frontal positions
"
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Fig. 2. Anemometer network (dots), and surface streamline and isotach analysis (mph) for 0800 EST on II March 1966 (from Loose and Bornstein, 1977)
...00
\D
190
D. Gaffen and R. D. Bornstein
0'100
0500
cwo
0800
0900
1000
1100
1100
11.D0
1100
'~OO
Fig. 3. Synoptic surface frontal isochrones (EST) on 11 March 1966 (from Loose and Bornstein, 1977)
(Fig. 3) from Loose and Bornstein (1977) show a
frontal movement of 100km in 11hr, yielding an
average frontal advance speed of only 2.5m s - 1.
This slow frontal movement, and the light and
variable post-frontal surface winds, allowed the
large urban surface roughness to retard the movement of the surface front over the most built-up
area of the city, i.e., Manhattan. Thus by 0800
EST the surface frontal shape was greatly distorted, as its speed of movement over NYC was
nearly zero.
Urban surface roughness also affected vertical
frontal structure; analyses of the pibal wind and
helicopter soundings suggest that the surface front
never passed Manhattan. The upper segment of
the front over the city was "torn off' from its
surface section. The upper segment then "rode
over" the city, subsequently (by 0900 EST) rejoining the surface front segments that had continued moving southward in the area both east
and west of Manhattan. The surface frontal frag-
ment which remained stagnated and looped
around Manhattan eventually dissipated. A similar effect at mountain ranges was first described
by Bjerknes and Solberg (1921).
Evidence for the above phenomenon is seen in
both the pibal wind and helicopter soundings. Pibal wind patterns on the non-frontal day are considerably less complicated than on the frontal day
and illustrate general PBL flow characteristics
(Gaffen, 1984). On the non-frontal day winds at
both pibal sites veered with height in accordance
with Ekman theory. Nocturnal jets formed at both
sites within the hours of midnight at an average
height of 500 m on each test day (Figs. 4 and 5).
Maximum speedsvaried between 6 and 16m s - I,
and averaged II m S-I.
Frontal passage on 11 March complicated this
pattern. At NYU (Fig. 4), frontal passage caused
winds to veer from westerly to northerly between
0900 EST (at 350m) and 1100 EST (at 750m).
Thus frontal passage at these levels occurred 2.5
191
Case Study of Urban Interactions with a Synoptic Scale Cold Front
NYU
11
MARCH
NYU
DIRECTION
1966
1000
1000
14
11 MARCH
SPEED
1966
2 8 6 4
1 10'
\, ,I ,I
16
.I,
I I I
II,
I I I
I I I
,I II ,,
750
,I,
III,,, ,
27
:J
en
::?;
:J
en
::?;
~
2
/'
I
2
I
:.€
Q)
Q)
-:;;500
.5
.s
~
~co
2
-:;;500
co
"ijj
"ijj
:J:
:I:
250
n
{34
2
12
Time
18
24
(LST)
00
06
2
:2 2
12
Time
18
24
(LST)
Fig. 4. Wind speed (m S-l) and direction (tens of degrees) time sections at NYU for 11 March 1966. Pibal soundings (solid
arrowheads) and surface frontal passage (open arrowhead) are shown
and 4.5 hr, respectively, after the 0630 EST surface
frontal passage in this area (Fig. 3).
At RB 4, near surface (pibal) wind speeds were
less than 2 m s- 1 as the surface front passed at
0800 EST (Fig. 5). Passage at the 350 m level occurred only about an hour later, indicating a
steeper slope at RB 4 than at the more upwind
NYU site.
Upper level progression of the frontal towards
the urban center, in conjunction with its surface
stagnation, first produced a totally vertical slope
and then a negative one. In such a situation cold
air overlays warm air, resulting in vertical mixing.
Singular points, with 1800 direction shifts in
association with zero wind speeds, have been associated with vertical mixing (Ackerman, 1974).
Such points do appear in the post-frontal isogon
analysis at RB 4 (Fig. 5) due to increased convection associated with the frontal "tear". The eight
hour post-frontal pibal data gap due to precipitation at RB 4 makes this interpretation somewhat
tenuous; nevertheless, rain fell earlier and for a
longer period at RB 4 than at NYU, suggesting
greater convective activity at the former site.
Helicopter SO2 soundings provide further evidence for this near surface frontal "tear" and reformation. Frontal passage brought either decreasing or slightly increasing SO2concentrations
below 200 m (Fig. 6) at all helicopter sounding
sites, except GID where they increased, depending
on the degree of urbanization under the surface
trajectory advecting post-frontal air to a particular
site (Fig. 2). This pattern is consistent with the
concurrent surface SO2 analysis of Bornstein and
Thompson (1981). They found surface concentration decreases over most of the area, and increases
only in a small area centered at GID and parallel
to (and within) the looped segment of 0800 EST
surface frontal position (Figs.2 and 3). The air
within this loop was not replaced by post-frontal
air as the surface front never passed through that
area.
This analysis also explains the concentration
increases above 200 m at all sounding sites (Fig. 4).
~
192
D. Gaffen and R. D. Bornstein
RS 4
11 MARCH 1966
RS 4
DIRECTION
30
11 MARCH 1966
SPEED
1000
1000
27
4
4
:J
:J
en
(f)
:::;:
:::;:
~
~
Q)
Q; 500
Q)
Q; 500
S
S
.s::
'"
'0;
I
.s::
'"
'0;
I
12
09
250
6
06
03
12
Time
18
24
00
(LST)
06
12
18
24
Time (LST)
Fig.S. Wind speed (m-I) and direction (tens of degrees) time sections at RB4 for 11 March 1966. Pibal soundings (solid
arrowheads) and surface frontal passage (open arrowhead) are shown
800
600
E
~
N
-ElL
---FWP
-GID
--155
400
200
0
-0.2
-0 I
0.4
0
~s (ppm)
Fig. 6. Change with time in SO2 concentration through PBL
due to frontal passage on 11 March 1966
As the surface frontal loop was closing inward,
polluted surface urban air was forced upwards, as
within the warm sector of an occluding synoptic
frontal system. The outward spreading of this air
aloft produced increasing concentrations aloft at
all sites (within and outside of the loop).
Frontal slope estimates provide yet further evidence for the steepening and destruction of the
near surface front. Surface frontal positions and
frontal heights aloft (second technique described
in Part 3) combined to yield slope estimates between - 1 : 1and 1 : 110(Table 2). Extremely steep
slopes of l: 7 and l: 2 were computed for the
earliest soundings (about 0700 EST), when frontal
height above these stations was low. By 0752 EST
(at NCK), the cold air had overrun the warm air,
producing a negative slope. After 0905 EST, the
frontal slope had an average value of 1 : 73. Thus
the re-formed near surface front was significantly
less steep than before its tear.
Linear regression, using helicopter sounding
data at about 1000 EST (first technique in Part
2), yielded a slope of 1 : 60 in the layer between
250 and 700 m, with a correlation coefficient of
-
0.90 (Fig. 7). Pibal wind data (third technique)
yielded frontal slopes of 1 : 53 and 1 : 60 at RB 4
and NYU, respectively in the 350 to 700 m layer,
in good agreement with both the linear regression
result and the near surface slopes of the re-formed
front. The nearly uniform slope of the re-formed
.,...
,'«
Case Study of Urban Interactions with a Synoptic Scale Cold Front
Table 2. Frontal Slopes on 11 March 1966
Site
Time
(EST)
Height
(km)
Distance
(km)
Frontal
Slope
155
FWP
NCK
155
FWP
40th
ElL
GID
CHR
CHR
FWP
0707
0711
0752
0905
0910
0917
0930
0935
0953
0959
1053
0.19
0.20
0.40
0.50
0.47
0.43
0.27
0.34
0.24
0.32
0.61
1.2
0.5
-0.5
29.8
31.5
23.8
19.5
18.3
26.5
27.8
48.3
I: 7
1: 2
-1: 1
I: 60
1:67
1: 55
1: 72
I: 54
1: 110
1: 88
1: 79
800
0
0155
Single
l>
Multiple
0
Average
600
l>
-5
193
steepened the frontal slope until a negative slope
resulted when the cold air over-rode the warm air.
Then the upper segment of the front over the city
was "torn off" from its surface section and "rode
over the city". This section finally rejoined the
surface frontal segments that had continued moving southward in the areas both east and west of
Manhattan.
Evidence for the above phenomenon was seen
in both pibal wind and helicopter (temperature
and SO2)soundings. The helicopter SO2soundings
showed either decreasing or slightly increasing SO2
concentrations below 200 m at all sites, except the
one within the looped segment of the front remaining around the southern half of Manhattan.
Surface concentrations within the loop did not
decrease, as the surface front never passed through
that area and this air was not replaced by postfrontal air. In addition, as the surface frontal loop
was closing inward, polluted surface urban air was
forced upwards, as within the warm sector of an
occluding synoptic frontal system. The outward
spreading of this air aloft produced increasing concentrations at heights above 200 m at all sites.
l-
I
400
Acknowledgements
(!)
W
I
l>
The authors would like to thank John Chamberlain and Nick
Powell for their analyses of wind time sections, Ms. Donna
Hurth for typing the manuscript, and Lynn Baxter-Poh1 for
drafting the figures.
200
References
0
0
10
20
30
40
DISTANCE (km)
Fig. 7. Frontal inversion heights versus horizontal distance
at about 1000 EST on 11 March 1966. Regression line is
through single inversions heights (circles) and average
(squares) of twin inversions (triangles)
front near the surface and aloft indicates that frictional effects are less important in the downwind
non-urban area.
5. Conclusion
A weak, slow moving synoptic scale cold front
was highly distorted in three-dimensions as it
passed through the PBL over NYC. Urban surface
friction greatly distorted the surface frontal shape
by reducing its speed of movement over Manhattan to a nearly zero value. The roughness effect
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D. Gaffen and R. D. Bornstein: Case Study of Urban Interactions
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Authors' addresses: Ms. Dian Gaffen, National Oceanic and
Atmospheric Admini~tration, R/PDC, 6010 Executive Blud.,
Rockville, MD 20857, U.S.A., and Prof. Robert D. Bornstein,
Department of Meteorology, School of Science, San Jose
State University, San Jose, California, U.S.A.
t.
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