Boundary-Layer MeteorolDOI 10.1007/s10546-017-0289-3

RESEARCH ARTICLE

Observations of Lake-Breeze Events During the Toronto2015 Pan-American Games

Zen Mariani1 · Armin Dehghan1 ·Paul Joe1 · David Sills1

Received: 20 March 2017 / Accepted: 1 August 2017© Springer Science+Business Media B.V. 2017

Abstract Enhanced meteorological observations were made during the 2015 Pan and Para-pan American Games in Toronto in order to measure the vertical and horizontal structureof lake-breeze events. Two scanning Doppler lidars (one fixed and one mobile), a C-bandradar, and a network including 53 surface meteorological stations (mesonet) provided pres-sure, temperature, humidity, and wind speed and direction measurements over Lake Ontarioand urban areas. These observations captured the full evolution (prior, during, and after)of 27 lake-breeze events (73% of observation days) in order to characterize the convectiveand dynamic processes driving lake breezes at the local scale and mesoscale. The dominantsignal of a passing lake-breeze front (LBF) was an increase in dew-point temperature of2.3±0.3 ◦C, coinciding with a 180◦ shift in wind direction and a decrease in air temperatureof 2.1 ± 0.2 ◦C. Doppler lidar observations over the lake detected lake breezes 1 hour (onaverage) before detection by radar and mesonet. On days with the synoptic flow in the off-shore direction, the lidars observedwedge-shaped LBFswith shallow depths, which inhibitedthe radar’s ability to detect the lake breeze. The LBF’s ground speed and inland penetrationdistance were found to bewell-correlated (r = 0.78), with larger inland penetration distancesoccurring on days with non-opposing (non-offshore) synoptic flow. The observed enhancedvertical motion (>1 m s−1) at the LBF, observed by the lidar on 54% of lake-breeze days,was greater (at times >2.5 m s−1) than that observed in previous studies and longer-lastingover the lake than over land. The weaker and less pronounced lake-breeze structure over land

B Zen Marianizen.mariani@canada.ca

Armin Dehghanarmin.dehghan@canada.ca

Paul Joepaul.joe@canada.ca

David Sillsdavid.sills@canada.ca

1 Cloud Physics and Severe Weather Research Section, Environment and Climate Change Canada,Toronto, Canada

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is illustrated in two case studies highlighting the lifetime of the lake-breeze circulation andthe impact of propagation distance on lake-breeze intensity.

Keywords Convection · Doppler lidar · Doppler radar · Lake breeze · Mesonet · PanAmgames

1 Introduction

Environment and Climate Change Canada (ECCC) conducted enhanced weather monitoringin the Toronto urban area in the summer of 2015 that coincided with the 2015 Pan andParapan American Games. This included the deployment of new meteorological instrumentsfor nowcasting the weather and enhanced air quality monitoring and forecasting throughoutsouthern Ontario (Joe et al. 2017). During the Games (10 July to 15 August 2015), 53surface meteorological weather stations (mesonet), a 14-station three-dimensional lightningmapping array, four new air-quality stations, a mobile air-quality laboratory, two wave-spectra buoys, twometeorological supersites, and fourmobileweather stationswere deployedthroughout the Toronto area. In addition, two scanning lidars and an existing C-band radarprovided Doppler observations in near-real time. These observations were used to identifyand characterize the vertical and horizontal wind structure of lake-breeze events over LakeOntario and the urban environment.

The dynamics of lake breezes, which are the lake counterpart of sea breezes, have beenstudied previously using theoretical, empirical, and modelling methods (e.g., Lyons 1972;Atkinson 1981; Pielke 1974, 1984; Simpson 1994; Sills et al. 2011). The lake breeze is athermally-direct circulation that arises from the differential heating between the lake andsurrounding land, producing a mesoscale horizontal pressure gradient in the lower tropo-sphere. Figure 2 of Sills et al. (2011) provides a detailed illustration of a typical lake breeze.A prominent diurnal cycle is characteristic of the lake breeze, where the surface onshore flowmoves inland during the daytime until the land breeze forms at night. The lake-breeze front(LBF) is the leading edge of the onshore flow and can have an impact on local severe weather(e.g., thunderstorms) and air quality.

The narrow region of convergence and lift at the LBF can initiate thunderstorms as wellas influence air-pollution transport, sometimes rapidly reducing air quality along the LBF,resulting in enhanced smog formation (Lyons 1972; Sills 1998; Hastie et al. 1999; Clappieret al. 2000; King et al. 2003; Brook et al. 2013; Wentworth et al. 2015). A passing LBF istypically associated with a decrease in air temperature, increase in dew-point temperature,and onshore flow near the surface. The changes to air temperature and dew-point temperatureoften become more subtle as the LBF progresses inland due to the modification of the lake-breeze air mass over land (Lyons 1972). Depending on its strength, the lower troposphericsynoptic-scale flow can alter the lake breeze’s location, shape, vertical structure, and/ormodify the inland penetration distance. For instance, a light onshore synoptic-scale flowincreases inland penetration, while a stronger offshore synoptic-scale flow prevents a LBFfrom reaching the shore at all (Sills et al. 2011).

Several studies have analyzed lake-breeze events in the Great Lakes region using radar andmesonet observations. For instance, during the Effects of Lake-breezes onWeather (ELBOW)1997 (King et al. 1998) and ELBOW 2001 (Sills et al. 2002) campaigns, several case studiesof the LBF affecting thunderstorm development in southern Ontario were analyzed (Sills1998). During the 2007 Border Air Quality and Meteorological Study (BAQS-Met) cam-

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paign, Great Lakes lake-breeze events in south-western Ontario were found to occur morefrequently than previously reported, penetrating up to 200km inland (Sills et al. 2011). Thesame observational techniques have been used to study lake breezes in other regions, suchas lake breezes in Manitoba (e.g., Curry et al. 2015; Kehler et al. 2016).

Previous studies of the Lake Ontario lake breezes in the Greater Toronto Area (GTA),however, are few (e.g., Comer and McKendry 1993; Wentworth et al. 2015). Characteristicsof the LBF, such as its horizontal and vertical wind structure over the water, are highlyvariable and difficult tomeasure accurately (Darby et al. 2002). Here, we utilize Doppler lidar(hereafter referred to as lidar) technology in conjunction with radar andmesonet observationsto characterize the LBF in an urban environment.

Lidars have been used to study the characteristics of lake/sea breezes elsewhere. The lifecycle of the land-breeze and sea-breeze systems at Monterey Bay, California, was analyzedusing lidar measurements and Regional Atmospheric Modelling System simulations (Darbyet al. 2002). The role of sea-breeze dynamics on pollution transport in the Marseille area,France, was investigated using meteorological surface stations and lidars (Bastin et al. 2005).A mobile lidar was applied to the measurement of sea breezes during the Qingdao Interna-tional Regatta, China, in 2006, to map and characterize winds during the sailboat racingevents (Sheng et al. 2007). The multilayered structure of larger, regional-scale sea breezescovering the Tokyo metropolitan area, Japan, were analyzed using a ground-based coherentlidar (Tsunematsu et al. 2009).Most recently, a lidar was used to study LBF shape and verticalvelocities in southern Manitoba (Curry et al. 2017).

This is the first study to utilize observations from a mesonet, satellite, weather radar, andtwo lidars to identify, track, and characterize the full evolution of the lake breeze (prior,during, and after) over Lake Ontario and over land. Section 2 describes the instrumentationused to measure the atmospheric state and wind field during lake-breeze events, while Sect. 3describes the sampling strategy for observing the LBF and subsequent analysis using theintegrated observations. Section 4 presents a summary of all lake-breeze observations duringthe observational period including two case studies of lake-breeze observations over LakeOntario and over land. The impact of lake-breeze propagation on lake-breeze intensity is alsodiscussed. A discussion of results and conclusions are provided in Sect. 5.

2 Instrumentation

2.1 Doppler Lidars

Scanning Doppler lidar is a relatively new technology that has only recently been made moreaffordable. The instrument emits a pulsed laser and measures the radial velocity compo-nent of the wind towards/away from the lidar (assuming the target aerosols are followingthe wind) using the Doppler shift of the backscatter. Real-time observations during clear,cloudy, and light precipitation conditions at any elevation or azimuth are possible. The lidarmeasurement technique is similar to radar, except with the added benefit that measurementscan be taken at low elevation angles along the surface since the lidar’s narrow beam removesthe issue of ground clutter. This makes lidar measurements useful in complex terrain (e.g.,Banta et al. 1997, 1999; Darby et al. 1999; Fast and Darby 2003). Notable disadvantagescompared to weather radars include the lidar’s lower scan speed (time required to performa horizontal/vertical beam sweep) and decreased measurement range by a factor of 5–60,depending on the system’s power.

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Table 1 Technical specifications for the HAN and MOB lidars

Technical specification HAN lidar MOB lidar

Manufactured date 2014 2011

Deployment site Hanlan’s Point, TorontoCentre Island

Mobile truck at several sites

Laser pulse energy 80 µJ 15 µJ

Maximum range 10kma 7.5kma

Minimum measureable signal −30.5 dB −26 dB

Scan cycle 11min 14min

Laser wavelength 1.5 µm

Laser pulse rate 10 kHz

Beamwidth 56 mm at aperture

Beam divergence 50µrad

Detector type PIN diode

Nyquist velocity ±19.4 m s−1

Doppler velocity resolution ± 0.04 m s−1

Range resolution Customizable; operated with 3 m overlapping range gates

a Note the typical maximum range strongly depends on atmospheric conditions

Two scanning lidars, designated HAN (for Hanlan’s Point) and MOB (for mobile), weredeployed during the observational period and conducted continuous, automated measure-ments. The lidar technical specifications are listed in Table 1; both lidars areHALO-PhotonicsStream Line models. The HAN lidar had 65µJ additional pulse energy, improved thermalstability, and was smaller and lighter compared to the MOB lidar.

Both lidars use fibre optics to prevent alignment issues with temperature fluctuations andtheir measurements are range-corrected and atmospheric-absorption corrected. The lidarswere subjected to quality control procedures based on the signal-to-noise ratio (SNR) withineach range gate. While there are a number of quality-control studies (e.g., Frehlich andYadlowsky 1994; Dabas 1999), Päschke et al. (2015) demonstrated that the manufacturer’sthreshold is too conservative, limiting data availability, using the same HALO lidar model.Their analysis determined an SN R + 1 threshold value ≥1.008 provides almost entirely‘good’ velocity measurements and this was the SN R + 1 threshold value used for bothlidars. All lidar scans were conducted using overlapping 60-m range gates, providing a rangeresolution of 3 m. It should be noted that since aerosols are the primary scattering targets forlidars, the backscatter signal strength (and thus lidar maximum range) primarily depends onthe aerosol concentration and relative humidity, along with additional aerosol characteristics(Fast and Darby 2003). The typical lidar maximum range varied from 2 to 5km during theobservational period.

The HAN lidar was deployed at Hanlan’s Point, which is located on Toronto Centre Island(43.6◦N, 79.4◦W), andwas installed on an elevated platformwithin anOntarioMinistry of theEnvironment andClimate Change air-qualitymonitoring station and remained at this locationthroughout the observational period, as shown in Fig. 1. The MOB lidar was installed in theback of a pickup truck enabling mobility (Fig. 1), with the truck equipped with a generator,cell phone modem, and a Vaisala WXT520 instrument (with temperature, T , pressure, P ,and relative humidity, RH, measurements). No significant difference in vertical velocity was

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Fig. 1 HAN lidar (upper-left background) and MOB lidar (centre-right foreground) during installation andside-by-side comparison testing on June 26 2015, at Hanlan’s Point

observed between the two lidars during side-by-side vertical stare comparisons (Fig. 1);the two lidar vertical velocities agreed to within <±0.5 m s−1 up to 1.8km above groundlevel (a.g.l.) with root-mean-square velocity errors <0.3 m s−1. This agreement is similar toprevious studies comparing in-situ wind observations and a lidar (Durran et al. 2002) andradiosonde/wind profiler radar/lidar measurements (Päschke et al. 2015).

2.2 Mesonet Data

The mesonet consisted of 53 new ground-based compact meteorological stations that com-plemented existing weather stations in the region. Each station was equipped to providefully automated continuous monitoring of surface temperature, pressure, relative humidity,dew-point temperature, and wind speed and direction at a temporal resolution of 1min. Themajority of these stations comprised compact stations and some were installed on rooftops ofexisting buildings (Joe et al. 2017). Mesonet observations were quality controlled in real timeusing standardized quality-control procedures to filter outliers based on a 95% confidenceinterval. Nine mesonet stations made measurements at standard heights of 1.5 m (T, P, RH)and 10 m (wind speed and direction). Emphasis was placed on observations from six of thesenine stations, numbered 1–6 (south to north) in Fig. 2, which were located along an analysistransect defined for this study (black line in Fig. 2) to obtain precise meteorological data asthe LBF moved inland. This analysis transect was selected over alternatives since it passedthrough the largest number of mesonet stations while being perpendicular to the shoreline.

2.3 King City Radar

The King City weather radar station is located just north of the city of Toronto in King City,Ontario, as indicated in Fig. 2. The dual-polarization Doppler radar is an operational radarthat is also used for radar research purposes; it has a frequency of 5625 MHz, wavelength of53.3 mm, antenna diameter of 6.1 m, and beamwidth of 0.62◦. Volume scan data (Doppler

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Fig. 2 Satellite image of southern Ontario (left), including the location of the King City C-band radar. Thedashed box outlines the zoomed-in area of the GTA (right). The magenta circle indicates the HAN lidarobservation site on Toronto Centre Island, yellow circles indicate MOB lidar observation sites (red outlinesindicate primary observation sites), numbered green markers indicate the six primary mesonet surface stationsused in this study, and the black line indicates the LBF analysis transect. © 2015 Terrametrics, © 2016 Google

and dual-polarization) are collected to a range of 250km (Doppler range ≈113km) in 10-min intervals (Hudak et al. 2006; Boodoo et al. 2010). Radar fine lines caused by insectsalong narrow regions of lift were used together with GOES-13 visible satellite imagery andmesonet data to track the position of LBFs (Wilson et al. 1994; Russell and Wilson 1997;Sills et al. 2011).

3 Data Analysis

3.1 Lake-Breeze Identification

A list of LBF criteria for satellite, radar, and surface station observations are provided in Sillset al. (2011) and were used in this study to identify the presence of the lake breeze. LBFanalyses were undertaken following the approach described in Sills et al. (2011); as shownin Wentworth et al. (2015), this method is highly reliable and made easier via the networkof observations installed during the Games (Joe et al. 2017). These datasets were combinedand visually inspected at hourly and 5-min intervals using the Aurora workstation (Greaveset al. 2001). The positions of LBFs and other low-level boundaries such as synoptic frontsand thunderstorm outflow boundaries were manually tracked within <±5 km (or <±1 kmif a radar fine line existed) throughout each lake-breeze day to provide a detailed mappingof their evolution. Uncertainties are due to cloud-line displacement relative to the front andparallax due to the satellite viewing angle. Note that while these analyses were applied overthe entire southern Ontario region and included lake-breeze detections from various lakes,only days with a LBF passing through the centre of the GTA (approximately outlined inFig. 2, right) were analyzed.

Lidar-based lake-breeze identification criteria were developed using the approach devel-oped by Sills et al. (2011), as shown in Table 2. This enabled lidar observations to be usedto precisely identify and time the location of the LBF. The lidar can detect the LBF signal

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Table 2 Lake-breeze identification criteria for the different types of lidar wind observations

Lidar observation Positive factors Negative factors Ambiguous

Horizontal winds(near-surface PPIscan)

Roughly 180◦ rotation inwind direction fromoffshore to onshore flow

Offshore winddirection

Rotation of wind direction<100◦

Radial velocities decrease to<4 m s−1 during LBFpassage

Synoptic-scale flow causingthe onshore flow

Radial velocities increaseafter LBF passage, typicallydoubling

Variability in onshore flowdirection

Vertical winds(RHI and/or starescans)

Enhancement of verticalvelocity (>1 m s−1) nearthe LBF

Offshore winddirection nearsurface

Low radial velocities<2 m s−1

Onshore flow depth<900 m a.g.l.

Downdraft near theLBF

Vertical velocity >1 m s−1

not near the LBF

Offshore (return) flow aboveonshore flow

No clear separation betweenonshore/offshore flow

Radial velocities increaseafter LBF passage, typicallydoubling

and measure distinct features of the LBF’s vertical and horizontal structure at high resolution(e.g., Banta et al. 1993; Darby et al. 2002; Bastin et al. 2005; Tsunematsu et al. 2009). Notethat the lidar lake-breeze identifications were independent of the mesonet lake-breeze iden-tifications. The MOB truck’s observations of the surface temperature, pressure, and relativehumidity could also independently verify the presence and passage of the LBF to the MOBlidar operators in real-time.

3.2 Lidar Observations of the Lake Breeze

Plan position indicator (PPI) scans of constant elevation (5◦ elevation, 3◦ azimuth) wereperformed for a full 360◦ horizontal sweep with the MOB lidar. The HAN lidar conductedscans (1◦ elevation, 1◦ azimuth) only in the southern sector (over Lake Ontario) to avoidblockage by nearby trees. Range-height indicator (RHI) scans of constant azimuth wereperformed by both lidars in the north-south direction at every 2◦ in elevation, with 3-beamDoppler beam swinging (DBS) and 8-beam velocity azimuth display (VAD) scans performedto collect wind profiles above the lidar. The scan sequence repeated every 10 and 14min,respectively, for the HAN and MOB lidars.

When the MOB lidar’s truck parked at a pre-determined observation site, it conductedvertical, PPI, RHI, DBS, and VAD scans. Several observation sites were pre-selected priorto the Games based on their geographic location and sightlines, as shown in Fig. 2. TheMOB lidar was parked at one of the three southern sites each morning to measure windspeed and direction over the lake prior to the LBF’s formation. If no LBF was detected, orif the LBF remained stationary south of the shoreline, the MOB lidar remained stationary.If the LBF moved inland, the MOB lidar relocated (while continuing to conduct verticalstare measurements in-transit) to a northern site such as Downsview (Fig. 2, right) where it

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repeated its scan operations. This ensured that the MOB lidar sampled the evolution of thelake breeze at both the shoreline as well as inland throughout the day.

3.3 Measuring Lake-Breeze Characteristics

Once the presence of a LBF was identified by the lidar using the criteria in Table 2, sevenmain characteristics of the lake breeze’s horizontal and vertical structure were measured toquantify the lake-breeze structure. These were:

1. Change in surface temperature: Tmax – Tmin temperature when the LBF passed overhead(within a 40-min interval centred on the time of LBF passage) as measured by mesonetsurface station 1 close to the shoreline,

2. Change in dew-point temperature (using the same methodology as for surface tempera-ture),

3. Enhancedvertical velocity near theLBF: a yes/no criterionwas assignedbasedonwhetherenhancedvertical velocity near theLBFexisted (>1 m s−1 during the time theLBFpassesthe lidar) as measured by either lidar’s vertical scan,

4. Lake-breeze flow depth: the onshore flow height as measured by the HAN lidar RHI scan1-h after the LBF passed overhead,

5. Lake-breeze radial velocity: the average radial velocity inside the lake breeze asmeasuredby the HAN lidar RHI or PPI scan near the surface 1 h after the LBF passed overhead,

6. Lake-breeze ground speed: the average velocity at which the LBF progressed inland asmeasured by selecting the initial and final LBF positions, D, over a time period, t , whenthe LBF was moving continuously inland (typically over several hours) and computingits velocity, v (v = Dfinal − Dinitial/t), and,

7. LBF inland penetration distance: the maximum distance the LBF travelled inland (fromthe shoreline).

The LBF ground speed and lake-breeze inland penetration distance were calculated alongthe LBF analyses transect (black line in Fig. 2) using the position and time of the LBF asmeasured by the satellite, radar, and mesonet analysis.

4 Results

4.1 Summary of Lake-Breeze Observations

A complete summary of all lake-breeze days and lake-breeze characteristics is provided inTable 3. This analysis of lake-breeze characteristics includes observations from the lidardata and the mesonet analysis. Averages and standard errors (σ /

√N ) of all lake-breeze

characteristics are also provided. The Lake Ontario lake breeze was observed for 27/37days (73%) within the study domain (central Greater Toronto Area) during the observationalperiod. The lidars were unable to identify the presence of three lake-breeze events (22 July, 1and 11 August) that occurred outside of the lidar range; only the mesonet analysis data wereused for these days. Note that there were five Lake Ontario lake breezes that did not travelthrough the GTA during the observational period; these were not included in this analysis.Thus the lake-breeze occurrence rate for the entire Lake Ontario region (not just for LBFspassing through Toronto) during the observational period was higher: 87% as measured bythe mesonet analysis.

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Table3

Lake-breeze

characteristicsforeach

LakeOntario

lidar-detectedlake-breezedaybetween10

July

and15

August2

015

Date(201

5)Temperature

change

(◦C)

Dew

-point

temp.

change

(◦C)

Enh

anced

(>1m

s−1)

verticalvelocity

atLBF

Lake-breeze

flow

depth(m

)Lake-breeze

radial

velocity

(averaged)

[ms−

1]

LBFgrou

ndspeed

[ms−

1]

LBF

inland

penetration

distance

(km)

July

10−1

+1.8

N39

07.9

0.8

18

July

11−1

.2+3

N42

08.3

1.6

24

July

12−2

.5+1

.8N

780

3.7

1.9

63

July

15−1

.3+3

Y300

3.1

0.5

7

July

18−1

.7+1

.1Y

720

4.4

1.0

23

July

19−3

.9+3

.5Y

750

6.2

0.4

6

July

20−1

.5+1

.6Y

900

8.0

0.4

7

July

22−1

.9+2

.5N/A

N/A

N/A

0.3

3

July

23−3

.1+9

.7Y

240

6.3

0.5

10

July

24−0

.8+1

.6Y

840

8.9

1.6

19

July

25−1

.2+1

.4Y

720

7.1

1.7

27

July

26−1

.9+1

.9Y

390

6.1

1.7

34

July

27−1

.2+4

Y81

06.6

1.2

21

July

28−2

.5+2

N78

04.2

0.8

35

July

29−1

.8+1

N75

08.8

1.4

57

July

31−1

.4+1

.7Y

180

5.1

2.7

30

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Table3

continued

Date(201

5)Temperature

change

(◦C)

Dew

-point

temp.

change

(◦C)

Enh

anced

(>1m

s−1)

verticalvelocity

atLBF

Lake-breeze

flow

depth(m

)Lake-breeze

radial

velocity

(averaged)

[ms−

1]

LBFgrou

ndspeed

[ms−

1]

LBF

inland

penetration

distance

(km)

Aug

1−4

.3+3

N/A

N/A

N/A

0.7

3

Aug

2−1

.5+0

.9N

290

7.3

1.2

27

Aug

3−0

.6+0

.6Y

350

3.2

0.4

0

Aug

4−3

.7+2

.1N

600

5.5

1.2

17

Aug

5−3

.0+2

.2N

150

5.7

0.4

0

Aug

6−3

.6+1

.6N

300

5.3

0.7

17

Aug

9−2

.4+1

.3N

600

5.5

3.2

61

Aug

11−2

.3+2

.1N/A

N/A

N/A

0.8

13

Aug

13−0

.8+2

.4Y

890

6.3

1.7

38

Aug

14−2

.2+1

Y690

3.6

0.1

1

Aug

15−3

.5+2

.4N

510

5.6

1.4

23

AVG

−2.1

±0.2

◦ C+2

.3±0

.3◦ C

54%

Y56

0±5

0m

5.6±0.4ms−

11.1±0.2ms−

122

±4km

Descriptio

nsof

each

lake-breezecharacteristicareprovided

inSect.3

.3.A

veragesandstandard

error(σ

/√ N

)areinclud

edatthebo

ttom.C

asestud

ydays

arebo

lded

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The dominant meteorological signal of the lake breeze was a sharp increase in dew-pointtemperature (average increase of 2.3 ± 0.4 ◦C), coinciding with a decrease in temperature(average decrease of 2.1± 0.2 ◦C) and offshore wind direction. The average change in dew-point temperature in a recent lake-breeze study of Curry et al. (2017) was 2.5 ◦C, which is ingood agreement with our results, but the average change in surface temperature was 0.5 ◦C,which is significantly smaller than the present in study. This difference could be due to theshallower LBF and higher LakeWinnipeg temperatures that cause smaller lake–land thermalcontrasts when compared to Lake Ontario. Overall the results in Table 3, particularly the LBFinland penetration distance and ground speed, agree well with long-term observations of lakebreezes from several different lakes in Manitoba (Curry et al. 2015; Kehler et al. 2016).

The changes in temperature, dew-point temperature, and relative humidity typically took1–5min to occur, providing a clear indication that the LBF passed the mesonet station. Onmost days with offshore synoptic flow, the LBF would sit along the shoreline above mesonetstation 1, producing oscillations and more gradual changes that might last for up to 40minuntil the LBF began to move inland. Precise estimates of the width of the LBF are requiredin order to interpret these oscillations, which is the subject of future work.

Enhanced vertical motion near the LBF was observed for 54% of the lake-breeze days;this enhancement was particularly strong when measured along the shoreline (periods of>2.5 m s−1). The magnitudes of the vertical velocities at the LBF are in good agreementwith recent observations by Curry et al. (2017) of 2–3 m s−1. The observed range in lake-breeze flow depths of 150–900 m with mean lake-breeze flow depth of 560±50m is in goodagreement with Lyons (1972), who reported a range of 100–1000 m, and mean of 500 m, inthe Lake Michigan area. After the lake breeze moved inland and matured in the afternoon,the lake-breeze radial velocity measured over the lake typically doubled in magnitude overthe course of 2–3h.

The largest variability in lake-breeze characteristics relates to the penetration distance.Observed lake-breeze penetration distances from the Great Lakes and several lakes in Man-itoba have ranged from near zero to >200 km (Lyons 1972; Comer and McKendry 1993;Sills 1998; King et al. 2003; Sills et al. 2011; Curry et al. 2015). For Lake Ontario, the lake-breeze inland penetration distances ranged from 6.5 to 63 km. The ability to observe lakebreezes with minimal penetration distances (<15 km) arises from lidar observations of theLBF close to the shoreline and the increased density of mesonet stations near the shoreline.The observed LBF ground speeds are consistent with previous numerical and observationalstudies of LBF propagation (Bechtold et al. 1991; Simpson 1994; Bastin et al. 2005; Curryet al. 2015).

Figure 3a illustrates the positive correlation between the LBF ground speed and inlandpenetration distance (r = 0.78, where r is the Pearson product-moment correlation coeffi-cient) found using the mesonet analysis data. A negative correlation was found between thesynoptic wind speed, obtained at 10 m a.g.l. by nine WMO-standardized mesonet stations(stations inside the lake breeze were filtered out), and inland penetration distance (r = 0.63)as shown in Fig. 3b. Wind direction is shown as degrees from north to illustrate the impactof non-northerly flow. The presence of even a weak (1–3 m s−1) northerly synoptic flow wasfound to inhibit lake-breeze inland penetration, with stronger (>3 m s−1) northerly synopticflow limiting lake-breeze penetration to≤10km inland. The synoptic flow direction explainsoutliers in Fig. 3; on the two days with large lake-breeze penetration distances (top-middleof Fig. 3a), a moderate (around 2.5 m s−1) synoptic-scale flow was from the south-west(12 July) and south (29 July). The southerly component of the flow on these days aidedin transporting the lake-breeze air further inland despite the lower LBF ground speed. Nocorrelation was found between the lake-breeze flow depth, synoptic-scale wind speed, radial

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Fig. 3 Correlation between the lake-breeze inland penetration distance and, a LBF ground speed, b synoptic-scale wind speed, for 27 lake-breeze days observed during the Games. Data points are coloured based on thesynoptic-scale wind direction at the time of lake-breeze formation. The black line indicates the linear fit andr is the Pearson product-moment correlation coefficient. The points circled in red indicate the two case studylake-breeze days

velocity, and LBF ground speed. In the next two sub-sections, two case studies are presentedto illustrate the extreme differences in observed lake-breeze characteristics: a slow-movingand shallow-penetrating LBFmeasured along the shoreline on 15 July (red circle, bottom-leftof Fig. 3a), and a fast-moving and deeply-penetrating LBF measured well inland on August9 (red circle, top-right of Fig. 3a).

4.2 15 July 2015: Lake-Breeze Observations Over the Lake

Measurements of the vertical and horizontal wind structure of a LBF during opposing (off-shore) synoptic flow over Lake Ontario were made on 15 July 2015. On this day, overnightcumulus clouds gave way to clear skies at sunrise that lasted for the entire day, with theexception of small cumulus clouds forming along thewestern edge of the LBF. A north/north-north-east synoptic flow near the surface of around 6 m s−1 persisted throughout much of thesouthern Ontario region. Maximum inland air temperatures ranged between 13 and 21◦C,while the average Lake Ontario temperature measured near Toronto Centre Island was 20◦C.The Lake Ontario LBF was detected offshore at 1400 UTC by the HAN lidar and reachedthe shoreline by 1510. It slowly progressed inland, pausing frequently and reaching a maxi-mum inland penetration distance of only 7.5km by 2000 UTC. The Georgian Bay and LakeSimcoe LBFs were detected at 1900 having travelled to the south, aided by the northerly syn-optic flow, but did not interact with the Lake Ontario lake breeze. At 0004 UTC+1, offshoresurface-flow observations by the lidar signalled the end of the lake-breeze event.

4.2.1 Horizontal and Vertical Structure on 15 July

The complete evolution of the lake-breeze horizontal structure as measured by the HANand MOB lidars on 15 July is shown in Fig. 4. The MOB lidar was located at Leslie Spiton this day, slightly north and 3.7km to the east of the HAN lidar. The LBF first formedoffshore and was detected by the HAN lidar at 1400 UTC (Fig. 4b, red arrow), as offshorewinds (red) met onshore winds (blue) 300 m south-east of the lidar. Note that the mesonetanalysis did not detect the lake breeze for another hour due to the scanning angle of the King

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Fig. 4 HAN (H) and MOB (M) lidar PPI scans during a passing LBF on 15 July 2015 at, a 1345, b 1400, c1427, d 1537, e 1633, and f 2257. Negative (blue) velocities represent winds towards the lidar; positive (red)velocities represent winds away from the lidar. Black arrows indicate the deduced wind direction from thelidar PPI scan. The red arrow in b indicates the location of the approaching LBF. All times are UTC

City Radar (as will be discussed). As the LBF continued moving onshore, it passed directlyover the MOB lidar at 1537 UTC (Fig. 4d). At 1633 UTC (Fig. 4e), the radial velocitiesclose to the shore increased from <4 to >6 m s−1 and a complete 180◦ reversal of the near-surface flow at Leslie Spit was observed. Radial velocities continued to increase as the lakebreeze matured until MOB lidar measurements ended at 2257 (Fig. 4f). The slight clockwiserotation of the lake-breeze wind direction from Fig. 4e, f can be attributed to bulging effectsof the lake breeze due to small variations in the shoreline and synoptic-scale flow and, to a

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Fig. 5 Mesonet analysis at 2300 UTC (same time as Fig. 4f) on 15 July 2015. Meteorological data fromindividual mesonet stations are displayed on the map (white) overlaid with radar and satellite observations.LBFs, such as the Lake Ontario LBF (along the shoreline), are indicated by purple triangles. Also visible arethe Lake Simcoe andGeorgian Bay LBFs (centre-left and top-right, respectively). Lidar locations are indicatedby the yellow circles

lesser extent, the Coriolis force. The LBF remained stationary above mesonet station 2 dueto the northerly synoptic-scale flow inhibiting its progression further north as illustrated inthe mesonet analysis in Fig. 5.

RHI scans provided in Fig. 6 illustrate the evolution of the LBF vertical structure overLake Ontario as measured by the HAN lidar. In Fig. 6a, the LBF is present during offshoresynoptic flow; in Fig. 6b the LBF is detected 80 m offshore fromHanlan’s Point with onshoreand offshore flows converging; in Fig. 6c, d the LBF had passed inland and strengthened,with the maximum radial velocity occurring near the surface and increasing from <3 to>4 m s−1. The wedge-shaped LBF (approximated by the green outline in Fig. 6b) differsfrom the idealized plume LBF shape in Fig. 2 of Sills et al. (2011). Low-altitude onshoreradial velocities were of similar magnitude to the higher-altitude offshore velocities, whilethe radial velocities between the two airflows were near zero over a relatively large span (60m).

Strong radar fine lines were observed for the Georgian Bay and Lake Simcoe LBFs whilealmost no radar fine line was observed for the Lake Ontario LBF (Fig. 5). The lack of aradar fine line along the shoreline is likely due to the King radar overshooting the LBF. Atthe shoreline, the radar 0.5◦ PPI scan is roughly 650 m a.g.l. (depending on atmosphericconditions); hence it can only detect LBFs with a lake-breeze flow depth close to this valuealong the shoreline. While the lake-breeze flow depth increased from 180 to 290 m in almostan hour as the lake breeze matured (Fig. 6c, d, white double arrow), it remained below theradar PPI scans (even below the hourly 0.3◦ PPI scan), failing to detect enhanced clear-airreflectivity. Note that for lake-breeze days with a lake-breeze flow depth >650 m, radar finelines were observed when the LBF was near the shoreline.

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Fig. 6 HAN lidar RHI scans southward (180◦ azimuth) over Lake Ontario during a passing LBF on 15 July2015: a before the lake breeze at 1357, b first detection of the wedge-shaped (green outline) LBF at 1417, cafter the LBF passed overhead at 1437, and d as the LBF matured at 1547. Negative (blue/green) velocitiesrepresent winds towards the lidar; positive (orange/red) velocities represent winds away from the lidar. Blackarrows depict northerly flow andwhite arrows depict lake-breeze onshore flow.Dotted arrows depict northerly(black) and lake-breeze return flow (white) along the LBF. The green dashed line and white double arrow inc and d depict the lake-breeze flow depth. All times are UTC

Figure 7 provides snapshots of the wind profiles before and after the LBF passed theMOB lidar at Leslie Spit (similar results were obtained by the HAN lidar). A relatively stablewind profile with northerly (offshore) winds was observed at 1541 UTC (Fig. 7a), and inFig. 7b, thewind direction changed near the surface at 1815UTC,with southerly near-surfaceflow and northerly flow above the lake-breeze flow (>300 m a.g.l.). The onshore flow’s radialvelocity was limited to<4.5 m s−1 whereas the higher-altitude offshore winds ranged from 3to 11 m s−1. From Fig. 7a, b, the wind-speed maximum shifted from 300 to 700 m a.g.l., wellabove the lake-breeze flow. In addition, the offshore flow (from >300 m to approximately1.1km) occurred at almost three times the lake-breeze flow depth, which is a larger differencethan predicted by Lyons (1972). The return flow from the lake-breeze circulation is likelyembedded in the shallowest layer (300 m) of the offshore flow as depicted in Fig. 7b. Withinthe lake breeze, a large gradient in the wind direction below 300 m and a multi-layeredstructure of vertical velocity is evident (Fig. 7b), as previously reported (e.g., Darby et al.2002; Tsunematsu et al. 2009).

A strong enhancement in vertical velocity was measured by both lidars at the time theLBF passed overhead. Observations from the HAN lidar vertical stare scan are provided inFig. 8 from 1000 to 2200 UTC (the LBF reached the HAN lidar at 1425). The lake-breezeflow depth (Fig. 8, black line) increased from 90 to 360 m, varied around 300 m a.g.l. forthe majority of the day, then decreased (not shown) as the lake breeze ended. Two uniquefeatures persist for this case: (1) a significant decrease in vertical motion for the remainderof the day, particularly within the lake-breeze flow depth. Given that the circulation withinthe lake breeze is mostly horizontal and thermals are not generated over water, the mutedvertical motion is expected; (2) pockets of increased vertical velocity are observed above

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Fig. 7 MOB lidar wind speed (left panels, red) and wind direction (right panels, blue) profiles before andafter a passing LBF on 15 July 2015 at, a 1541 and b 1815. All times are UTC

(but not inside) the lake breeze, which are indicative of thermals that have been advectedover top of the lake breeze. Such thermals form over land but, due to the lake breeze andoffshore synoptic-scale wind, may have been shifted over the lake and to higher altitudes.This transport of thermals above the lake breeze is made easier by the wedge-shaped LBF.

4.3 9 August 2015: Lake-Breeze Observations Over Land

Observations of the LBF over land were performed on several days during the observationalperiod; 9 August 2015 will be used as a case study of a deeply penetrating lake breeze.The LBFs observed over land, hours after they moved onshore, had significantly different

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Fig. 8 15 July 2015 HAN lidar vertical velocity measurements before, during (1425—green vertical bar) andafter the LBF passed. White vertical gaps indicate periods where the lidar was conducting measurements inother scan modes. The lake-breeze flow depth is also shown (black line). Negative (blue) velocities representflow towards the lidar (surface); positive (red) velocities represent flow away from the lidar (surface)

characteristics than when they were newly formed over Lake Ontario. The presence of anonshore synoptic flow also resulted in several unique differences in LBF structure, as willbe discussed below. The 9 August lake breeze occurred during east/east-north-east synopticflow of around 3 m s−1, which is rare for the southern Ontario region. Clear skies persistedthroughout most of this day, with the exception of deepening cumulus clouds along the LakeOntario LBF in the afternoon as it moved inland. Maximum inland air temperatures rangedbetween 12 and 23◦C, while the average Lake Ontario temperature measured near TorontoCentre Island was 19◦C. The Lake Ontario LBF was detected offshore at 1405 UTC by theHAN lidar and reached the shoreline by 1500; at 2000UTC, the LakeOntario LBF intersectedand continued through the Lake Simcoe LBF south of Lake Simcoe, but this intersection didnot result in the development of thunderstorms. The Lake Ontario LBF progressed inlandat a propogation speed of 3.2 m s−1, reaching its maximum inland penetration distance of60.5km by 2300 UTC. Soon after 2300 UTC, the land breeze was detected via offshoresurface-flow observations by the HAN lidar (or, alternatively, the offshore synoptic flow wasobserved as the lake breeze detached from the lake), signalling the end of the lake-breezeevent.

4.3.1 Horizontal and Vertical Structure on 9 August

Lidar observations during the morning of the 9 August lake breeze over Lake Ontario weresimilar to that for the 15 July case (Fig. 4). As the LBF progressed inland, the MOB lidarrelocated to the 400N site 31km north of the shoreline and conducted PPI scans before,during, and after the LBF reached the site. Unlike the morning lake-side observations, theMOB lidar PPI scans at the 400N site are of a weaker, poorly-defined lake breeze. Surfaceradial velocities were less over land than over the lake (at the HAN site), and decreased onlyslightly (from 6 to 5 m s−1) as the LBF passed over the MOB lidar at 1824 UTC. The winddirection shifted <100◦ and no increase in surface radial velocity was observed following

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Fig. 9 As in Fig. 5, except for before the LBF reached the 400N site at 1800 UTC on 9 August 2015

LBF passage. The onshore surface flow also had a poorly-defined horizontal velocity gradientcompared to observations along the shoreline. Themesonet analysis at 1800 UTC is providedin Fig. 9 to illustrate the progression of the LBF far inland as it approached the 400N site(yellow circle, centre-left).

The LBF at the 400N site had less pronounced vertical structure with no observed offshoreflow (Fig. 10), unlike 15 July observations. The shallow circulation consists of negativeradial velocities inside the lake breeze with a mixture of away/towards velocities above dueto thermals being advected above the lake breeze. The lack of offshore flow is due to thesynoptic flow in a non-offshore direction and the weaker lake-breeze circulation limitingthe return flow. The LBF shape was a rounded wedge, somewhat resembling the idealized‘plume’ (roughly approximated by the green outline in Fig. 10) depicted in Sills et al. (2011).In agreementwith simulatedLBFevents in Fig. 11.14 of Simpson (1994), the lake-breezeflowon 9August was deepest behind the LBF and then tapered back. Only limited enhancement invertical velocity (around 0.9 m s−1) was observed at 1825 UTC (not shown), approximatelythe time when the LBF passed over the lidar.

5 Discussion and Conclusions

Two scanning lidars, a C-band radar, and a 55-station mesonet provided observations of thevertical and horizontal structure of lake breezes during the 2015 Pan and Parapan AmericanGames in Toronto (10 July to 15 August 2015). New lake-breeze identification criteria werecreated for vertical and horizontal lidar observations and the first measurements of the LBFstructure over Lake Ontario were conducted using the lidars. The lidars detected the lakebreeze on average 1 h before the mesonet and radar, demonstrating early detection when

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Fig. 10 As in Fig. 5, except MOB lidar RHI scan southward (180◦ azimuth) 31km inland at the 400N site at1806 UTC on 9 August 2015. The approaching plume-shaped LBF can be seen<60 m from the lidar (roughlyapproximated by the green curve). Black arrows depict northerly flow being uplifted and white arrows depictlake-breeze onshore flow

deployed near the shoreline. The Lake Ontario lake-breeze occurrence frequency is morethan double the estimate of 32% provided in Comer and McKendry (1993), likely due to dif-ferences in study duration (the Comer and McKendry analysis included time periods outsideof the summermonths, which experience fewer lake-breeze events) and instrumentation. It is,however, similar to occurrence frequencies of 56–74% provided in Wentworth et al. (2015)using one summer of data from 2010. The continuous, autonomous, and maintenance-freeoperation of the lidars demonstrated their reliability and the applications for meteorologicaloperations.

There were several days where two or more lake breezes intersected (typically the LakeOntario lake breeze moving north and the Georgian Bay lake breeze moving south); despitetheir intersection, no significant thunderstorm activity was observed, likely due to persis-tent low convective available potential energy or large convective inhibition environments.Note the observational period encountered significantly less total precipitation in the GreaterToronto Area (22 mm in July 2015), which is almost one-third the climatological normal.

The dominant signal of a passing LBF was an average 2.3 ± 0.3 ◦C increase in dew-point temperature, coinciding with a 180◦ shift in wind direction and an average decreasein temperature of 2.1 ± 0.2◦C. Enhanced (>1 m s−1) vertical motion along the LBF wasobserved by both lidars in 54% of lake-breeze cases. The lake-breeze flow had diminishedvertical velocities, while enhanced vertical motion (thermals) was observed above the lake-breeze flow near the shoreline. For lake breezes measured near the shoreline, the largestvertical velocities occurred <600 m a.g.l. around the time of LBF passage and exceed thepeak vertical velocity during LBF passage measured in the Monterey Bay area, likely due tothe stable lower troposphere at the California coast (Banta et al. 1993). This altitude range ofenhanced vertical velocities is below typical convective cloud bases due to the wedge-shapedLBF.

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The LBF ground speed and synoptic-scale wind speed correlatedwell with the lake-breezemaximum inland penetration distance (r = 0.78 and r = 0.63, respectively), with maximumpenetration distances occurring on days with non-opposing (non-offshore) synoptic flow.This relationship can be used to forecast LBF location and potential convergence in theafternoon (assuming constant synoptic flow) by measuring the LBF ground speed earlier inthe day. While this analysis uses the average LBF ground speed to obtain this relationship,the LBF ground speed changes through the day in a predictable way (Simpson 1994) andcan be accounted for when forecasting LBF location. This can have important implicationsfor predicting thunderstorm development and inland pollution transport (Lyons and Olsson1973; Clappier et al. 2000; Wentworth et al. 2015) and is the focus of future research.

As seen in Fig. 4, the HAN lidar observed changes due to the lake breeze about 30minbefore the MOB lidar at Leslie Spit. Given that the HAN lidar site is more exposed to LakeOntario, located south of Leslie Spit, and the variability in the LBF angle of approach, thistemporal offset was common throughout the observational period. As measured by bothlidars, it typically took around 2h for a full 180◦ shift in wind direction to occur. Whenmeasured at one of the inland sites, the time for a complete shift in wind direction wastypically less (0.5–2h) due to the LBF’s slower progress over the lake compared to over land.Similarly, the pattern of diminishing surface radial velocities as the LBF passed over thelidar (Fig. 4c–e), followed by increased surface velocities after the LBF passed (Fig. 4e, f),occurred near the shoreline but not when measured at the inland sites. Similar increases inradial velocities of 1–2 m s−1 within the leading edge of the LBF along the shoreline wereobserved in Curry et al. (2017).

Several differences in lake-breeze characteristics were observed between 15 July and9 August lake breezes, such as greater LBF ground speed (3.2 m s−1 on 9 August versus0.5 m s−1 on 15 July) and lake-breeze inland penetration depth (60.5 km on 9 August versus7.5km on 15 July). The lake breeze on 15 July had surface radial velocities >6 m s−1,whereas the deeply-penetrating lake breeze on 9 August produced radial velocities<5 m s−1

over land. The greater radial velocities measured over Lake Ontario are likely due to lesssurface roughness and a shallower boundary-layer height compared to over land (Darbyet al. 2002). Almost no radar fine line was observed on 9 August even as the LBF movedwithin range of the radar. Due to the lack of a northerly synoptic flow, the LBF ground speedwas exceptional and convergence at the LBF was limited, producing negligible uplift, likelyinhibiting the concentrations of insects to enhance the clear-air reflectivity along the LBF.King radar observations were also found to overshoot the LBF when the lake-breeze flowdepth was<650 m a.g.l. near the shoreline, highlighting the significance of the synoptic flowon lake-breeze detection, structure, and characteristics.

A wedge-shaped LBF was observed by both lidars when conducting measurements overLake Ontario for all lake-breeze days with northerly synoptic flow; these are among the firstobservations of a wedge-shaped LBF using lidar (Curry et al. 2017). Generally speaking, theability of the LBF to form a wedge shape is dependent on the presence of moderate to strongnortherly synoptic flow to tilt theLBFbackwards and/or aweak lake-breeze environment (lesstemperature contrast between the lake and land). Conversely, a light to moderate synopticflow in a strongly forced lake-breeze environment (larger temperature contrast) produces amore plume-shaped LBF. The implication of awedge-shaped LBF is the production of a tiltedupdraft that can have a diminished impact on the upward motion of air parcels to the level offree convection. This inhibits the formation of cumulus clouds along the LBF and limits theLBF’s ability to aid in convection initiation. The wedge-shaped LBF also causes a shallowerlake-breeze flow near the LBF with increasing lake-breeze flow depth in time, exacerbatingthe issue of King radar overshooting the LBF. This LBF structure is in agreement with lake-

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breeze flow-depth observations in the Monterey Bay area (Banta et al. 1993; Darby et al.2002) and lake-breeze simulations during a headwind described in Simpson (1994). Sincethe LBF is an area known for pollutant transport, an increased lake-breeze flow depth nearplume-shaped LBFs can encourage the transport of pollutants and aerosols higher into theatmosphere, the implications of which are the focus of future studies.

Lidar measurements of the LBF vertical structure indicate a clear separation betweenonshore and offshore flow; the average lake-breeze flow depth of 560 ± 50 m is in closeagreement to a climatology provided in Lyons (1972). In several cases an increase in theoffshore flow speed was observed directly above the onshore flow, indicative of the synopticflow merging with the lake-breeze return flow. While the return flow can be difficult tomeasure since distinguishing it from the synoptic flow is ambiguous, the enhanced windspeed observed between 400 and 600m (directly above the lake-breeze flow depth) in Fig. 7a,b is likely the result of an additive effect of the synoptic flow and the return flow merging inthe same layer, as discussed in Banta et al. (1993) and recently observed using a lidar (Curryet al. 2017). The presence of the return flow would complete the lake-breeze flow cycle oflow-altitude onshore flow, updraft at the LBF, and offshore return flow above the onshoreflow as depicted in Fig. 2 of Sills et al. (2011). This accelerated offshore flow directly abovethe LBF is a commonly observed feature during lake-breeze events measured near the lake,but not over land; a potential reason for this is the weaker lake-breeze circulation over landinhibiting the return flow.

A likely mechanism for the diminishing lake breeze over land is provided in Herkoff(1969), who showed rapid heating of the near-surface lake-breeze air as the LBF progressedinland. A diminishing lake breeze over land was frequently observed during this study as wellas in previous studies (e.g., Curry et al. 2015; Kehler et al. 2016); for instance, the 9 AugustLBF had weaker horizontal and vertical structure over land, inhibiting vertical motion at theLBF. In addition, the greater turbulence intensity over land during the day increases verticalmixing within the boundary layer, preventing prolonged enhancements in vertical velocitynormally associated with the LBF. This highlights the impact lake-breeze propagation awayfrom the source region has on lake-breeze intensity.

The lidar, radar, and mesonet observations provide a means of evaluating and improvingthe high-resolution (250m) numerical weather prediction model deployed during the Games.Lidarmeasurements of convection, particularly along the LBF,will also be used to investigatethe effects of lake-breeze structure on air quality and transport of ozone plumes observedduring the Games.

Acknowledgements Special thanks to the PanAm Science Team and, in particular, the AMMOS and RSDteams (particularly Neil Taylor) for coordinating LBF detection efforts.MOB lidarmeasurements conducted atLeslie Spitweremade possible thanks to the support of Ports Toronto and theToronto andRegionConservation.Thanks to MOB lidar operators Eva Mekis and Reno Sit for conducting lidar observations of the lake breezethroughout the Games. Thanks to Christopher Zaworski, Sylvie Leroyer, and Emma Hung for developing newlidar analysis products, refining lidar quality control algorithms, providing assistance with model data, andpreparing RSD plots.

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