Evaluation of Modeled Lake Breezes Using an Enhanced Observational Network in Southern Ontario: Case Studies

Canadian Global Environmental Multiscale (GEM) numerical model output was compared with the meteorological data from an enhanced observational network to investigate the model’s ability to predict Lake Ontario lake breezes and their characteristics for two cases in the Greater Toronto Area—one in which the large-scale wind opposed the lake breeze and one in which it was in the same direction as the lake breeze. An enhanced observational network of surface meteorological stations, a C-band radar, and two Doppler wind lidars were deployed among other sensors during the 2015 Pan and Parapan American Games in Toronto. The GEM model was run for three nested domains with grid spacings of 2.5, 1, and 0.25 km. Comparisons between the model predictions and ground-based observations showed that the model successfully predicted lake breezes for the two events. The results indicated that using GEM 1 and 0.25 km increased the forecast accuracy of the lake-breeze location, updraft intensity, and depth. The accuracy of the modeled lake breeze timing was approximately ±135 min. The model underpredicted the surface cooling caused by the lake breeze. The GEM 0.25-km model significantly improved the temperature forecast accuracy during the lake-breeze circulations, reducing the bias by up to 72%, but it mainly underpredicted the moisture and overpredicted the surface wind speed. Root-mean-square errors of wind direction forecasts were generally high because of large biases and high variability of errors.

Spatiotemporal Characteristics of Lake Breezes over Lake Taihu, China

Lake Taihu is a shallow lake located in the Yangtze River delta region in eastern China. Lake breezes and their interactions with urban heat islands are of great importance to air quality and weather forecasting. In this study, surface observations at a dense network and Wind Profile Radar measurements were utilized to characterize the lake breezes at Lake Taihu and assess the impact of geophysical factors on the development and intensity of the lake breezes. The lake breezes were characterized by a low occurrence frequency of 12%–17% (defined as the percentage of days with lake breezes in a given month), weak speed (annual mean ranging from 1.5 to 3.3 m s−1), late onset [average onset around 1110 local standard time (LST), with a range of 0900–1300 LST], short duration (annual mean 3.5 h), and low circulation depth (average depth of 400 m from 1200 to 1400 LST). The lake breezes were greatly suppressed when the geostrophic winds were higher than 4.1 m s−1. The low heat capacity of shallow water (mean depth 2.0 m) led to small temperature differences between the land and the lake, which was the main factor responsible for the low occurrence frequency along Lake Taihu. All of the characteristic parameters showed distinct seasonal variations. Increased frequencies, earlier onset times, and longer durations on the northern lakeshore were indicative of the impact of the urban heat island on the lake breezes.

8. Localized weather

Although certain weather events, such as violent tornadoes, affect relatively small areas on the ground, there are a number of effects that are localized in their influence. ‘Localized weather’ first considers fog, which may be associated with widespread anticyclonic conditions leading to a significant drop in temperature at night, and relatively quiet, or windless, conditions. The two common forms of fog are radiation fog and advection fog. Haze and smog are also discussed along with local winds divided into two groups: sea, land, and lake breezes; and valley and mountain winds. Katabatic winds, föhn conditions, lake effect snow, ice storms, and glaze (or ‘black ice’) are also considered.