Aircraft observations and sub‐km modelling of the lake–land breeze circulation over Lake Victoria

Abstract

The lake–land breeze circulation over Lake Victoria was observed in unprecedented detail with a research aircraft during the HyVic pilot flight campaign in January 2019. An evening and morning flight observed the lake and land breezes respectively under mostly dry conditions. The circulation was observed at various heights along a transect across the lake and onshore in Tanzania. Profiles of the lower troposphere were recorded by dropsondes over the lake and land. Convection‐permitting MetUM simulations with different horizontal grid‐spacings (including sub‐km) were run for the flight periods. During the evening flight, the aircraft crossed the lake breeze front over land at 1627 LT, approximately 50 km to the east of the lake shore, recording a 6 g·kg−1 decrease in specific humidity and reversal in wind direction over ∼5 km. During the morning flight, a shallow land breeze was observed across the eastern shore at 0545 LT. At least one region of increased and deeper moisture (previously seen in simulations but never observed) was sampled over the lake surface between 0527 and 0855 LT. This bulge of moisture was likely formed from the lifting of near‐surface moist air above the lake by low‐level convergence. The observations and model simulations suggest that low‐level convergence occurred at the leading edge of the land breeze, which had detached from the main land breeze and independently propagated westward across the lake with wave‐like characteristics. The MetUM simulations were able to reasonably reproduce the lake breeze front, bulge feature, and its propagation, which is a major achievement given the sparse observational data for model initialisation in this region. However, some timing, resolution and boundary‐layer depth biases require further investigation. Overall, this pilot campaign provided an unprecedented snapshot of the Lake Victoria lake–land breeze circulation and motivates a more comprehensive field campaign in the future.

Assessment of convection‐permitting versions of the Unified Model over the Lake Victoria basin region

“…The HIGH Weather impact lAke sYstem (HIGHWAY) project has had a huge focus on enhancing the observation network as well as rehabilitating the radar in Mwanza on the southern shore of Lake Victoria. Parallel to this, the HyVic 1 -pilot flight campaign in January 2019 (Woodhams et al, 2021) also provided 1 Hydroclimate Project for the Lake Victoria basin.…”

“…The HIGH Weather impact lAke sYstem (HIGHWAY) project has had a huge focus on enhancing the observation network as well as rehabilitating the radar in Mwanza on the southern shore of Lake Victoria. Parallel to this, the HyVic‐pilot flight campaign in January 2019 (Woodhams et al ., 2021) also provided the first observations from over the lake. Data collected in both enhanced and standard observing periods are eminently valuable for understanding and validating model behaviour and there is reason to be optimistic that there will be better observational coverage in the future.…”

The Met Office winter testbed 2020/2021: Experimenting with an on‐demand 300‐m ensemble in a real‐time environment

“…It is likely that with increases in computer power, greater use will be made of very high‐resolution (sub‐km) NWP models in future operations. Sub‐km models have better representation of meteorological processes, such as convection, which is explicitly represented and better resolved than in models with km‐scale grid spacing (Hanley et al, 2021; Stein et al, 2015; Woodhams et al, 2021). Features such as orographic enhancement, cold pools in valleys (Vosper et al, 2014), fog (Smith et al, 2021), gust fronts and downdrafts are better simulated as the grid scale becomes closer to the length scales of meteorological processes in the real world.…”

Observation and Process Understanding of Typical Cloud Holes Above Lakes Over the Tibetan Plateau

“…Finally, the lake breeze in C1 is thought to account for the observed cloud ring around the lake. The cool lake breeze is like a front, called the lake breeze front (Woodhams et al., 2021). When the front makes landfall, cool air enters the bottom and the original warm air is lifted up.…”

“…Tsujimoto and Koike (2013) show that there are more clouds over a Cambodian lake than surrounding land in the early morning during December. In Lake Victoria, an upheaval of the contour line of humidity and much higher lightning density is observed over the lake than on land during the night (Roberts et al., 2022; Woodhams et al., 2021). The gradient of temperature between lake and land drives this lake‐land breeze.…”