Yesterday Hadleigh and I (Hilary) arrived at our field site at 1800 UTC for the first shift and it was completely sunny with a blue sky! Conditions were much the same throughout the day with a couple brief periods of rain.
Temperatures were around 15°C. Clouds were variable throughout the day and at certain points it looked like there was going to be good convection moving in from the north and that it would come right above us, however it always moved around to the west towards the mountain ridge. No big convection for us! Winds were light with some moderate gusts. Overall it was a very lovely day to spend sitting outside.
Selina and Cécile took over from us around 2200 UTC. They reported that conditions were quite nice during the evening and there was no precipitation. Winds were very light with a few moderate gusts and scattered clouds. They left the field site at 0300 UTC on May 9 after spending a lovely evening.
Grizzly bears were spotted by both shifts on the way to and from the site along the Sinclair Pass.
André and Aurélie arrived at Fortress station at 1950 UTC as we were expecting some convective cells from then on through to the afternoon and until the evening.
A few minutes after our arrival, graupel was observed for a short time period, and afterwards we had a beautiful sunny and blue sky. We started taking the observations at 2000 UTC until 0200 UTC as convective precipitation and clear weather were constantly alternating. As a matter of fact, the same weather patterns were observed repeatedly along the convective cells crossover.
Coming from the north, convective cells were bringing large grey clouds, progressively bringing overcast skies towards the South.
Sky over the station was then covered throughout this process. Associated fog was often observed at the tops of mountains, both on west and east side of the valley.
Wind speed increased, but never exceeded medium intensity.
Presence of precipitation was observed during those convective events, while intensity and hydrometeor type varied from one episode to another.
The systems evolved as bright blue and sunny skies were appearing from the north, dissipating clouds and progressively taking the place of precipitations clouds.
The strongest precipitation event happened around 2220 UTC. Aggregates more than 1 cm and rimed dendrites were observed (see figure below). Hydrometeors were photographed, and the precipitation rate was such that the velvet pad was completely covered in 8 sec.
Other precipitation events occurred at 2000 and 2330 UTC, respectively, characterized by a mix of graupel, large aggregates, rimed and irregular dendrites and by a mix of aggregates, dendrites and rimed dendrites.
We took the opportunity during one of these calm atmosphere and sunny sky episodes to paint the wood board white, which is used to measure precipitation accumulation, so that the albedo of the board is similar to that of snow.
As it was yesterday, high temperatures have made photography difficult, especially for early episodes of dendrites and aggregates which were melting too fast, even with the camera set-up outside the tent.
Graupel was also hard to photograph as they were frequently bouncing off the velvet pad.
We conducted snow-water-equivalent (SWE) measurements requiring the use of ground-snow cores. We saw the different layers of the snowpack created by the last precipitation episodes (see the figures below).
We also noted that the melting over the last few warm days led to the enhancement of the snowpack density, resulting in a SWE larger than 100 mm, corresponding to the maximum measurement that our instrument could record.
Juris and Charlie spent the afternoon at the Coldwater Lab to upload the data. Afterwards they went to Fortress Junction Service to download data and to look at the instrument box where the ECCC brick computer is housed. They arrived at 0040 UTC to take over observations so Andre and Aurelie could leave.
No precipitation was observed since their arrival at 0250 UTC. Skies started to clear and Strathmore radar showed dissipating convection. Some cells were observed to the north, but were slow moving and decreasing in intensity and would bring minimal precipitation if it were to pass over. Consequently, both sides decided it was time to head home and call it a day
- Aurélie Desroches-Lapointe
Field Participants: Aurélie Desroches-Lapointe, André Bertoncini, Charlie Hebert-Pinard & Juris Almonte