Arctic observations and numerical simulations of surface wind effects on Multi-Angle Snowflake Camera measurements

Abstract

Abstract. Ground-based measurements of frozen precipitation are heavily influenced by interactions of surface winds with gauge-shield geometry. The Multi-Angle Snowflake Camera (MASC), which photographs hydrometeors in free-fall from three different angles while simultaneously measuring their fall speed, has been used in the field at multiple midlatitude and polar locations both with and without wind shielding. Here, we present an analysis of Arctic field observations – with and without a Belfort double Alter shield – and compare the results to computational fluid dynamics (CFD) simulations of the airflow and corresponding particle trajectories around the unshielded MASC. MASC-measured fall speeds compare well with Ka-band Atmospheric Radiation Measurement (ARM) Zenith Radar (KAZR) mean Doppler velocities only when winds are light (≤5ms-1) and the MASC is shielded. MASC-measured fall speeds that do not match KAZR-measured velocities tend to fall below a threshold value that increases approximately linearly with wind speed but is generally <0.5ms-1. For those events with wind speeds ≤1.5ms-1, hydrometeors fall with an orientation angle mode of 12∘ from the horizontal plane, and large, low-density aggregates are as much as 5 times more likely to be observed. Simulations in the absence of a wind shield show a separation of flow at the upstream side of the instrument, with an upward velocity component just above the aperture, which decreases the mean particle fall speed by 55 % (74 %) for a wind speed of 5 m s−1 (10 m s−1). We conclude that accurate MASC observations of the microphysical, orientation, and fall speed characteristics of snow particles require shielding by a double wind fence and restriction of analysis to events where winds are light (≤5ms-1). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined.

Raindrop fall velocity in turbulent flow: an observational study

“…7a) and increase nearly linearly for u < 2 m s −1 . The latter two points are consistent with Fornari et al (2016) who used direct numerical simulation (DNS) for heavy rigid spheres in turbulent flow. They state that ".…”

“…This non-linear drag force has a vertical component which "slows" down on average the drop settling speed. Now this non-linear drag is present even in non-turbulent flows but there is an enhancement in turbulent flow which goes as (u /V t ) 2 (Fornari et al, 2016). Figure 5a shows another example of a rain event (08:00-09:00 UTC on 9 April 2020) where the fall speeds of the 3 mm drops on average are close to the terminal fall speed shown by the black dashed line.…”

“…7a). Fornari et al (2016) also obtained /V t of around 10 % for u /V t in the range 0.2 to 0.3 but they did not model the turbulent scales reflective of atmospheric turbulence.…”

“…minimum of 80 at 23:45 UTC and its shape is anti-correlated with ε. From the theory and numerical calculations in Fornari et al (2016), when S v > 1 then gravity dominates the settling behaviour of heavy particles falling in the turbulent flow leading to reduction of the mean fall (settling) velocity relative to the terminal velocity in still air with the fractional reduction depending on the turbulent intensity (u /V t ) as well as the Re. For 2 (3 mm) drop diameters the corresponding Re is 800 (1600) using the terminal velocity.…”

Bringing Microphysics to the Masses: The Blowing Snow Observations at the University of North Dakota: Education through Research (BLOWN-UNDER) Campaign

“…Further, many of the current instruments are flawed when it comes to sampling snowfall or blizzard environments. Accurate sizing of a broad range of particles during falling or blowing snow events is an issue for instruments like the Parsivel 2 (Battaglia et al 2010;Loeb and Kennedy 2021) while instruments with enclosed sampling volumes like the MASC raise the likelihood of interactions with the environmental wind field (Fitch et al 2021 The project also tested the recently upgraded balloon-borne Particle Size and Velocity (PASIV)…”