Wintertime precipitation over the Australian Snowy Mountains: Observations from an Intensive Field Campaign 2018

Abstract

Understanding the key dynamical and microphysical mechanisms driving precipitation in the Snowy Mountains region of southeast Australia, including the role of orography, can help improve precipitation forecasts, which is of great value for efficient water management. An intensive observation campaign was carried out during the 2018 austral winter, providing a comprehensive range of ground-based observations across the Snowy Mountains. We used data from three vertically pointing rain radars, cloud radar, a PARSIVEL disdrometer, and a network of 76 pluviometers. The observations reveal that all of the precipitation events were associated with cold front passages. About half accumulated during the frontal passage associated with deep, fully glaciated cloud tops; while the rest occurred in the post-frontal environment and was associated with clouds with supercooled liquid water (SLW) tops. About three quarters of the accumulated precipitation were observed under blocked conditions, likely associated with blocked stratiform orographic enhancement. Specifically, more than a third of the precipitation resulted from moist cloudless air being lifted over stagnant air, upwind from the barrier, creating SLW-top clouds. These SLW-clouds then produced stratiform precipitation mostly over the upwind slopes and mountain tops, with hydrometeors reaching the mountain tops mostly as rimed snow. Two precipitation events were studied in detail, which showed that during unblocked conditions, orographic convection invigoration and unblocked stratiform enhancement were the two main mechanisms driving the precipitation; with the latter being more prevalent after the frontal passage. During these events, ice particle growth was likely dominated by vapor deposition and aggregation during the frontal periods, while riming dominated during the post-frontal periods.

Radar‐Derived Snowfall Microphysical Properties at Davis, Antarctica

“…Maahn and Kollias (2012) developed an algorithm to improve the sensitivity of the older MRR-2 instrument from a minimum of 3 dBZ to −10 dBZ, allowing for a much improved detection of snowfall. We implemented a modified version of the Maahn and Kollias (2012) code for our MRR-PRO instrument, using the method developed by Ackermann et al (2021).…”

“…We implemented a modified version of the Maahn and Kollias (2012) code for our MRR‐PRO instrument, using the method developed by Ackermann et al. (2021).…”

Microphysical mechanisms of wintertime postfrontal precipitation enhancement over the Australian Snowy Mountains

“…Air masses arriving from the Southern Ocean are often highly pristine before they cross into southeastern Australia (Huang et al, 2017, 2021), featuring few ice‐nucleating particles (INP, McCluskey et al, 2018). These weather systems are renowned for favoring the generation of supercooled liquid water (SLW)‐topped clouds across high‐elevation regions of southeastern Australia and Tasmania, when the pristine postfrontal maritime air mass brings moisture from the Southern Ocean and encounters the orographic barriers (Ackermann et al, 2021; Chubb et al, 2012; Morrison et al, 2013; Osburn et al, 2016). Cloud microphysical processes associated with those postfrontal clouds and precipitation have been considered by previous observational studies.…”

“…Cloud microphysical processes associated with those postfrontal clouds and precipitation have been considered by previous observational studies. In particular, observations from a 2018 ASM wintertime intensive field campaign showed that the postfrontal clouds precipitate primarily due to riming processes (Ackermann et al, 2021). Prolonged precipitation events were mostly associated with stratiform ascent, while the cloud thickness and precipitation intensity were highly sensitive to the potential blocking and stability of the low‐level upstream flow (Ackermann et al, 2021; Manton et al, 2017; Sarmadi et al, 2017).…”

“…In particular, observations from a 2018 ASM wintertime intensive field campaign showed that the postfrontal clouds precipitate primarily due to riming processes (Ackermann et al, 2021). Prolonged precipitation events were mostly associated with stratiform ascent, while the cloud thickness and precipitation intensity were highly sensitive to the potential blocking and stability of the low‐level upstream flow (Ackermann et al, 2021; Manton et al, 2017; Sarmadi et al, 2017). Gevorgyan et al (2022) showed that the crossbarrier component of the Integrated Vapor Transport (IVT) of the upstream flow can also be used to explain the enhancement of precipitation over the windward slopes of the ASM.…”