Atmos. Chem. Phys., 21, 11955–11978, 2021 https://doi.org/10.5194/acp-21-11955-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Impact of wind pattern and complex topography on snow microphysics during International Collaborative Experiment for PyeongChang 2018 Olympic and Paralympic winter games (ICE-POP 2018) Kwonil Kim1, Wonbae Bang1, Eun-Chul Chang2, Francisco J. Tapiador3, Chia-Lun Tsai1, Eunsil Jung4, and Gyuwon Lee1 1Department of Astronomy and Atmospheric Sciences, Center for Atmospheric REmote sensing (CARE), Kyungpook National University, Daegu, Republic of Korea 2Department of Atmospheric Sciences, Kongju National University, Gongju, Republic of Korea 3Earth and Space Sciences Research Group, Institute of Environmental Sciences, University of Castilla-La Mancha, Toledo, Spain 4Department of Advanced Science and Technology Convergence, Kyungpook National University, Sangju, Republic of Korea Correspondence: Gyuwon Lee ([email protected]) Received: 13 February 2021 – Discussion started: 12 March 2021 Revised: 23 June 2021 – Accepted: 6 July 2021 – Published: 10 August 2021 Abstract. Snowfall in the northeastern part of South Ko- ward side, resulting in significant aggregation in the coastal rea is the result of complex snowfall mechanisms due to a region, with riming featuring as a primary growth mechanism highly contrasting terrain combined with nearby warm wa- in both mountainous and coastal regions. The cold-low pat- ters and three synoptic pressure patterns. All these factors to- tern is characterized by a higher snowfall rate and vertically gether create unique combinations, whose disentangling can deep systems in the mountainous region, with the precipi- provide new insights into the microphysics of snow on the tation system becoming shallower in the coastal region and planet. This study focuses on the impact of wind flow and to- strong turbulence being found in the layer below 2 km in the pography on the microphysics drawing of 20 snowfall events mountainous upstream region (linked with dominant aggre- during the ICE-POP 2018 (International Collaborative Ex- gation). The warm-low pattern features the deepest system: periment for PyeongChang 2018 Olympic and Paralympic precipitation here is enhanced by the seeder–feeder mecha- winter games) field campaign in the Gangwon region. The nism with two different precipitation systems divided by the vertical structure of precipitation and size distribution char- transition zone (easterly below and westerly above). Overall, acteristics are investigated with collocated MRR (micro rain it is found that strong shear and turbulence in the transition radar) and PARSIVEL (particle size velocity) disdrometers zone is a likely reason for the dominant riming process in installed across the mountain range. The results indicate that the mountainous region, with aggregation being important in wind shear and embedded turbulence were the cause of the both mountainous and coastal regions. riming process dominating the mountainous region. As the strength of these processes weakens from the mountainous region to the coastal region, riming became less significant and gave way to aggregation. This study specifically analyzes 1 Introduction the microphysical characteristics under three major synoptic patterns: air–sea interaction, cold low, and warm low. Air–sea Understanding the developing mechanism of heavy snowfall interaction pattern is characterized by more frequent snow- in the eastern part of the Korean Peninsula (Gangwon region) fall and vertically deeper precipitation systems on the wind- could have a great impact on improving the accuracy of fore- Published by Copernicus Publications on behalf of the European Geosciences Union. 11956 K. Kim et al.: Impact of wind pattern and complex topography on snow microphysics casts in winter. The snowfall in this region is known to be different to the lake-effect snow (Laird et al., 2009; Minder characterized by strong orographic effects due to complex et al., 2015; Kristovich et al., 2017; Wiley and Mercer, 2020) terrain in the west (Taebaek Mountains) and by the air-mass except in that the source of moisture and heat fluxes is here transformation in the nearby ocean in the east (East Sea). The the ocean and not a lake. orographic enhancement and sea effect induce heavy snow- It is also known that low-pressure systems passing through fall in this region (Chung et al., 2004; Cheong et al., 2006; the southern part of the Korean Peninsula support the easterly Cho and Kwon, 2012; Jung et al., 2012, 2014). The com- or northeasterly flow over the East Sea in the later part of the plexity of the snowfall mechanism arises from the steepness low-pressure passage and supply abundant moisture from the of terrain from the Taebaek Mountains to the ocean (hori- Southern Ocean, leading to heavier snowfall (Seo and Jhun, zontal distance of about 20 km and vertical heights of 1000– 1991; Park et al., 2009; Gehring et al., 2020). Song et al. 2000 m) in combination with air-mass transformation over (2016) classified the synoptic environments when Siberian the warm ocean. Thus, complex airflows of this region affect highs expand eastward with and without low-pressure sys- microphysics, in particular the snow growth process above tems. They showed that the mean precipitation amount in- the melting layer. creased by about 45 % in the presence of both Kaema high The details of the interactions between airflows and mi- and low pressure. crophysical processes have been partially investigated in the The complex and steep orography of the Taebaek Moun- past. Indeed, it is well recognized that the dynamics associ- tain ranges is another important factor for both the dynamics ated with the shear have important implications for two pri- and the precipitation system (amount, distribution, and du- mary snow growth processes (i.e., aggregation and riming). ration). The orographic effect on airflow and precipitation is Under strong shear conditions, the turbulent cell and induced difficult to characterize because of the many different mech- updraft are favored, being responsible for the production of anisms, as many as 20, following Houze(2012). Yu et al. a considerable amount of supercooled water that promotes (2007) also pointed out that multiple precipitation cells pro- both aggregation and riming as they increase the probability duced by different mechanisms can appear simultaneously in of collision between hydrometeors, in particular, snowflakes complex terrain. and supercooled water droplets (Houze and Medina, 2005; Previous studies on the impact of the Taebaek Mountains Barnes et al., 2018). Colle et al.(2014) also investigated the in the precipitation systems were carried out mainly by nu- relationship between degree of riming and radar measurables merical simulation. Lee and Kim(2008) examined the effect (Doppler radial velocity, spectrum width, etc) from vertically of the Taebaek Mountains on the distribution and intensity pointing radar and determined the degree of riming every 15– of snowfall by removing their topography in numerical ex- 30 min for 12 cyclone events. They reported turbulent mo- periments. They showed that, with the mountains, updraft tions and strong updrafts and downdrafts in heavy riming is strengthened from 0.2 to 1.2 ms−1, and the accumulated events. Further insight into the role of the warm conveyor belt precipitation amount increases 8.5 times more in mountain- and its dynamics on the snow microphysics in the Gangwon ous areas than the experiment without mountains. They also region was provided by Gehring et al.(2020), who argued found that a topographic blocking by the Taebaek Mountains that ascents in the warm conveyor belt contributed to per- causes horizontal convergence and stronger updraft along sisting production of supercooled liquid water and that the the coastal region, resulting in heavy snowfall. Lee and Lee process enhances the riming process during extreme snow- (2003) also investigated the effect of the Taebaek Moun- fall events. tains by numerical simulation for two heavy snowfall events. The mean airflow governed by the synoptic patterns con- They reported that the Froude number, which can represent trols snowfall in the region. Numerous studies have reported the blocking degree, is important to determine whether the that favorable synoptic conditions for heavy snowfall are the precipitation amount is more in the coastal region or in the eastern expansion of Siberian high pressure and a cyclone mountainous region. An insufficient Froude number favors passing through the central or southern part of the Korean an occurrence of convergence between blocked and envi- Peninsula (Lee and Kim, 2008; Park et al., 2009; Jung et al., ronmental flow and a shift in maximum precipitation to the 2012; Cho and Chang, 2017; J. Kim et al., 2019). The eastern coastal area (Ohigashi et al., 2014; Veals et al., 2020). On expansion of Siberian high pressure (the so-called “Kaema the other hand, when the Froude number is large enough, the high pressure”) is responsible for snowstorms by air-mass maximum precipitation tends to occur over the mountainous transformation, which is induced by cold easterly or north- or inland area (Veals et al., 2020). easterly flow over the East Sea. Sea-effect snow (Estoque It is also known that the wind flow plays a critical role and Ninomiya, 1976; Nakamura and Asai, 1985; Ikeda et al., in the precipitation patterns of this region. In particular, 2009; Kindap, 2010; Nam et al., 2014; Bao and Ren, 2018; easterly flow or northeasterly (or sometimes northerly) flow Steenburgh and Nakai, 2020) is the same kind of mesoscale is thought to be necessary for the development of heavy precipitation caused by the air-mass transformation of cold snowfall (Nam et al., 2014; Cho and Chang, 2017; Tsai air over warm ocean. It develops over the East Sea and affects et al., 2018; J. Kim et al., 2019). The term Korea easter- downstream of the wind flow. This mechanism is not greatly lies (Kor’easterlies hereafter) was proposed by Park and Park Atmos. Chem. Phys., 21, 11955–11978, 2021 https://doi.org/10.5194/acp-21-11955-2021 K.