Microphysics and dynamics of snowfall associated with a warm conveyor belt over Korea

Gehring, Josué; Oertel, Annika; Vignon, Étienne; Jullien, Nicolas; Bešič, Nikola; Berne, Alexis

doi:10.5194/acp-20-7373-2020

Cited by 36 publications

(41 citation statements)

References 52 publications

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“…While slantwise WCB ascent leads to large-scale stratiform precipitation and the formation of widespread regions with low-PV air at upper levels, convective WCB ascent goes along with peaks of particularly strong surface precipitation and the formation of mesoscale upper-level PV dipoles, including regions with negative PV (Oertel et al, 2020). Finally, Gehring et al (2020) investigated the snowfall microphysical processes in a strongly precipitating wintertime WCB over the Korean Peninsula that was observed with radar data, radio soundings and snowflake photographs. They showed how the WCB created ideal conditions for rapid precipitation growth, including the formation of supercooled liquid water in the strongly ascending air masses, which favoured intense riming and aggregation.…”

Section: Introductionmentioning

confidence: 99%

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Vertical cloud structure of warm conveyor belts – a comparison and evaluation of ERA5 reanalysis, CloudSat and CALIPSO data

Binder

Boettcher

Joos

et al. 2020

Weather Clim. Dynam.

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Abstract. Warm conveyor belts (WCBs) are important cyclone-related airstreams that are responsible for most of the cloud and precipitation formation in the extratropics. They can also substantially influence the dynamics of cyclones and the upper-level flow. So far, most of the knowledge about WCBs is based on model data from analyses, reanalyses and forecast data with only a few observational studies available. The aim of this work is to gain a detailed observational perspective on the vertical cloud and precipitation structure of WCBs during their inflow, ascent and outflow and to evaluate their representation in the new ERA5 reanalysis dataset. To this end, satellite observations from the CloudSat radar and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar are combined with an ERA5-based WCB climatology for nine Northern Hemisphere winters. Based on a case study and a composite analysis, the main findings can be summarized as follows. (i) WCB air masses are part of deep, strongly precipitating clouds, with cloud-top heights at 9–10 km during their ascent and an about 2–3 km deep layer with supercooled liquid water co-existing with ice above the melting layer. The maximum surface precipitation occurs when the WCB is at about 2–4 km height. (ii) Convective clouds can be observed above the inflow and during the ascent. (iii) At upper levels, the WCB outflow is typically located near the top of a 3 km deep cirrus layer. (iv) There is a large variability between WCBs in terms of cloud structure, peak reflectivity and associated surface precipitation. (v) The WCB trajectories with the highest radar reflectivities are mainly located over the North Atlantic and North Pacific, and – apart from the inflow – they occur at relatively low latitudes. They are associated with particularly deep and strongly precipitating clouds that occur not only during the ascent but also in the inflow and outflow regions. (vi) ERA5 represents the WCB clouds remarkably well in terms of position, thermodynamic phase and frozen hydrometeor distribution, although it underestimates the high ice and snow values in the mixed-phase clouds near the melting layer. (vii) In the lower troposphere, high potential vorticity is diabatically produced along the WCB in areas with high reflectivities and hydrometeor contents, and at upper levels, low potential vorticity prevails in the cirrus layer in the WCB outflow. The study provides important observational insight into the internal cloud structure of WCBs and emphasizes the ability of ERA5 to essentially capture the observed pattern but also reveals many small- and mesoscale structures observed by the remote sensing instruments but not captured by ERA5.

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Section: Introductionmentioning

confidence: 99%

“…the tropopause and the downstream weather evolution (e.g. Wernli and Davies, 1997;Grams et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

Vertical cloud structure of warm conveyor belts – a comparison and evaluation of ERA5 reanalysis, CloudSat and CALIPSO data

Binder

Boettcher

Joos

et al. 2020

Weather Clim. Dynam.

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“…Before performing these conversions, radar reflectivity was corrected for attenuation due to absorption by atmospheric gases and cloud liquid water and to scattering by ice particles. Absorption by atmospheric gases is calculated based on Rosenkranz (1998) for water vapor and Schwartz (1998) for oxygen with input for geophysical parameters interpolated from the Modern Era Reanalysis for Research and Applications Version 2 (MERRA-2) (Gelaro et al, 2017). Absorption by cloud liquid water is computed using the liquid water path derived by the method described later in this section and assuming cloud liquid water uniformly distributed vertically in the radar echo layer.…”

Section: Retrieved Microphysical Variablesmentioning

confidence: 99%

“…They showed that wet-bulb temperature is a key parameter for separating solid and liquid precipitation, and the low-level temperature lapse rate also affects the precipitation phase. Geophysical parameters from MERRA-2 reanalysis (Gelaro et al, 2017) were used in this study as input for the Sims and Liu (2015) scheme. In addition, we use a near-surface reflectivity higher than −20 dBZ as the criterion for snowfall detection; all radar data analyzed for snow clouds in the following sections have a near-surface radar reflectivity greater than −20 dBZ.…”

Section: Snow Cloud Detectionmentioning

confidence: 99%

Microphysical properties of three types of snow clouds: implication for satellite snowfall retrievals

et al. 2020

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Abstract. Ground-based radar and radiometer data observed during the 2017–2018 winter season over the Pyeongchang area on the east coast of the Korean Peninsula were used to simultaneously estimate both the cloud liquid water path and snowfall rate for three types of snow clouds: near-surface, shallow, and deep. Surveying all the observed data, it is found that near-surface clouds are the most frequently observed cloud type with an area fraction of over 60 %, while deep clouds contribute the most in snowfall volume with about 50 % of the total. The probability distributions of snowfall rates are clearly different among the three types of clouds, with the vast majority hardly reaching 0.3 mm h−1 (liquid water equivalent snowfall rate) for near-surface, 0.5 mm h−1 for shallow, and 1 mm h−1 for deep clouds. However, the liquid water paths in the three types of clouds all have the substantial probability to reach 500 g m−2. There is no clear correlation found between snowfall rate and the liquid water path for any of the cloud types. Based on all observed snow profiles, brightness temperatures at Global Precipitation Measurement Microwave Imager (GPM/GMI) channels are simulated, and the ability of a Bayesian algorithm to retrieve snowfall rate is examined using half the profiles as observations and the other half as an a priori database. Under an idealized scenario, i.e., without considering the uncertainties caused by surface emissivity, ice particle size distribution, and particle shape, the study found that the correlation as expressed by R2 between the “retrieved” and “observed” snowfall rates is about 0.32, 0.41, and 0.62, respectively, for near-surface, shallow, and deep snow clouds over land surfaces; these numbers basically indicate the upper limits capped by cloud natural variability, to which the retrieval skill of a Bayesian retrieval algorithm can reach. A hypothetical retrieval for the same clouds but over ocean is also studied, and a major improvement in skills is found for near-surface clouds with R2 increasing from 0.32 to 0.52, while a smaller improvement is found for shallow and deep clouds. This study provides a general picture of the microphysical characteristics of the different types of snow clouds and points out the associated challenges in retrieving their snowfall rate from passive microwave observations.

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“…In radar observations it has been identified through increased horizontal heterogeneity of the radar reflectivity, the absence of a well-defined bright band and the occurrence of narrow plumes of enhanced radar reflectivity (Neiman et al, 1993;Crespo and Posselt, 2016;Oertel et al, 2019). Compared to deep convective storms, where reflectivities exceed 40-50 dBZ in the upper troposphere and updrafts exceed 10-15 m s −1 (e.g., Carbone, 1982;Miller et al, 1988;Steiner et al, 1995), the ascent within embedded convection was reported to be slower with 1-5 m s −1 with radar reflectivities reaching approximately 20-30 dBZ (Crespo and Posselt, 2016;Oertel et al, 2019;Binder et al, 2020;Gehring et al, 2020;Blanchard et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Oertel¹,

Sprenger²,

Joos³

et al. 2020

Preprint

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Abstract. Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB's large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposhere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyses the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact EXperiment (NAWDEX) field campaign (IOPs 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behaviour of the WCB trajectories reveals that very intense convection with ascent rates of 600 hPa in 30–60 min occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection – moderate and intense convection – is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: Intense convection produces locally very intense surface precipitation with peak values exceeding 6 mm in 15 minutes and large amounts of graupel of up to 2.8 g kg−1 in the middle troposphere (compared to 3.9 mm and 1.0 g kg−2 for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study – in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet – the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications.

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Microphysics and dynamics of snowfall associated with a warm conveyor belt over Korea

Cited by 36 publications

References 52 publications

Vertical cloud structure of warm conveyor belts – a comparison and evaluation of ERA5 reanalysis, CloudSat and CALIPSO data

Vertical cloud structure of warm conveyor belts – a comparison and evaluation of ERA5 reanalysis, CloudSat and CALIPSO data

Microphysical properties of three types of snow clouds: implication for satellite snowfall retrievals

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

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