On 28 February 2018, 57 mm of precipitation associated with a warm conveyor belt (WCB) fell within 21 h over South Korea. To investigate how the large-scale circulation influenced the microphysics of ...this intense precipitation event, we used radar measurements, snowflake photographs and radiosounding data from the International Collaborative Experiments for Pyeongchang 2018 Olympic and Paralympic Winter Games (ICE-POP 2018).
The WCB was identified with trajectories computed with analysis wind fields from the Integrated Forecast System global atmospheric model. The WCB was collocated with a zone of enhanced wind speed of up to 45 m s−1 at 6500 m a.s.l., as measured by a radiosonde and a Doppler radar. Supercooled liquid water (SLW) with concentrations exceeding 0.2 g kg−1 was produced during the rapid ascent within the WCB. During the most intense precipitation period, vertical profiles of polarimetric radar variables show a peak and subsequent decrease in differential reflectivity as aggregation starts. Below the peak in differential reflectivity, the specific differential phase shift continues to increase, indicating early riming of oblate crystals and secondary ice generation. We hypothesise that the SLW produced in the WCB led to intense riming. Moreover, embedded updraughts in the WCB and turbulence at its lower boundary enhanced aggregation by increasing the probability of collisions between particles.
This suggests that both aggregation and riming occurred prominently in this WCB. This case study shows how the large-scale atmospheric flow of a WCB provides ideal conditions for rapid precipitation growth involving SLW production, riming and aggregation. Future microphysical studies should also investigate the synoptic conditions to understand how observed processes in clouds are related to large-scale circulation.
Secondary ice production (SIP) has an essential role in cloud and precipitation microphysics.
In recent years, substantial insights were gained into SIP by combining experimental, modeling, and ...observational approaches. Remote sensing instruments, among them meteorological radars, offer the possibility of studying clouds and precipitation in extended areas over long time periods and are highly valuable to understand the spatiotemporal structure of microphysical processes.
Multi-modal Doppler spectra measured by vertically pointing radars reveal the coexistence, within a radar resolution volume, of hydrometeor populations with distinct properties;
as such, they can provide decisive insight into precipitation microphysics.
This paper leverages polarimetric radar Doppler spectra as a tool to study the microphysical processes that took place during a snowfall event on 27 January 2021 in the Swiss Jura Mountains during the ICE GENESIS campaign.
A multi-layered cloud system was present, with ice particles sedimenting through a supercooled liquid water (SLW) layer in a seeder–feeder configuration.
Building on a Doppler peak detection algorithm, we implement a peak labeling procedure to identify the particle type(s) that may be present within a radar resolution volume.
With this approach, we can visualize spatiotemporal features in the radar time series that point to the occurrence of distinct mechanisms during different stages of the event.
By focusing on three 30 min phases of the case study and by using the detailed information contained in the Doppler spectra, together with dual-frequency radar measurements, aircraft in situ images, and simulated profiles of atmospheric variables, we narrow down the possible processes that could be responsible for the observed signatures.
Depending on the availability of SLW and the droplet sizes, on the temperature range, and on the interaction between the liquid and ice particles, various SIP processes are identified as plausible, with distinct fingerprints in the radar Doppler spectra. A simple modeling approach suggests that the ice crystal number concentrations likely exceed typical concentrations of ice-nucleating particles by 1 to 4 orders of magnitude.
While a robust proof of occurrence of a given SIP mechanism cannot be easily established, the multi-sensor data provide various independent elements each supporting the proposed interpretations.
Modelling and forecasting wind-driven redistribution of
snow in mountainous regions with its implications on avalanche danger,
mountain hydrology or flood hazard is still a challenging task often ...lacking
in essential details. Measurements of drifting and blowing snow for
improving process understanding and model validation are typically limited
to point measurements at meteorological stations, providing no information
on the spatial variability of horizontal mass fluxes or even the vertically
integrated mass flux. We present a promising application of a compact and
low-cost radar system for measuring and characterizing larger-scale
(hundreds of metres) snow redistribution processes, specifically blowing
snow off a mountain ridge. These measurements provide valuable information
of blowing snow velocities, frequency of occurrence, travel distances and
turbulence characteristics. Three blowing snow events are investigated, two
in the absence of precipitation and one with concurrent precipitation.
Blowing snow velocities measured with the radar are validated by comparison
against wind velocities measured with a 3D ultra-sonic anemometer. A minimal
blowing snow travel distance of 60–120 m is reached 10–20 % of the
time during a snow storm, depending on the strength of the storm event. The
relative frequency of transport distances decreases exponentially above the
minimal travel distance, with a maximum measured distance of 280 m. In a
first-order approximation, the travel distance increases linearly with the
wind velocity, allowing for an estimate of a threshold wind velocity for
snow particle entrainment and transport of 7.5–8.8 m s−1, most
likely depending on the prevailing snow cover properties. Turbulence
statistics did not allow a conclusion to be drawn on whether low-level,
low-turbulence jets or highly turbulent gusts are more effective in
transporting blowing snow over longer distances, but highly turbulent flows
are more likely to bring particles to greater heights and thus influence
cloud processes. Drone-based photogrammetry measurements of the spatial snow
height distribution revealed that increased snow accumulation in the lee of the
ridge is the result of the measured local blowing snow conditions.
Polarimetric radar systems are commonly used to study the microphysics of precipitation. While they offer continuous measurements with a large spatial coverage, retrieving information about the ...microphysical processes that govern the evolution of snowfall from the polarimetric signal is challenging. The present study develops a new method, called process identification based on vertical gradient signs (PIVSs), to spatially identify the occurrence of the main microphysical processes (aggregation and riming, crystal growth by vapor deposition and sublimation) in snowfall from dual-polarization Doppler radar scans. We first derive an analytical framework to assess in which meteorological conditions the local vertical gradients of radar variables reliably inform about microphysical processes. In such conditions, we then identify regions dominated by (i) vapor deposition, (ii) aggregation and riming and (iii) snowflake sublimation and possibly snowflake breakup, based on the sign of the local vertical gradients of the reflectivity ZH and the differential reflectivity ZDR. The method is then applied to data from two frontal snowfall events, namely one in coastal Adélie Land, Antarctica, and one in the Taebaek Mountains in South Korea. The validity of the method is assessed by comparing its outcome with snowflake observations, using a multi-angle snowflake camera, and with the output of a hydrometeor classification, based on polarimetric radar signal. The application of the method further makes it possible to better characterize and understand how snowfall forms, grows and decays in two different geographical and meteorological contexts. In particular, we are able to automatically derive and discuss the altitude and thickness of the layers where each process prevails for both case studies. We infer some microphysical characteristics in terms of radar variables from statistical analysis of the method output (e.g., ZH and ZDR distribution for each process). We, finally, highlight the potential for extensive application to cold precipitation events in different meteorological contexts.
Radar-based hydrometeor classification typically comes down
to determining the dominant type of hydrometeor populating a given radar
sampling volume. In this paper we address the subsequent problem ...of inferring
the secondary hydrometeor types present in a volume – the issue of
hydrometeor de-mixing. The present study relies on the semi-supervised
hydrometeor classification proposed by Besic et al. (2016) but nevertheless
results in solutions and conclusions of a more general character and
applicability. In the first part, oriented towards synthesis, a bin-based
de-mixing approach is proposed, inspired by the conventional coherent and
linear decomposition methods widely employed across different remote-sensing
disciplines. Intrinsically related to the concept of entropy, introduced in
the context of the radar hydrometeor classification in Besic et al. (2016), the
proposed method, based on the hypothesis of the reduced random interferences
of backscattered signals, estimates the proportions of different hydrometeor
types in a given radar sampling volume, without considering the neighboring
spatial context. Plausibility and performances of the method are evaluated
using C- and X-band radar measurements, compared with hydrometeor properties
derived from a Multi-Angle Snowflake Camera instrument. In the second part,
we examine the influence of the potential residual random interference
contribution in the backscattering from different hydrometeors populating a
radar sampling volume. This part consists in adapting and testing the
techniques commonly used in conventional incoherent decomposition methods to
the context of weather radar polarimetry. The impact of the residual
incoherency is found to be limited, justifying the hypothesis of the reduced
random interferences even in a case of mixed volumes and confirming the
applicability of the proposed bin-based approach, which essentially relies on
first-order statistics.
Intense snowfall sublimation was observed during a precipitation event over Davis in the Vestfold Hills, East Antarctica, from 08 to 10 January 2019. Radar observations and simulations from the ...Weather Research and Forecasting model revealed that orographic gravity waves (OGWs), generated by a north‐easterly flow impinging on the ice ridge upstream of Davis, were responsible for snowfall sublimation through a foehn effect. Despite the strong meridional moisture advection associated with an atmospheric river (AR) during this event, almost no precipitation reached the ground at Davis. We found that the direction of the synoptic flow with respect to the orography determined the intensity of OGWs over Davis, which in turn directly influenced the snowfall microphysics. We hypothesize that turbulence induced by the OGWs likely enhanced the aggregation process, as identified thanks to dual‐polarization and dual‐frequency radar observations. This study suggests that despite the intense AR, the precipitation distribution was determined by local processes tied to the orography. The mechanisms found in this case study could contribute to the extremely dry climate of the Vestfold Hills, one of the main Antarctic oases.
Plain Language Summary
A case study of a snowfall event over Davis, Antarctica is presented. Despite the strong precipitation, snowfall did not reach the ground due to intense sublimation (transition from solid to gas state). Meteorological radar observations and atmospheric model simulations revealed that a dry downslope wind was responsible for the sublimation of snowfall below the cloud base. Despite the intense transport of moisture associated with the low pressure system during this event, almost no precipitation reached the ground at Davis. We found that the wind direction with respect to the main ridge upstream of Davis determined the intensity of the sublimation. This study suggests that despite favorable large‐scale conditions for intense snowfall at Davis, local processes related to the topography determined how much precipitation reached the ground. The mechanisms found in this case study could contribute to the extremely dry climate of the Vestfold Hills, one of the main ice‐free regions of Antarctica.
Key Points
Despite intense moisture advection by an atmospheric river, foehn winds tied to orographic gravity waves (OGWs) led to snowfall sublimation
The flow direction determined the intensity of the foehn and hence the temporal and spatial precipitation variability
The event can be divided into three phases during which the features of the OGWs influenced the observed microphysics in distinct ways
Meridional atmospheric transport is an important process in the climate system and has implications for the availability of heat and moisture at high latitudes. Near-surface cold and warm temperature ...advection over the ocean in the context of extratropical cyclones additionally leads to important air–sea exchange. In this paper, we investigate the impact of these air–sea fluxes on the stable water isotope (SWI) composition of water vapour in the Southern Ocean's atmospheric boundary layer. SWIs serve as a tool to trace phase change processes involved in the atmospheric water cycle and, thus, provide important insight into moist atmospheric processes associated with extratropical cyclones. Here we combine a 3-month ship-based SWI measurement data set around Antarctica with a series of regional high-resolution numerical model simulations from the isotope-enabled numerical weather prediction model COSMOiso. We objectively identify atmospheric cold and warm temperature advection associated with the cold and warm sector of extratropical cyclones, respectively, based on the air–sea temperature difference applied to the measurement and the simulation data sets. A Lagrangian composite analysis of temperature advection based on the COSMOiso simulation data is compiled to identify the main processes affecting the observed variability of the isotopic signal in marine boundary layer water vapour in the region from 35 to 70∘ S. This analysis shows that the cold and warm sectors of extratropical cyclones are associated with contrasting SWI signals. Specifically, the measurements show that the median values of δ18O and δ2H in the atmospheric water vapour are 3.8 ‰ and 27.9 ‰ higher during warm than during cold advection. The median value of the second-order isotope variable deuterium excess d, which can be used as a measure of non-equilibrium processes during phase changes, is 6.4 ‰ lower during warm than during cold advection. These characteristic isotope signals during cold and warm advection reflect the opposite air–sea fluxes associated with these large-scale transport events. The trajectory-based analysis reveals that the SWI signals in the cold sector are mainly shaped by ocean evaporation. In the warm sector, the air masses experience a net loss of moisture due to dew deposition as they are advected over the relatively colder ocean, which leads to the observed low d. We show that additionally the formation of clouds and precipitation in moist adiabatically ascending warm air parcels can decrease d in boundary layer water vapour. These findings illustrate the highly variable isotopic composition in water vapour due to contrasting air–sea interactions during cold and warm advection, respectively, induced by the circulation associated with extratropical cyclones. SWIs can thus potentially be useful as tracers for meridional air advection and other characteristics associated with the dynamics of the storm tracks over interannual timescales.
This article describes a 4-month dataset of precipitation and cloud measurements collected during the International Collaborative Experiments for PyeongChang 2018 Olympic and Paralympic winter games ...(ICE-POP 2018). This paper aims to describe the data collected by the Environmental Remote Sensing Laboratory of the École Polytechnique Fédérale de Lausanne. The dataset includes observations from an X-band dual-polarisation Doppler radar, a W-band Doppler cloud profiler, a multi-angle snowflake camera and a two-dimensional video disdrometer (https://doi.org/10.1594/PANGAEA.918315, Gehring et al., 2020a). Classifications of hydrometeor types derived from dual-polarisation measurements and snowflake photographs are presented. The dataset covers the period from 15 November 2017 to 18 March 2018 and features nine precipitation events with a total accumulation of 195 mm of equivalent liquid precipitation. This represents 85 % of the climatological accumulation over this period. To illustrate the available data, measurements corresponding to the four precipitation events with the largest accumulation are presented. The synoptic situations of these events were contrasted and influenced the precipitation type and accumulation. The hydrometeor classifications reveal that aggregate snowflakes were dominant and that some events featured significant riming. The combination of dual-polarisation variables and high-resolution Doppler spectra with ground-level snowflake images makes this dataset particularly suited to study snowfall microphysics in a region where such measurements were not available before.
Abstract An international field experiment took place in the Swiss Jura in January 2021 as a milestone of the European ICE GENESIS project ( www.ice-genesis.eu/ ), which aims to better measure, ...understand, and model the ice/snow particle properties and mechanisms responsible for icing of rotor-craft and aircraft. The field campaign was designed to collect observations of clouds and snowfall at a prescribed range of temperatures (−10° to +2°C). The suite of in situ and remote sensing instruments included airborne probes and imagers on board a SAFIRE ATR-42 aircraft, able to sample liquid and ice particles from the micron to the millimeter size range, as well as icing sensors and cameras. Two 95 GHz Doppler cloud radars were installed on the SAFIRE ATR-42, while six Doppler weather radars operating at frequencies ranging from 10 to 95 GHz (and one lidar) were ground based. An operational polarimetric weather radar in nearby France (Montancy) complements the coverage. Finally, observations of standard meteorological variables as well as high-resolution pictures of falling snowflakes from a multiangle snowflake camera were collected at the ground level. The campaign showed its full potential during five (multihourly) flights where precipitation was monitored from cloud to ground. The originality of this campaign resides in the targeted specific temperature range for snowfall and in the synchronization between the ground-based remote sensing and the aircraft trajectories designed to maximize the collection of in situ observations within the column above the radar systems.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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