Introduction
Cloud microphysical properties (e.g., phase composition, cloud particle number
concentrations and size distributions) next to dynamical processes are key
parameters for the cloud's lifetime, the cloud extent and the intensity of
precipitation they produce . In particular, orographic
precipitation plays a crucial role for the world's water resources, as the
headwaters of many rivers are located in alpine regions . In the
midlatitudes, mixed-phase clouds (MPCs) consisting of a mixture of ice
crystals and supercooled liquid droplets produce 30 to 50 % of liquid
precipitation due to the rapid growth of ice crystals to
precipitation size in the presence of supercooled liquid droplets. This is
due to a higher saturation vapor pressure over liquid water than over ice, and
thus ice crystals grow at the expense of evaporating cloud droplets. This
process was first described in the works of , and
and is referred to as the Wegener–Bergeron–Findeisen (WBF)
process. As such, correctly representing the fraction of ice in orographic
MPCs is crucial for accurate weather and water resource predictions in alpine
terrain.
In situ measurements are important to further improve our understanding of
the microphysical properties and fundamental processes of orographic MPCs
and are frequently conducted at mountain-top research stations.
Despite an improved understanding of the origin of ice crystals from
nucleation and secondary
ice-multiplication processes , the source of most of the ice
crystals observed at mountain-top stations and their impact on the
development of the cloud remains an enigma .
In situ observations with aircraft usually observe ICNCs on the order of
1–10 L-1 , whereas at mountain-top research stations
(e.g., Elk Mountain, USA or Jungfraujoch, Switzerland) or near the snow
surface in the Arctic ICNCs of several hundreds to thousands per liter are
frequently reported , which exceeds the number of
measured ice nuclei by several orders of magnitude (see
Fig. ). This discrepancy between ice nuclei and ICNC may
be explained by so-called secondary ice-multiplication processes. A commonly
accepted secondary ice-multiplication process to enhance ICNCs in free-floating clouds is the rime-splintering or Hallett–Mossop process. This
process describes the production of small splinters after the impact of cloud
droplets on ice crystals and a subsequent burst of the cloud droplet during
its freezing process. It is active only in a small temperature range of
-3 to -8 ∘C and the presence of small (< ∼ 13 µm)
and large (> ∼ 25 µm) cloud droplets is required
. Another secondary ice-multiplication process is the
fracturing of fragile ice crystals upon collision with other solid cloud
particles . Although this process has been studied in the
lab and is expected to occur at temperatures of ∼ -15 ∘C,
there is little evidence from field measurements for this process to
significantly contribute to the ICNC e.g.,. Other
processes that produce secondary ice crystals are associated with the
freezing of cloud droplets and the subsequent breakup or ejection of small
spicules . In previous studies, the described secondary ice-multiplication in free-floating clouds like fragmentation or
the Hallett–Mossop process is usually ruled out as the source
of the observed ice crystals due to the absence of large ice crystals
necessary for fragmentation or the absence of large cloud droplets and the
right temperature range necessary for the Hallett–Mossop process. Instead,
surface processes are proposed to produce such enormous ICNCs.
suggested two possible processes as a source of the observed ICNC: riming on
trees, rocks and the snow surface or the lofting of snow particles from the
surface, i.e., blowing snow. Riming as a surface process is similar to the
previously described rime-splintering process in free-floating clouds. For
this process to be active, cloud droplets need to be present near the
surface, as is typically the case with orographic mixed-phase clouds. Blowing
snow, on the other hand, can also occur without a cloud present and can be
frequently observed visually in winter at mountain ridges . In
addition, suggested hoar frost as a wind-independent surface
process causing ICNCs larger than 100 L-1 for which they did not
observe a wind speed dependency as expected for blowing snow. Hoar frost
describes the formation of vapor-grown ice crystals on the crystalline snow
surface, which may be detached due to mechanical fracture. Although different
studies are strife about the mechanisms to explain the measured high ICNCs,
they agree on a strong influence by surface processes.
While the influence of surface processes on ICNCs observed at mountain-top
stations has received more attention in recent years , the
impact of surface processes on the development of supercooled orographic
clouds, e.g., a more rapid glaciation and enhanced precipitation, has not been
studied extensively . Whether the proposed surface processes have the
potential to impact the development of a cloud depends primarily on the
penetration depth of the resuspended particle into a cloud, i.e., the maximum
height above the surface to which the particles get lofted.
The height dependence of blowing snow has been studied in the context of snow
redistribution (“snow drift”) and reduced visibility due to resuspended
ice crystals by observing ice crystals up to several meters above a snow
surface . It has been reported that blowing snow occurs
above a certain wind speed threshold. This threshold varies between
4 and 13 ms-1 because the concentration of blowing snow depends on snowpack
properties (e.g., snow type, density, wetness) in addition to atmospheric
conditions (e.g., wind speed, temperature, humidity) .
observed resuspended ice crystals from the surface up to a
height of 9.6 m and found that the ICNCs usually decreased to as low
as 1–10 particles per liter. Meanwhile, during a precipitation event, the
relative importance of the small ice crystals (< 100 µm) decreases
from nearly 100 % at 1.1 m to below 20 % at
9.6 m. The rapid decrease in ICNC with height observed in these
studies may limit the impact of blowing snow on orographic clouds. The
applicability of these results to orographic cloud may be restricted because
most of these studies were conducted in dry air conditions under which ice crystals
undergo rapid sublimation , and lofting of ice crystals is more
likely in orographic terrain because updrafts are higher than over flat
surfaces.
Sketch of the experimental setup and the surrounding structures
(a) with their heights relative to the bottom of the measurement terrace.
Aerial image of the Sonnblick Observatory (b, courtesy of Michael
Staudinger, ZAMG) and a topographic map of the Hohen Tauern region surrounding
the Sonnblick Observatory (c, https://basemap.at/, last access: 24 September 2017).
Setup of the elevator with the holographic imager HOLIMO mounted on
the meteorological tower at the SBO (courtesy of Monika Burkert, ETH Zürich).
The red lines and numbers indicate the five different heights at which the elevator
was repeatedly positioned to obtain vertical profiles of the ICNC. The reference
height of 0.0 m is the bottom of the measurement platform (green line).
Examples of 2-D images taken by the holographic imager HOLIMO for the
three subclassifications of ice crystal habits (irregular, regular and aggregates).
The images are a collection recorded during the field campaign at different
heights of the elevator.
suggested that vapor-grown ice crystals on the crystalline
surface of the snow cover, i.e., hoar frost, may be detached by mechanical
fracturing due to turbulence, independent of wind speed. To our knowledge,
only one modeling study exists that assesses the impact of hoar frost on
the development of a cloud. increased the IN concentration and
simulated secondary ice processes in the WRF (Weather Research and
Forecasting) model to produce such high ICNCs measured at the Jungfraujoch by
. In addition, they implemented a flux of surface hoar crystals
based on a frost flower aerosol flux. They concluded that an increased IN
concentration can better represent the high ICNCs observed at the
Jungfraujoch, but also removed the liquid water from the model and prevented
the existence of mixed-phase clouds. They also found that secondary ice
processes are not sufficient to explain such high ICNCs at cold temperatures.
However, they found that a flux of surface-based ice crystals, i.e., hoar
frost, provided a good agreement with the ICNCs measured by . On
the other hand, surface-based ice crystals are not advected high into the
atmosphere and as such have a limited impact on orographic clouds. To verify
their findings regarding the impact of a surface flux on orographic clouds,
more measurements of ice crystal fluxes from the snow-covered surface are
necessary .
In contrast to these findings, several remote sensing (i.e., satellite, lidar
and radar) studies measured ice crystals advected as high as 1 km
above the surface that suggest an impact of surface-originated ice
crystals on clouds e.g.,. Satellite observations of
blowing snow from MODIS and CALIOP over Antarctica observed ice
crystals up to 1 km above the surface with an average height of
120 m for all observed blowing snow events. Similar observations from
lidar measurements exist from the South Pole with observed ice crystal
heights of usually less than 400 m, with some rare cases when a
subvisible layer exceeded a height of 1 km . However,
the suspension of clear-sky precipitation could not be ruled out as a source
of the observed ice crystal layers. Radar measurements of ice crystals from
an aircraft in the vicinity of the Medicine Bow Mountains
detected subvisible ground-layer snow clouds most of the time. Indeed,
presented evidence for ice crystals becoming lofted up to
250 m in the atmosphere by boundary layer separation behind terrain
crests and by hydraulic jumps. They proposed that these ice crystals from the
surface may lead to a rapid glaciation of supercooled orographic clouds and
enhanced precipitation. However, they also mentioned the limitation of radar
and lidar measurements to separate the small ice crystals produced by surface
processes from the larger falling snow particles and more abundant cloud
droplets. They even concluded that “to explore BIP (blowing snow ice
particles) lofting into orographic clouds, ice particle imaging devices need
to be installed on a tall tower, or on a very steep mountain like the
Jungfraujoch”.
In this study we assess the influence of surface processes on in situ cloud
observations at mountain-top stations and the potential impact on orographic
mixed-phase clouds. Vertical profiles of the ICNC up to a height of
10 m above the surface were for the first time observed on a
high-altitude mountain station with the holographic imager HOLIMO
. HOLIMO is capable of imaging ice crystals larger than
25 µm and the shape of these ice crystals can be analyzed.
Overview of the meteorological and microphysical parameters on
4 February 2017. Meteorological measurements are 1 min averages except
for the maximum wind speed (ws), which corresponds to the maximum wind
speed observed during a 1 min average. The shaded areas represent
intervals with ice crystal measurements with the SBO in cloud (gray) and
not in cloud (blue). Shown are the temperature and relative humidity (a),
wind speed (b) and wind direction (c). A wind rose
plot is shown in the bottom panel. The ICNC measurements
(d) are averages for each height level during a single profile.
As Fig. , but for 17 February 2017. On this
day temperature and wind measurements are available from the SBO and the 3-D
sonic anemometer. Shown are the temperature and relative humidity (a),
wind direction (b), a comparison of the horizontal wind
speed from the SBO and the 3-D sonic anemometer (c), and detailed
wind speed from the 3-D sonic anemometer (d) and the ICNC (e).
Results
The data presented were observed on 4 and 17 February 2017. Figures
and show an overview of the
meteorological conditions on both days. The main difference is the wind
direction, which was southwesterly on most of 4 February and northerly on
17 February.
ICNCs as a function of the height of the elevator for four different
time intervals during 4 February, representing different conditions
(Fig. ). From the 24 profiles observed on 4 February 2017
the individual profiles (left) show only 16 for better readability of the
figure. The circles indicate the mean and the error bars the standard error
of the mean. The shaded areas extend from the minima to the maxima of the
measured ICNCs. Each color represents one profile with the elevator in the
corresponding time interval. The box plots (right) show a summary of all 24
profiles in the respective time interval as in Fig. .
Case study on 4 February 2017
On 4 February a low-pressure system tracked eastwards from the Atlantic
Ocean over northern France and reached western Germany, where it slowly
dissipated. Influenced by this low-pressure system, the wind at the SBO
predominantly came from the west–southwest with wind speeds between 10 and
25 ms-1 (Fig. b and c). By late afternoon at around 19:00 UTC,
the low-pressure system dissipated over western Germany, the wind
direction shifted to the north and wind speeds decreased to a minimum of 5 ms-1.
After 19:00 UTC the wind speed increased again to up to 15 ms-1.
Due to riming of the 3-D sonic anemometer, 1 min
averages of wind speed and direction from the ZAMG measurements were used.
The temperature remained between -10 and -9 ∘C until 19:00 UTC
when the wind direction shifted to the north and the temperature decreased to
-11 ∘C by 22:00 UTC. The SBO was in cloud for most of the
measurements, except for a short time interval between 19:10 and 20:20 UTC.
Vertical profile of the concentration (a) and the fraction
(b) of individual ice crystal habits for the profiles between 19:10 and
20:20 UTC on 4 February 2017. The concentration of regular crystals and
aggregates is below 1 L-1 for all heights. For the fraction, the
ICNCs of individual habits were divided by the total ICNCs. The circles represent
the mean and the error bars represent the standard error of the mean.
Figure shows a summary of the height dependence of ICNCs
and CDNCs for all 24 profiles. Averaged over the time period of a single
measurement at an individual height, the ICNC reached a maximum of
200 L-1 at 2.5 m above the surface and decreased by a
factor of 2 at a height of 10 m, while the median decreased by a
factor of 4 in the same height interval. The CDNC, on the other hand, stayed
constant with height. The decrease in ICNCs with height and the height
independence of CDNCs suggest that surface processes strongly influence the
ICNC close to the surface. For a more detailed presentation of the results,
the measurement period is divided into four time intervals representing
different meteorological conditions as indicated by the shaded areas in
Fig. . The most important features of the profiles for
the different time intervals are summarized in Table .
Summary of important features of the ICNC profiles observed on
4 February (Fig. ) and 17 February (Fig. )
2017. ICNC‾max refers to the observed maximum of the
mean ICNC over height. ICNC‾10 refers to the average
ICNC at 10 m.
Time
Height of
Processes
interval
ICNC‾max
ICNC‾max
ICNC‾10
involved
(UTC)
(m)
(l-1)
(l-1)
from Fig.
4 February 2017
08:30–11:00
4.1
150
50
(a, c)
12:00–15:00
2.5
800
250
(a, c)
19:10–20:20
2.5
100
10
(a)
20:30–22:00
4.1
300
30
(b, d)
17 February 2017
18:00–20:00
4.1–6
300
100
(b, d)
Between 19:10 and 20:10 UTC (Fig. , third row) there were
clear-sky conditions at the SBO. The ICNC reached a maximum instantaneous
value of 600 L-1 in a single hologram at 2.5 m. The large
shaded area represents the high variability of the ICNC over the
respective measurement period. The average ICNC of all clear-sky data in this
time period decreases by a factor of 10 within 7.5 m, and more than
98 % of the observed ice crystals had irregular shapes
(Fig. b).
Since there were clear-sky conditions in this
time interval, the ice crystals had to originate from the surface.
As Fig. , but for ICNC as a function of the
horizontal wind speed for the time periods between 08:30 and 15:00 UTC when
the wind direction was from the west–southwest (a) and between 19:10 and
22:00 UTC when the wind direction was from the north (b). The ICNCs from
HOLIMO are 1 min averages and the wind speeds from the SBO are the maxima
in the respective 1 min intervals.
As Fig. , but for ICNC as a function of
height observed on 17 February 2017.
In the morning between 08:30 and 11:00 UTC when the SBO was in cloud, the
observed mean ICNCs (Fig. , first row) decreases by a
factor of 2 between the height of 2.5 m and the top of the
elevator. The ICNCs in this time period are much lower than between 12:00 and
15:00 UTC, although wind speeds were as high as 20 ms-1. A
possible reason is that the last snowfall was observed 3 days before the
measurements. During this time, the loose ice crystals at the surface were
blown away and the snowpack was solidified by temporal melting and
refreezing. Consequently, fewer ice crystals are expected to be resuspended
from the surface e.g.,.
The highest ICNCs were observed in the time period between 12:00 and 15:00 UTC
(Fig. , second row) when the SBO was in cloud. The ICNC
reached its maximum at 2.5 m with a mean value of 800 L-1
and decreased by a factor of 2 within 7.5 m (Fig. b,
second row). Between different profiles
ICNCs changed by a factor of up to 2; however, consistently for all profiles
a decrease in ICNCs with height was observed.
As Fig. , but only for 17 February and four
different wind speed measurements: (a) 1 min averages of the horizontal
wind speed from the SBO, (b) maximum wind speed of the corresponding time
interval in (a), (c) 1 s averages of the horizontal wind speed and (d) 1 s
average of the vertical wind speed from both 3-D sonic anemometers.
Between 20:30 and 22:00 UTC (Fig. , last row), when the
SBO was in cloud again, the maximum instantaneous ICNC is observed at a
height of 4.1 m. In this time period the ICNC decreased by a factor
of 9 between 4.1 m and the top of the tower. The observation of a
maximum in ICNCs at an elevated level of 4.1 m is not in agreement
with the expectations about the height dependence of blowing snow and will be
further discussed in Sect. and .
The correlation between wind speed and ICNCs for 1 min time intervals is
shown in Fig. . Instead of the average wind speed,
the maximum wind speed is used because gusts, i.e., the highest wind speed in
a time interval, are most relevant for resuspending ice crystals from the
surface. From 08:30 to 15:00 UTC, when the wind direction was west–southwest,
no correlation is observed between ICNC and wind speed for wind speeds higher
than 14 ms-1 (Fig. a). However,
between 19:10 and 22:30 UTC, when the wind direction was north, a much more
pronounced dependency of the ICNC on wind speed is observed for wind speeds
lower than 14 ms-1 (Fig. b).
This suggests that in strong winds (here greater than 14 ms-1)
the ICNC near the surface due to surface processes is saturated and no longer
increases with increasing wind speed. A further discussion of the wind
dependence of surface processes follows in Sect. .
Vertical profile of the concentration (a) and the fraction
(b) of individual ice crystal habits for the profiles on 17 February 2017. For the
fraction, the ICNCs of individual habits were divided by the total ICNC. The
circles represent the mean and the error bars represent the standard error of the mean.
As Fig. , but only for 17 February 2017 for
the ICNCs of different ice crystal habits as a function of the vertical wind
speed. Aggregates are not shown because of their very low concentrations.
Case study on 17 February 2017
On 17 February a cold front over northern Europe was dropping southwards,
producing northerly flow across the Alps and at the SBO (Fig. b).
Wind speeds observed at the SBO in the time interval
between 18:00 and 20:00 UTC were between 5 and 10 ms-1. During
this period, the temperature decreased by 1 K from -12.5 to
-13.5 ∘C. The SBO was in cloud starting at 13:00 UTC with
varying visibility between several meters up to several hundreds of meters.
Some snowfall was observed in the afternoon between 13:00 and 15:00 UTC.
For this day, wind data from the 3-D sonic anemometer were available, which
allow for a more detailed analysis of the correlation between the observed
ICNCs and wind speed. However, only four vertical profiles were obtained due
to hardware problems with the computer. The first profile was measured in the
morning at 12:00 UTC when the SBO was not in cloud and no ice crystals were
observed. Three more profiles were taken in the evening starting at 18:00 UTC.
For these profiles the ice crystals were manually classified into three
categories: regular, irregular and aggregates.
For the three profiles, the ICNC reached its maximum instantaneous value of
several hundreds per liter at the height level of 2.5 or 6 m
(Fig. a).
The minimum ICNC was consistently observed at the
top of the elevator with values less than 150 L-1. As such, the
ICNCs decreased by a factor of 2 to 4 in the observed height interval (Fig. ).
The habits of the ice crystals are primarily
irregular, which is in agreement with ground-based observations at the
Jungfraujoch and with observations in free-floating clouds
e.g.,. In addition, the observed fraction of
regular to irregular ice crystals stays constant with height (Fig. b). This implies that either
surface processes also produce a significant number of regular-shaped ice
crystals or additional processes need to be active to explain these results.
A further discussion of these results follows in Sect. .
Number size distribution of the irregular (a) and regular
(b) ice crystals observed on 17 February 2017 as a function of height. The error
bars represent the standard error of the mean. Aggregates are not shown because
of their low concentrations.
Comparison of the height dependence of the ice water content for
the time intervals between 12:00 and 15:00 UTC (red box plots) and 19:10 and
20:20 UTC (blue box plots) using the blowing snow parameterization of .
The wind speed in the time interval between 12:00 and 15:00 UTC was around
20 ms-1 and between 19:10 and 20:20 around 10 ms-1.
The gray shaded area indicates the calculated ice water content from the
parameterization for wind speeds between 10 (blue line) and 20 ms-1 (red line).
In contrast to the data from 4 February, no correlation between ICNCs and the
horizontal wind speed is observed (Fig. 11). This holds true for the 1 min
mean and maximum wind speeds observed by the SBO and the 1 s averages
observed with the 3-D sonic anemometer. However, ICNCs increased with vertical
wind speed. The wind speed dependence of irregular and regular ICNCs is
comparable, and both increase by a factor of 2 when the vertical wind speed
increases from 0–2 to 4–6 ms-1 (Fig. ). Whereas the shape of the size distribution
of the irregular ice crystals hardly varies with height, larger regular ice
crystals are more strongly reduced with height than smaller regular ice
crystals (Fig. ).
Illustration of surface and near-surface processes that impact the
measured ICNC at mountain-top research stations. Panels (a) and (b) illustrate the
difference in the height dependence of blowing snow over a surface with a
gentle and a convergence effect on the ICNC of regular and irregular ice
crystals if a cloud is forced over a mountain.
Discussion
Sources of enhanced ICNC observed at mountain-top research stations
To disentangle possible sources and mechanisms that
enhance the observed ICNCs at mountain-top research stations, the following
discussion will be based on the observed height profile of the ICNC and the
observed ice crystal shape.
In the context of snow redistribution, blowing snow has been studied
thoroughly. For blowing snow, two main layers are distinguished. In the
saltation layer, with a typical thickness of 0.01–0.02 m, snow
particles are lofted and follow ballistic trajectories. Depending on the
crystal size, the crystals in the saltation layer either impact onto the
snow surface or are transported by turbulent eddies into the suspension layer
e.g.,, which can extend up to a height of several tens
of meters above the surface. and observed the
height dependence of blowing snow up to 10 m over a flat surface in
the Arctic and in Antarctica and found that particles reaching layers higher
than 1 m above the surface are usually smaller than 100 µm,
and the particle concentration gradually decreases with height
(Fig. a). Similar to blowing snow we expect such a
height dependence for any other surface process. As such, a gradual decrease
in ICNCs with height is expected for any surface process and no height
dependence is expected for ice crystals produced in free-floating clouds.
While ice crystals observed in free-floating clouds have mainly (> 80 %)
irregular habits e.g.,, no studies have
investigated the ice crystal shape produced by surface processes like hoar
frost, blowing snow or riming on trees, rocks or the snow surface. We expect
irregular shapes for resuspended ice crystals, i.e., blowing snow, due to
mechanical fracturing upon their impact on the surface or due to the successive
melting and freezing of the ice crystals on the snow surface.
Ice crystals originating as hoar frost grow in regular shapes on the snow
surface. Whether these vapor-grown ice crystals keep their regular shape depends
on the exact physical process through which they are detached from the surface. While
some ice crystals may keep their initial regular habit, for other ice
crystals this regular habit may be destroyed when they are detached from the
surface due to mechanical fracturing as described by . Similar
to blowing snow, the ICNC from hoar frost is likely to be increased near the
surface because only smaller ice crystals are lofted higher up. In this
layer ice crystals are likely to collide and fracture. On the one hand, this
reduces the probability to observe regular ice crystals from surface
processes. On the other hand, if small regular and irregular ice crystals
(µm) are produced, they have the potential to grow into larger
regular-shaped ice crystals being observed at the measurement location.
Impact of surface processes
During the clear-sky period on 4 February 2017
between 19:10 and 20:20 UTC the observed ice crystals have predominantly
irregular habits (Fig. ). Since no cloud was
present at or above the SBO, the observed ice crystals have to originate from
the surface. As such, this observation confirms our expectations that surface
processes mainly produce irregular ice crystals.
The observed ICNCs on 4 February 2017 before 20:30 UTC gradually decrease
with height as expected for surface processes, and this is consistent with
observations of blowing snow in the Arctic and Antarctica. The
west–southwest wind during this time period (Fig. c)
transported the ice crystals from an area with a gentle slope towards the
station. The gentle nature of the slope may explain why the height dependence
of the ICNC close to the surface is similar to observations of blowing snow
over a flat terrain (Fig. a).
reported the height dependence of ice water content for different wind
speeds up to a height of 10 m. The observations between 12:00
and 15:00 UTC and between 19:10 and 20:20 UTC are compared to a parameterization by
for the ice water content as a function of wind speed and
height for blowing snow observed in the Arctic (Fig. ).
Although there is roughly an order of magnitude difference between the IWC
predicted by the parameterization and the observations, the decrease in IWC
as a function of height is consistent. Thus, the height dependence of the
ICNC at the SBO is likely due to blowing snow or a similar surface-based process.
The maximum in the ICNCs at an elevated level as observed on 4 February 2017
between 20:30 and 22:00 UTC and on 17 February 2017 cannot be explained by
surface processes over a gentle slope. However, these observations correspond
to periods when the wind direction was from the north
(Fig. c). With the change in wind direction the
location of the origin of the lofted ice crystals also changed. While the terrain to
the west has a gentle slope, the terrain to the north is characterized by a
steep wall (Fig. ). If the wind direction is north, a wind
rotor may develop to the lee of the ridge, where the elevator was located in
north wind cases, and shadow the lower levels of the vertical profiles. This
possibly explains the elevated maximum of ICNCs
(Fig. b), as observed during north wind events (4 February
between 20:30 and 22:00 UTC; see Fig. ).
Illustration of the challenges of observing the wind dependence of
blowing snow (Sect. ). The two squares represent
two air parcels with a duration Δt of 10–15 s, which
is typical for a gust. Parcel 2 represents a gust with a higher wind speed
v2 than the average wind speed v1 in parcel 1. At the location of
particle lofting (a) ice crystals are lofted due to the high turbulence
in air parcel 2. On the way to the measurement location (b) some of the
ice crystals were transported to the other air parcel, e.g., by sedimentation
or turbulence. Also, the effect of different averaging times is illustrated (b).
Regardless, the majority of crystals produced by surface processes are
expected to have irregular habits. As such, it is expected that the ratio of
irregular to regular ice crystals decreases with height if the irregular
crystals are solely produced through surface processes. This is in contrast
to our observations on 17 February 2017, when the fraction of irregular to
regular crystals remained constant with height (Fig. b). Although the contribution of regular ice
crystals from surface processes like hoar frost and riming cannot be
completely excluded, we give an additional and/or alternative explanation
for these observations in the following section.
Impact of near-surface processes
Two near-surface processes are proposed
to potentially modify ICNCs in the presence of a cloud in contact with a
mountain. When a cloud is forced over a mountain and the cloud base is below
the mountain top, a convergence zone of enriched ICNCs could develop if cloud
particles below the mountain top are entrained into the flow over the
mountain (Fig. d). If irregular and regular ice
crystals are well mixed within the cloud below the mountain top, they will
maintain their respective fractions in such a convergence zone. The fraction
of irregular ice crystals on 17 February 2017 is also in good agreement with
observations in free-floating clouds in the Arctic
. Thus, this near-surface process could explain the
height independent of irregular and regular ice crystals that was observed on
17 February 2017.
Additionally, sedimenting ice crystals originating in cloud may remain
lofted near the surface in a turbulent layer (Fig. c),
similar to the lofting of ice crystals in the suspension layer of blowing
snow. However, in this case the sedimenting particles may maintain their
habits because they do not reach the surface. Therefore, such an effect can
enrich both irregular and regular ice crystals near the surface, resulting
in the same gradual decrease in the ICNC, while keeping the ratio of regular
to irregular crystals constant.
Ice crystal number concentrations (ICNCs; orange and brown)
measured on the elevator at the SBO are compared to the ICNCs (blue) and
ice nucleating particle (INP) concentrations (red) measured at the High
Altitude Research Station, Jungfraujoch (JFJ). The ICNCs at the JFJ were
measured with HOLIMO II during the winter 2012 and 2013 .
Each box represents a cloud case. The INP concentrations were measured with
the Horizontal Ice Nucleation Chamber and additional measurements
at a relative humidity with respect to water (RHw) of 103–104 %.
The three boxes on the left were taken during (from the left) Sahara dust
events, summer seasons and winter seasons at a temperature of -31 ∘C,
but shifted slightly to visualize them. The measurements at -25 ∘C
were taken during a summer season. The left and right edges of each box represent
the 25th and 75th percentiles, the circle is the mean value and the small dots are outliers.
The mechanisms illustrated in Fig. cannot only
be considered separately, but can also occur in combination with each other.
For example, the observed decrease in the ICNC with height during the first
two time periods on 4 February 2017 (Fig. ) could be
blowing snow over a moderate slope in the presence of a cloud at the SBO
(Fig. a) and/or an enrichment by cloud ice
crystals captured in a turbulent layer near the surface (Fig. c).
It is also important to note that the proposed
near-surface processes (Fig. c and d) could impact
the observed ICNC at mountain-top research stations even without a snow-covered surface.
Wind dependence of the observed ICNCs
Turbulent eddies near the surface are responsible
for the lofting of snow particles into the suspension layer (see Sect. ).
Observations in the Arctic or Antarctica
usually use wind measurements close to the surface (< 3 m) to
estimate these turbulent eddies using friction velocity. In this study, only
wind measurements on top of the meteorological tower at a height of 15 m
are available. For 4 February 2017 only the horizontal wind speed
averaged over 1 min is available from the 2-D sonic anemometer operated by
the SBO. On 17 February 2017, 1 s averages are also available for
horizontal and vertical wind speed from our own 3-D sonic anemometer.
Similar to , who observed a dependence of the observed ICNCs on
horizontal wind speed only for a small fraction of cloud events (27 % in 2013
and 13 % in 2014), we observed a dependence of ICNCs on horizontal wind speed
on 4 February only when horizontal wind speeds were less than
14 ms-1. At higher wind speeds and on 17 February such a
dependence was not observed. While proposed blowing snow to
explain observations when a correlation was observed between ICNCs and
horizontal wind speed, they proposed hoar frost to explain observations when
no such correlation was present. However, in our opinion the orography in the
proximity of the measurement site and the positioning of the different
measurement instruments (i.e., cloud probes and sonic anemometers) have an
impact on the observable correlation between ICNC and wind speed. As such, it
is much more difficult to distinguish between blowing snow and hoar frost as
the processes responsible for enhanced ICNCs.
For example, the lack of dependence of ICNC on horizontal wind speed on 17 February
may be explained by a process that lofts ice crystals from a steep
mountain slope to form a mountain-induced ice crystal convergence zone near
the surface on the leeward side of the mountain ridge
(Fig. ). In such a case, horizontal wind speed
may not be a good predictor for the presence of turbulent eddies near the
surface capable of lofting ice crystals from the surface, but vertical wind
speed may be a better indicator as was observed on 17 February. Additionally,
a dependence on horizontal wind speed may be lost due to the exact setup of
the measurement instruments at the measurement site
(Fig. ). In the following we discuss factors
that possibly mask the wind dependence of the observed ICNCs.
Firstly, ice crystals are resuspended from the surface by local turbulence
and captured in the same air parcel (Fig. b). For
a certain time after their resuspension, the ice crystals are transported in
the same air parcel as the local turbulence. However, with time they can
leave this air parcel, for example by sedimentation or through additional
turbulence. If the measurements are performed too far away from the place
where the ice crystals were resuspended, either the turbulence responsible
for the resuspension may have already dissipated or ice crystals may have
already been transported to other air parcels (Fig. b).
Both effects mask the correlation between the ICNC of resuspended
ice crystals and the responsible turbulence.
Secondly, at mountain-top research stations local turbulence is also created
by nearby structures. Since it is difficult to observe all of the turbulence
responsible for lofting ice crystals from the surface, wind measurements at a
height of 15 m above the surface are expected to be a good estimation
of the strength of such turbulence.
Thirdly, the averaging time is crucial to observe the correlation between
ICNCs and wind speed (Fig. b). If the averaging
time is too large, any correlation is averaged out. Meanwhile, if the
averaging time is too short, any enriched ICNCs can also be measured at lower
wind speeds due to a lag between the turbulence responsible for lofting the
crystals and the entrainment time required for the crystals to be established
in the flow. In this study we average over 10–15 s, which is the
expected timescale of gusts responsible for the resuspension of ice
crystals.
Finally, the ICNC of resuspended particles not only depends on wind
speed, but also on the age of the snow cover and atmospheric conditions, and a
possible correlation may be suppressed in a data set with different snowpack
and atmospheric conditions. Between 19:00 and 20:30 UTC on 4 February 2017
when no cloud was present at the SBO, a decrease in the ICNC was observed
with time at a height of 2.5 m although the wind speed remained
constant at approximately 10 ms-1. A possible reason for the
decrease in the ICNC in this time period is that the very loose ice crystals
on top of the snow cover were gradually blown away, changing the snowpack
properties over time.
Origin of crystals measured at mountain-top stations
The origin of ice crystals observed at mountain-top research stations is an
open question because the ICNC exceeds the measured ice nucleating particle
(INP) concentration by several orders of magnitudes
(Fig. ). Thus, additional processes, i.e., ice-multiplication as well as surface and near-surface processes,
have to contribute significantly to the ICNC.
The contribution of ice crystals from the surface is on the order of several
hundreds of ice crystals per liter, which is estimated from the measurements
after 19:10 UTC on 4 February 2017. Without cloud (19:10–20:20 UTC),
several hundred ice crystals of blowing snow were observed (see
Sect. and Fig. a).
With a cloud present (20:30–22:00 UTC), several hundred ice
crystals were also observed near the surface and only several tens to
100 L-1 above 8.1 m. Assuming that the upper ICNCs are
representative for the cloud, the contribution from the surface is similar.
An estimation of the impact of the proposed near-surface processes
(Fig. ) is difficult. The profiles observed on
17 February that are possibly affected by such a near-surface process still have
a decreasing tendency in the upper levels. Therefore, no information about
the ICNC in cloud is available (Fig. ). However, the
contribution of near-surface processes is at least on the order of 100–250 L-1.
The ICNCs near the surface at the SBO (2.5 m) are
comparable with similar measurements at the Jungfraujoch (JFJ) at a height of
2 m above the ground (Fig. ) , which indicates a similar origin of the observed ice crystals at
both stations.
The contribution of surface and near-surface processes of several hundreds
per liter can explain most of the gap between the measured INP concentration
and the observed ICNC (Fig. ). However, even at a height
of 10 m the observed ICNCs of several tens to 100 L-1
exceed the expected INP concentration (Fig. ). This
discrepancy is either because even at a height of 10 m the cloud is
influenced from the surface or ice-multiplication processes contribute
significantly.
Impact of the surface on ICNCs in clouds
To assess the impact of surface and near-surface processes on the properties
and the development of a cloud, understanding the height dependence of the
resuspended ice crystals is crucial. If ice crystals are lofted only several
meters off the surface, clouds are expected to be influenced only locally
. As most vertical profiles do not show constant ICNCs at the
top of the tower, it is likely that resuspended ice crystals reach heights
greater than 10 m where they can influence clouds. For example,
between 12:00 and 15:00 UTC (Fig. , second row) when
wind speeds are greater than 20 ms-1, ICNCs gradually decreased
with height and were larger than 300 L-1 at the top of the tower.
In this case it is likely that ice crystals from the surface were lofted
higher into the cloud than 10 m. As such, due to the limited vertical
extent of the profiles, it is unclear whether and to what extent clouds are
influenced by ice crystals produced by surface processes. Already a low
concentration of ice crystals from the surface can have a significant impact
on cloud properties, e.g., extent and lifetime. Therefore, a final statement
on the impact on clouds is not possible.
Conclusions
This study assessed the impact of surface and near-surface
processes on ICNCs measured at mountain-top stations and possible
implications on the atmospheric relevance of such measurements. To achieve
this, an elevator was attached to the meteorological tower of the SBO and
vertical profiles of the ICNC were observed with the holographic imager
HOLIMO on two days in February 2017. The main findings are the following.
ICNCs decrease with height. ICNCs near the ground are at least a factor
of 2 larger than at a height of 10 m if ICNCs near the ground are
larger than 100 L-1. The increase in ICNCs near the ground can be
up to an order of magnitude during cloud events and even 2 orders of magnitudes
during cloud-free periods. Therefore, in situ measurements of ICNCs at
mountain-top research stations overestimate ICNCs.
Some observations show a similar decrease in the ICNC of irregular and
regular ice crystals with height. This suggests that either surface processes
like hoar frost or enhanced rime splintering produce a significant number of
regular ice crystals or that alternative and/or additional processes need to
be active. In the presence of a cloud, two near-surface processes are proposed
as an alternative to enrich the ICNC of irregular and regular ice crystals near
the surface. Either sedimenting ice crystals are captured in turbulence near the
surface or ice crystals are enriched in a convergence zone when a cloud is
forced over a mountain. In both cases, the observed ICNC at mountain-top
research stations is not representative of the cloud further away from the
surface, even without the presence of a snow-covered surface.
On 4 February 2017 the observed ICNC shows a dependence on horizontal
wind speed for wind speeds up to 14 ms-1. On 17 February a
dependence of the ICNC on horizontal wind speed was not observed, but instead
on vertical wind speeds. Possibly, horizontal or vertical wind speeds measured
15 m above the surface are not a good estimate for the turbulent eddies
responsible for the resuspension of blowing snow particles.
The contribution of surface and near-surface processes to the observed
ICNC at mountain-top research stations is estimated to account for several
hundred ice crystals per liter. ICNCs in clouds without any contribution
from surface and near-surface processes are estimated to be several tens per
liter based on the observations between 20:30 and 22:00 UTC on 4 February 2017.
This is still orders of magnitude higher than the measured INP concentration
(Fig. ). As such, additional processes must be active,
e.g., ice-multiplication processes, and contribute significantly to the ICNC in orographic clouds.
The strong influence of surface and near-surface processes on the ICNC
measured at mountain-top stations limits the atmospheric relevance of such
mountain-top cloud measurements. However, the data set obtained is too small
to make a clear statement regarding under which conditions in situ measurements at
mountain-top research stations may represent the real properties of a cloud in contact with the surface.
To better understand the processes that are responsible for enhanced ICNCs
close to the surface and to further investigate the processes proposed in
this study, we suggest a more thorough field campaign with additional 3-D
sonic anemometers. Ideally, one 3-D sonic anemometer should be placed upwind
of the ICNC measurement to observe the turbulent eddies that are responsible
for the resuspension of ice crystals, one 3-D sonic anemometer should be
placed on the elevator and one on the top of the tower. This may help to
better understand the dependence of ICNC on wind and to find the origin of
the observed ice crystals. At best, three cloud-imaging probes would be part
of such a campaign and would be installed in parallel to the 3-D sonic
anemometers. In addition, to get a better estimate of the impact of
resuspended particles on cloud properties, especially for high wind speeds,
the vertical profiles have to be extended to larger heights above the
surface. Such a field campaign could be conducted using a tethered balloon
system equipped with cloud-imaging probes, which can be lofted several
hundred meters into the atmosphere.