Large surface radiative forcing from surface-based ice crystal events measured in the High Arctic at Eureka

Large surface radiative forcing from surface-based ice crystal events measured in the High Arctic at Eureka G. Lesins, L. Bourdages, T. J. Duck, J. R. Drummond, E. W. Eloranta, and V. P. Walden Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada Space Science and Engineering Center, University of Wisconsin, Madison, USA Department of Geography, University of Idaho, Moscow, ID, USA Received: 25 July 2008 – Accepted: 28 August 2008 – Published: 29 September 2008 Correspondence to: G. Lesins (glen.lesins@dal.ca) Published by Copernicus Publications on behalf of the European Geosciences Union.


Introduction
Ice crystals are a commonly observed atmospheric phenomenon occurring during the winter months at high latitudes in the presence of a strong surface-based temperature inversion.The Meteorology Service of Canada (MANOBS, 2006) classifies ice crystals (IC) as a type of frozen precipitation defined as "a fall of non-branched ice crystals, in the form of needles, columns or plates, often so tiny that they seem to be suspended in the air.These crystals may fall from cloud or from a cloudless sky. . . .Ice Crystals are a common form of precipitation at very low temperatures.This type of precipitation Introduction

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Full sky.The restriction to vision may or may not be severe."Ice crystals are also called diamond dust because of the scintillation observed when viewing the air towards a light source which results in forward scattering by the ice crystals.This ice crystal suspension is also referred to as "clear sky precipitation" to distinguish it from conventional precipitation which typically falls from clouds although it is often impossible to discern a fall speed for the ice crystals.It is generally accepted that diamond dust or ice crystal events are a result of ice nucleation occurring on ice nuclei near the Earth's surface directly from the vapor phase with subsequent depositional growth.Hence the ice crystals can appear in situ in otherwise perfectly clear sky conditions.
Early field measurements showed that surface based ice crystals can increase the downward infrared irradiance at the surface by as much as 80 W m −2 (Witte, 1968;Overland and Guest, 1991;Curry et al., 1996).Such large values for the radiative forcing would require a high optical depth of ice crystals and a strong temperature inversion so that much of the ice crystal resides at a considerably higher temperature than the surface.
Since ice crystals are so common in the Arctic, Blanchet and Girard (1995) suggested that they may be important in understanding the formation of continental Arctic air masses.They also hypothesized that diamond dust formation and sedimentation can dehydrate the Arctic boundary layer in the winter, which would reduce the downward flux of longwave radiation and promote surface cooling.This is opposite to the short-term effect of enhanced forcing when diamond dust of high enough optical depth is present.Model simulations by Girard and Blanchet (2001a, b) indicate that although diamond dust can add up to 60 W m −2 to the downward infrared irradiance at the surface, a portion of this forcing is due to liquid water in or above the diamond dust layer.
Using a more comprehensive suite of measurements, Intrieri and Shupe (2004) reported that during the SHEBA experiment (Uttal et al., 2002) in the western Arctic Ocean region from November 1997 to May 1998 diamond dust alone did not make a significant contribution to the surface radiative forcing, and indeed it was essentially Introduction

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Full indistinguishable from clear sky conditions.Significant surface radiative forcing was observed only when mixed phase clouds, precipitation or a thin liquid water cloud was also present in the boundary layer.
It is unknown to what extent the results from SHEBA, over the Arctic Ocean, can be translated to over land much further east in the High Arctic.Surface observations from land stations indicate that visibilities can be severely restricted in some ice crystal events which suggest that the radiative forcing might be more significant than observed in SHEBA.The main purpose of this paper is to investigate whether ice crystal events at Eureka, Canada have high enough optical depths to be radiatively important and to better understand their origins.
In Sect. 2 the observations from the weather station, satellites and the special surface-based instrumentation are described.In Sect. 3 the SBDART model is described as well as the anisotropy factor that is needed to convert zenith radiances into irradiances.Section 4 presents the results from the four cases studies of high optical depth ice crystal events.Section 5 contains the discussion where the important role of local topography in generating strong ice crystal events is highlighted.Final conclusions are given in Sect.6.

Background
In collaboration between the Canadian Network for the Detection of Atmospheric Change (CANDAC) and NOAA Study of Environmental Arctic Change (SEARCH) a suite of surface-based atmospheric instruments has been installed at Eureka.Eureka is located on Slidre Fiord on Ellesmere Island at 79.99 N and 85.93 W in the Canadian High Arctic.It is the second farthest north weather station in Canada, after Alert which is located at the northern end of Ellesmere Island at 82.52 N.
Figure 1 shows the topographic features in the vicinity of Eureka.Eureka is situated In December, January and February of 2000 to 2006, IC under clear sky conditions were reported 32.5% of the time (4521 h) while IC occurring simultaneously with surface observed clouds were reported 35.8% of the time (4985 h).If we select those cases when the horizontal surface visibility is reduced to less than 10 km then clear sky ice crystals were reported 3.3% of the time (453 h).A further breakdown of the number of cases are given in Tables 1 and 2.

Upper air soundings
Eureka is also an operational upper air station (ID 71917) releasing radiosonde balloons twice a day at 00:00 and 12:00 UT.The archive from Environment Canada is available only from January 1961.The upper air profiles are used to determine input profiles for the SBDART model and to assess the role of boundary layer winds in the transport of ice crystals.

AHSRL Lidar
The University of Wisconsin Arctic High Spectral Resolution Lidar (AHSRL) was deployed at Eureka in August 2005 and has been operating nearly continuously since.
The AHSRL is capable of separately measuring molecular scattering and particle scattering, which allows a measurement of the extinction profile (Eloranta, 2005).The outgoing laser light is circular polarized and the receiver determines the circular depolarization ratio which is related to the linear depolarization ratio using δ c =2δ l /(1-δ l ) (Mishchenko and Hovenier, 1995).The linear depolarization ratio is defined as the ratio of the perpendicular backscatter to the parallel backscatter.It is an important parameter to help distinguish non-spherical from spherical particles which is used here to identify ice crystals in the lower troposphere.
The laser is pointed 4 degrees off the zenith to reduce specular reflection from platelike ice crystals.The backscattering can be measured above a height of 75 m with a height resolution of about 7.5 m.In this work the minimum lidar height used is 100 m.The optical depth of the ice crystal layer is calculated from the particle extinction vertical profile.The optical depth profile is linearly extrapolated from 100 m height to the surface in order to calculate the total optical depth of the ice crystal layer.The average optical depth profile was calculated by averaging the 2.5 s profiles over the time period of the ice crystal event used for radiative forcing calculations.
Based on inspection of the hourly lidar vertical profiles of particle backscattering and depolarization during January, February and December 2006, ice crystal events in the absence of any clouds occurred for 212 h or about 11.4% of the time.It should be noted that these cases do not include situations where unattached, non-precipitating clouds occurred simultaneously above the ice crystal layer but were not directly interacting with it.These additional cases contributed 170 h of ice crystal events in the three winter months of 2006 or 9.1% of the total observations.Based on the total of 417 h of lidar measured ice crystals, the average height of the top of the layer was 400 m above ground.A further breakdown of the statistics is given in The frequency of ice crystal events as determined from hourly surface observations (Table 2) is much greater than the frequency determined from lidar profiles (Table 3).For example for the months of January, February and December in 2006 ice crystals were reported 65% of the time from hourly surface observations compared to 22% of the time from lidar profiles.For clear sky ice crystals these percentages drop to 30% and 11% respectively.This is likely a result of two factors: 1. many ice crystal events do not reach at altitude of over 70 m above the ground and hence are not detectable by the lidar, and 2. the ice crystal number concentration might be too low to be detected by the lidar (above 70 m) whereas they might be visible as scintillation in front of a light source by the human observer at the surface.

P-AERI spectroradiometer
The Polar Atmospheric Emitted Radiance Interferometer (P-AERI), operated by SEARCH, has been measuring the zenith spectral radiance from 3 to 20 microns at Eureka since March 2006.Although the instrument has a spectral resolution of 1 cm −1 we used the 25 cm −1 average spectrum for all calculations and comparisons because our focus is on broad-band energetics.For the broad-band calculation the spectrum was integrated from 5.56 to 20 µm to avoid the noisy channels that lie above and below this range.Each spectrum is derived from an average of eight interferometric co-additions, which are taken over a period of about 35 s.These spectra are then averaged over the time period used in computing the clear sky and ice crystal radiances, which is an average over about 10 to 40 spectra.The calibration of the P-AERI radiances is traceable to NIST standards, and the accuracy of these instruments is less than 1% of ambient radiance (Knuteson et al., 2004a, b).Details on the instrument performance in the polar regions are found in Walden et al. (2005Walden et al. ( , 2006)).visible and 1 km in the thermal infrared, for over half of the day from about 12:00 UT to 03:00 UT the following day.This makes it possible to track the time evolution of ice crystal events from space.Detection of the boundary layer ice crystals in otherwise clear skies during the winter when the sun is below the horizon is done by using the 11.03 µm brightness temperature which corresponds to the MODIS window channel 31.The strong surface based temperature inversion gives an emitting brightness temperature at the top of the ice crystal layer that is substantial warmer than the skin temperature of the ground in the surrounding clear regions making it possible to determine the ice crystal's regional extent if the skies are otherwise clear.In regions where the topography is high enough to raise the ground's skin temperature it becomes more difficult to identify ice crystals in the air.However these elevated regions are stationary and can be identified as a topographic influence by examining a sequence of brightness temperature maps.
One of cases studied in this paper occurred on 21 March 2007 which is at the spring equinox giving nearly 12 h of sunlight for that day.The MODIS near-IR channel 2 (0.841 to 0.876 µm) was used to view the area in reflected sunlight.This channel is used since it has a high horizontal resolution of 0.25 km at the nadir and also there is less atmospheric scattering compared to the shorter wavelengths.Even though it is usually very difficult to distinguish between snow on the ground and ice crystal clouds, the small solar elevation angle at Eureka helps to create shadows that can be used to identify plumes of ice crystals above the snow covered ground.Introduction

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Model description
The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model (Ricchiazzi et al, 1998) was used to compute thermal infrared spectral radiances and irradiances from 5.56 to 100 µm using 20 streams with a spectral resolution of 25 cm −1 to match the spectral resolution used from the P-AERI instrument.The temperature and water vapor profile was taken from the closest in time (within 6 h) operational radiosonde release from Eureka.Typically 65 height levels were used in specifying the profile with a vertical resolution increasing from 2 km above 30 km altitude to 50 m in the lowest 1 km closest to the ground.The CO 2 concentration was constant at 380 ppmv.
The ozone profile was taken from the closest in time ECC ozonesonde release from Eureka and no aerosols are input in the model.The optical depth and height of the ice crystal layer, which extends right down to the surface, is determined from the AHSRL.SBDART converts the 0.55 µm optical depth to the infrared optical depths assuming spherical ice particles with an effective radius of 50 µm.The extinction is assumed to be uniform within the ice crystal layer.The ice crystal size distribution used is the SBDART default modified gamma distribution (Ricchiazzi et al., 1998) given by where r o =r eff / (p+2) and p=7.

Anisotropy factor
In this paper the main use of the SBDART model is to determine the relationship between the radiance and irradiance for each wavelength.Following Ellingson and Introduction

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Interactive Discussion
Wiscombe (1996) the P-AERI zenith spectral radiances are converted to downward irradiance by applying an anisotropy factor, A λ , which is computed from SBDART using where the numerator is the model spectral downward irradiance and the denominator contains the downward spectral radiance from the model for a zenith angle of zero.
An example of the spectral dependence of the anisotropy factor for both clear and ice crystal sky conditions for the 6 February 2007 case study is given in Fig. 2. The model computed anisotropic factors are then applied to the P-AERI radiances to convert them to spectral downward irradiances.Finally the spectral irradiances are integrated from 5.56 to 20 µm to obtain the broad-band irradiance.
The anisotropy factor equals one when the downward radiance is isotropic, meaning it has a constant brightness temperature for all down-welling directions.Typically the anisotropy factor is greater than one (Fig. 2) indicating the actual irradiance is greater than the irradiance computed from an assumed isotropic radiance based on the zenith value.This is particularly evident at those wavelengths where the cloud-free atmosphere is relatively transparent because the zenith radiance is mostly determined by radiation emitted from the cold upper troposphere or higher compared to the strongly slanted radiance paths.It is interesting to note that the anisotropy factor can drop to slightly below one in cases where the wavelength is very strongly absorbing.In this case the strong surface based temperature inversion produces a warmer zenith radiance emitted by the warmer air at the inversion height compared to the slant paths which are mostly being emitted by the colder air below the inversion height.Introduction

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Full 4 Case studies of strong ice crystal events

Criteria
One purpose of this study is to determine the magnitude of the surface infrared radiative forcing from significant ice crystal events.Examination of the hourly surface observations and the continuous AHSRL profiles reveals that the ice crystal optical depth varies considerably and that there are many cases when ice crystals reported by a human observer is not seen by the AHSRL lidar.This could be because the ice crystals are below the minimum detection height (70 m) of the lidar or because the optical depth of the ice crystals is below the detectable limit of the lidar.In any event we consider cases where both the surface hourlies and the lidar agree that an ice crystal event has occurred.
It is of interest to determine how large the radiative forcing from such ice crystal events can get.This would provide some upper bounds on the importance of ice crystals from a radiative perspective and also allow for comparison with other field and modeling studies.The ice crystal events must be screened to eliminate cases which contain other phenomena which can also impact on the radiative forcing.This would include the presence of clouds, precipitation, fogs or any other obstruction to visibility.In this way the focus will be on the pure clear sky frozen precipitation or ice crystal events.
The following criteria were used in selecting the case studies.First, the hourly surface observations needed to report ice crystals in the absence of clouds, precipitation, drifting and blowing snow, and fog for at least a 3-h period.Second, the AHSRL was used to screen out cases in which clouds or precipitation were associated with the ice crystal event by rejecting cases where the lidar backscatter cross-section was greater than about 10 −6 m −1 str −1 above the ice crystal layer.Furthermore, the AHSRL linear particulate depolarization ratio, δ, had to be greater than 20% throughout the ice crystal layer to reduce the possibility of a mixed phase cloud.Finally the lidar extinction profile was examined to avoid cases with layers of enhanced extinction which might Introduction

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Full indicate the presence of liquid water.This criteria was strictly applied in selecting cases that were used in determine the radiative forcing by the ice crystals which required the SBDART model to compute the anisotropy factors.Additional constraints were used for these narrower time periods which included a fairly uniform lidar backscattering and a fairly uniform and distinct top to the ice crystal layer, to minimize the variability of the optical depth during the radiative calculation time periods.
The choice of δ=20% as the cutoff between ice crystal and mixed phase clouds is based on a number of theoretical and field studies.Sakai et al. (2003) summarized a number of studies and reported a range of δ=19 to 36% from theoretical studies, 28 to 33% from laboratory studies and 13 to 38% from field studies.Their quoted theoretical studies also showed that δ can drop to about 10% for ice crystals with a maximum dimension of only a few micrometers and close to 0% for horizontally oriented columns and plates.Their quoted laboratory studies suggest that δ can be as high as 33% for mixed phase scatterers.Chen et al. (2002) measured depolarization in cirrus clouds and obtained a range for average δ of 20 to 54% when binned by altitude.During SHEBA it was determined that δ=11% was the best value to distinguish between liquid water and ice phase (Intrieri et al., 2002).Our choice of δ=20% is deliberately conservative to minimize cases where both water and ice coexist in the same volume.
Based on these criteria the four highest optical depth cases were chosen which were also considered to be the best time periods with extended cloud-free conditions.For radiative forcing calculations a time average of 5 to 30 min, depending on the case, was used to reduce the effects of small scale fast fluctuations in the atmospheric radiance and to minimize the instrument variance of the measured signals.A summary of the conditions and calculations for the four cases are in Table 4. Figure 3 compares the ice crystal events for the four cases bracketing in time the full duration of the events.In the following four subsections the cases are described in more detail referencing to Figs.A significant boundary layer ice crystal event started by 03:00 UT on 10 December and lasted until 11:00 UT on 11 December (Fig. 3a).The ice crystal layer varied considerably in intensity and height, reaching above 1 km at times.During the time interval when the radiative forcing was calculated, 13:00 to 13:10 UT on the 10th, there were no clouds or precipitation detected above the uniform ice crystal layer (Fig. 4).By far most of the ice crystal layer was below 300-m altitude; however an optically thin layer of ice crystals appears to extend up to about 500 m. Figure 8 shows that the time averaged lidar linear particulate depolarizations were greater than 20% in the ice crystal layer which suggests that liquid water was not significant radiatively.The profiles shown in Fig. 8 are averages of the 2.5 s profiles during the period when the radiative forcing is calculated.The particle backscatter varied from 2.5 to 2.8×10 −5 m −1 sr −1 .Surface observations indicate clear skies with a visibility of 24 km and a wind speed of 1.5 m s −1 .

Case 2: 24 January 2007
A continuous boundary layer ice crystal event lasted from 00:15 to 22:50 UT on 24 January 2007 with the greatest optical and geometric thickness occurring from 19:00-22:00 UT.There was considerable variation in optical depth as the altitude of the top of the layer remained below 300 m until about 14:00 UT when it began to increase to 700 m (Fig. 3b).The sample time of 19:30 to 20:00 UT was chosen based on available data from P-AERI and it occurs when the layer has its greatest optical depth.No clouds or precipitation were measured above the ice crystal layer.
The detailed time-height lidar cross-section shows that the height of the layer contained some high frequency variation at the layer top from 500 to 700 m (Fig. 5).The lidar depolarization was greater than 35% throughout the profile suggesting that water droplets were not contributing to the radiances (Fig. 9).The particle backscatter varied from 3 to 4×10 1000 m and had its greatest optical depth from 03:00 to 08:00 UT on the 7th (Fig. 3c).
The radiance measurements were averaged from 21:50 to 22:10 UT on the 6th when the layer was particularly steady (Fig. 6) and also well in advance of some clouds which appeared after 07:00 UT on the 7th between 1 and 3 km.Linear particulate depolarizations were greater than 24% during the radiative calculation period (Fig. 10).
For about 10 min at 20:15 UT the lidar depolarization dipped to less than 5% at the top of the ice crystal layer which indicates a thin water layer, however this occurred nearly 2 h before the radiance measurement period.
The particle backscatter increased from 5.0×10 −5 m −1 sr −1 at 100 m to 1.7×10 −4 m −1 sr −1 at 400 m which resulted in an optical depth profile that increases nonlinearly with height.Surface observations from 21:00 to 23:00 UT report ice crystals under clear skies with the visibility varying from 6 to 16 km and calm winds.Figure 2 shows very good agreement between the P-AERI spectral radiance measurements and SBDART calculations which was also true for the other 3 cases (not shown).

Case 4: 21 March 2007
The ice crystal event began at about 08:00 UT on the 20th and persisted until 02:00 UT the following day.The height of the layer top gradually increased to 800 m by 16:00 UT and then decayed until the end of the event (Fig. 3d).The period from 00:50 to 01:00 Z on the 21st was chosen since the backscattering from the ice crystals was very uniform with the altitude of the layer top remaining very close to 300 m (Fig. 7).The linear Introduction

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Full depolarization ratio remained greater than 24% throughout the layer, while the particle backscattering cross-section varied from 6 to 7×10 −5 m −1 sr −1 .The optical depth increased nearly linearly with height (Fig. 11).
The lidar indicated an absence of any clouds above the ice crystal layer during the entire event.However surface observations report broken stratus, stratocumulus and altocumulus at various times during the event.This is likely because the daylight conditions at this time in March, unlike the other 3 case study times, allows the observer to detect clouds closer to the horizon away from the zenith direction which the lidar is sampling.The ridges and mountains that surround Eureka can be the foci of cloud formation while the station itself is clear overhead.Surface winds were less than 3 m s −1 with the horizontal visibility dropping as low as 4 km at 17:00 UT on the 20th.

Discussion
The occurrence of significant optical depth ice crystal events could be important in understanding the total radiative forcing exerted at the surface.Any increase in the downward infrared irradiance at the surface caused by the ice crystals which reside in the warmer air above the ground will impact how cold the surface temperatures can become.To quantify this impact the thermal infrared surface irradiances and forcings are given in Table 4 for the four case studies.
The ice crystal surface radiative forcing is computed by subtracting the clear sky downward irradiance from the irradiance with the ice crystal layer.additional ice crystal radiative forcing of not more than about 10% due to irradiance from 20 to 100 µm.Although this paper is not intended to investigate the source of differences between observed and modelled radiative forcings, a short summary of the possible biases will be discussed.The SBDART clear sky irradiance exceeds the P-AERI observations by an average of 2.9 W m −2 over the four cases.The SBDART ice crystal forcing exceeds the observed value by an average of 4.0 W m −2 over the four cases.There are a number of possible sources for this bias, in addition to the clear sky bias, which include: 1. incorrect ice crystal effective radius in SBDART, 2. incorrect single particle extinction cross-section used in SBDART due to nonspherical effects, 3. errors in the lidar determined optical depth, particularly in extrapolating from 75 m to the surface, 4. errors in the air temperature and water vapor profile used for the radiative calculations and 5. errors introduced from neglecting aerosols.
For case 3 SBDART was run using ice crystal effective radii of 25, 50 and 100 µm.The change in ice crystal forcing was less than 1 W m −2 .The effect of non-spherical particles is much more difficult to estimate given the lack of knowledge of the size and shape of the ice crystals.A parameterization developed by Yang et al. (2005) predicts a reduction in the mean particle extinction efficiency of at least 20% as the effective particle size decreases from 50 to 10 µm in the IR window spectral region for mid-latitude cirrus clouds.SBDART was also used to show that an estimated average aerosol optical depth of 0.08 produced a terrestrial IR forcing of less than 1 W m −2 .
The results here differ from those obtained further west and south over the Arctic Ocean during SHEBA where significant radiative forcing occurred only with mixed Introduction

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Full phase clouds characterized by a thin water cloud topping the diamond dust layer (Intrieri and Shupe, 2004).During their diamond dust events consisting of only ice crystals the optical depths were low enough so that the radiative forcing by the diamond dust was almost indistinguishable from clear sky conditions.This is in sharp contrast to the four cases of strong radiative forcing observed at Eureka in which the forcing was caused only by the ice crystals.
In a conventional diamond dust event the ice crystals form near the ground by deposition on ice nuclei under clear skies.They grow by continued deposition until the ice crystals become visible, especially when viewed towards the sun or other light source, and may slowly precipitate out as their terminal velocities increase.The high optical depths and also the vertical extent of the ice crystals in the four case studies at Eureka suggested that another more prolific source of ice crystals may be responsible.We were able to rule out precipitation of ice from high clouds using the lidar time series.We were also able to eliminate blowing snow from the surrounding ground as the source of the ice crystals since the wind were too light to lift the snow and also since the weather observers did not report any blowing or drifting snow.
The fact that the height of the top of the ice crystal layer varied from a couple hundred metres to close to 1 km was a clue since the surrounding ridges and hills varies in altitude up to about 800 m.It might be that the ice crystals were being blown from the tops of the ridges where the wind speed might be much larger than at the Eureka station.The dark winter skies and also the reduction in visibility from the ice crystals would make it impossible for the weather observers to ascertain whether snow was being blown off the ridge tops.
The frequent overpasses at the latitude of Eureka by the MODIS instrument on both the TERRA and AQUA satellite platforms provides a unique opportunity to look at the geographic extent and possible source of the ice crystals.The results for the March 20th case are detailed in the various panels of Fig. 12.The March 20th case is presented in greater detail here since the sun is above the horizon for about half the day and so both the thermal infrared and reflected solar channels can be investigated.ure 12 presents 6 vertical panels showing the time evolution for 20 March the ice crystal event day, in the top two horizontal panels, and for 21 March the next day which has clear skies without ice crystals, in the bottom two horizontal panels.We attempted to match the time of day as close as possible for the two days in each vertical panel set.The color images are the 11.03 µm brightness temperature from MODIS channel 31 which is in the transparent window region and is used to determine the surface skin temperature in clear skies.The 21 March clear sky picture in the IR shows warmer regions are caused by the topography.The strong temperature inversion results in warmer surface temperatures in higher terrain.Indeed the match between the brightness temperature and the topography is excellent and relatively constant during the course of the day on 21 March.However, by comparing the top and bottom IR images for each time of day it is apparent that on 20 March there were large additional areas also emitting at warmer temperatures that cannot be explained by the topography.Furthermore these warm regions occur as plumes aligned NNW to SSE and appear to the streaming off the tops of the ridges.This becomes more obvious when the time sequence is examined during the course of the day on the 20th.
In the center two images for each vertical panel the 0.858 µm albedo using MODIS channel 2 is shown.This channel is used because it has the highest horizontal resolution of 250 m at nadir and is preferred over the 0.645 µm albedo since it has slightly less solar scattering making the shadows more distinct.On the clear day, the 21st, the shadows from the ridges and hills show the topography very clearly.It compares very well with the location of the warm skin temperatures on the IR images below.However on the 20th the albedo image shows the presence of reflective plumes which are casting shadows and are aligned NNW to SSE, appearing to stream off the higher topographic features.
The images provided by MODIS show that the ice crystal event at Eureka on March 20 is caused by snow blown off the ridges to the NW and being advected towards Eureka in distinct plumes.This helps to explain the vertical extent of the ice crystal layer which matches the height range of the ridges.It also helps to explain why the ice Introduction

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Full crystal layer can change in optical thickness and altitude so rapidly since any variation in wind speed and direction can cause different portions of the plume to be over Eureka.
For the other three cases there are no solar reflectance images available because of the winter night.However the thermal infrared images were examined (not shown here) and the same pattern of warm plumes advecting off the ridges is clearly evident.
The radiosonde profiles for the four case studies are presented in Figs. 13 to 16.All four cases show a similar pattern.A very strong temperature inversion is present in the boundary layer and the wind speed increases significantly from the ground to 600 to 800 m corresponding to the ridge heights.Most importantly the wind direction is from the N to NW at 600 to 800 m for all four cases matching the direction of the plumes seen in the MODIS images.
The evidence presented in this study suggests the following explanation for the high optical depth cloud-free ice crystal events at Eureka.The winds are strong enough to lift the snow from the ridges and create blowing snow.Since the atmospheric layer from the ground to the ridge height is thermally very stable one expects vertical mixing to be strongly inhibited.It is possible that some of the wave-like patterns seen at the top of the ice crystal layer may be wind-shear induced Kelvin-Helmholtz type undulations.Since the vertical motion field is likely very weak within the stable boundary layer, the ice crystals must reach the surface simply from their terminal velocities.The largest ice crystals created by the blowing snow from the ridge tops will fall out the fastest and closest to the ridge.
The ice crystals which reach Eureka at the ground must first be advected by the strong NW winds at around 600 to 800 m.However their terminal velocity will allow them to fall out of the strong wind layer into the quiet air closer to the ground.At this point they can slowly settle out, although deposition, sublimation and accretion can be occurring during this fall-out depending on the ice saturation conditions in the boundary layer air.These events would still be classified as ice crystals by the surface weather observers since they would have no way of knowing that the ice crystals may have originated as blowing snow from the higher terrain.Furthermore the surface winds Introduction

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Full were too low at the observing station to result in blowing snow locally.
The classical diamond dust phenomenon is believed to contribute to a dehydration of the Arctic winter boundary layers which enhances the radiative cooling to space from the surface.This operates because the water mass associated with the diamond dust is provided by the water vapor in the boundary layer.The ice crystals slowly settle to the ground and hence the event is a net sink for water vapor in the boundary layer column.
For the high optical depth ice crystal events observed at Eureka, where the ice crystals are the settled remains of blowing snow streamers aloft, it cannot be argued that there is a net sink of water vapor.Indeed it is possible that the ice crystals are subliming in the warmer air near the height of the ridges and may be increasing the amount of the water vapor there.The dehydration effect may be completely absent and may even by replaced by a net moistening of the boundary layer resulting in less radiative cooling from the ground to space.
As part of the Canadian Arctic Shelf Exchange Study 2003-2004 expedition, Save-  2006) deduced that sublimation rates were low during blowing snow events.Although the high optical depth ice crystals events considered in our case studies originated as blowing snow from the surrounding ridge tops it is important to separate them from blowing snow events that are observed in situ and that are caused by sufficiently strong local surface winds to lift the local surface snow.The presence of strong local surface winds will modify the boundary layer mixing processes and surface latent and sensible heat fluxes compared to the quiescent boundary layers in our case studies.

Conclusions
Ice crystals are reported by surface weather observers almost continuously at Eureka during the winter.At times these events are characterized by optical depths as high as the ridges to the North-West of Eureka by strong winds at the altitude of the ridge tops.
The ice crystals eventually settled into the quiescent stable boundary layer below.The strong ice crystal events at Eureka are not the classical diamond dust phenomenon in which ice crystals undergo nucleation and depositional growth locally, but instead are caused by blowing snow from surrounding higher terrain at distances of at least 10 km.
The high optical depth ice crystal events result in large downward radiative surface forcing in the thermal infrared of up to 36 W m −2 or 36% of the downward irradiance.
This can have important consequences for the surface energy balance.Since the frequency and magnitude of these events is in large part determined by the local topography the effects on the radiative balance will be highly regional.In those regions where the events occur frequently we can expect that the local character of the stable winter-time boundary layer will be influenced by the presence of the wind swept ice crystals.
Since high optical depth ice crystal events are also caused by topography and wind profile conditions, instead of the classical diamond dust depositional method, a new assessment of the mechanisms associated with Arctic atmospheric dehydration by diamond dust, and the interaction between ice crystals, aerosols, radiation, wind regime and topography should be considered.studied with lidar measurements of particle extinction-to-backscatter ratio, particle depolarization, and water-vapor mixing ratio over Tsukuba, Appl. Optics, 42, 7103-7116, 2003.Savelyev, S. A., Gordon, M., Hanesiak, J., Papakyriakou, T., and Taylor, P. A.: Blowing snow studies in the Canadian Arctic Shelf Exchange Study, 2003-04, Hydrol. Process., 20, 817-827, 2006.−2 for the four case studies."Clear" denotes an atmosphere without clouds or ice crystals, "Total" denotes an atmosphere with the ice crystal layer present and "Ice Crystal" (IC) is the forcing by the ice crystal layer alone obtained by taking the difference between Total and Clear.The Time Period is used for averaging the AHSRL and P-AERI measurements.The upper air sounding used for the clear sky and ice crystal profiles are given (day of month is given in parenthesis).The P-AERI clear adjusted irradiance applies the SBDART clear sky difference between the measured clear and ice crystal sounding times to give an estimate of what the P-AERI clear sky irradiance would be during the ice crystal period."SBDART (20-100 µm)" shows the model estimate of the additional surface forcing by the ice crystal layer for wavelengths greater than 20 µm.The optical depth at 0.53 µm is determined from the AHSRL and includes the extrapolated region from the surface to 100 m altitude.The depolarization ratio is the linear type averaged over the time period of the ice crystal event.The depolarization ratio range is the variation with height within the ice crystal layer above 100 m altitude.The "Horizontal Surface Visibility" is taken from the hourly surface observations for Eureka."Height of IC layer" is the height above the ground of the top of the ice crystal layer which is assumed to extend down to the surface.The "Inversion Height" is the altitude of the first significant temperature maximum.The "IC Top Temperature" is the temperature at the top of the ice crystal layer determined by interpolation of the radiosonde observations at the nearest time.The "Max.Inversion Temp." is the temperature at the Inversion Height which is a maximum for the layer.The effective radius of the ice crystals used in SBDART is 50 µm.
−5 m −1 sr −1 .Surface observations at 19:00 and 20:00 UT show ice Introduction -based ice crystal layer developed by 16:00 UT on 6 February 2007 and persisted to 17:00 UT on 7 February.The layer top varied from around 500 to The qualitycontrolled P-AERI spectral range extends from 5.56 to 20 µm.The ice crystal radiative forcing from P-AERI ranged from 8.2 W m −2 with an optical depth of 0.20 to 35.8 W m −2with an optical depth of 1.65.The ice crystals accounted for up to 36% of the total downward irradiance over these four cases.Note that the air temperatures at the surface and the top of the ice crystal layer did not differ by a large amount between the cases and so it is the variation in optical depth that determines most of the difference in the radiative forcing by the ice crystals between the cases.SBDART predicts an lyec et al. ( 2.0 and vertical extents up to about 1 km.In a study of four such events from December 2006 to March 2007 it was determined that the ice crystals were being advected from Uttal, T., Curry, J. A.,McPhee, M. G., Perovich, D. K., et al.:  The surface heat budget of the Arctic Ocean, B. Am.Meteorol.Soc., 83, 255-275, 2002.Walden, V. P., Town, M. S., Halter, B., and  Storey, J. W. V.: First measurements of the infrared sky brightness at Dome C, Antarctica, Proc.Pac.Astron.Soc., 117, 300-308, 2005.Walden, V. P., Roth, W. L., Stone, R. S., and Halter, B.: Radiometric validation of the Atmospheric Infrared Sounder over the Antarctic Plateau, J. Geophys.Res., 111, D09S03, doi:10.1029/2005JD006357,2006.Witt, H. J.: Airborne observations of cloud particles and infrared flux density in the Arctic, MSc.Thesis, Department of Atmospheric Sciences, University of Washington, USA, 1968.Yang, P., Wei, H., Huang, H.-L., Baum, B. A., Hu, Y. X., Kattawar, G. W., Mishchenko, M. I., and Fu, Q.: Scattering and absorption property database for nonspherical ice particles in the near-through far-infrared spectral region, Appl.Optics, 44, 5512-5523, 2005

Fig. 2 .Fig. 3 .
Fig. 2. The upper portion shows the anisotropy factor as determined by SBDART for the 6 February 2007 Case 3 study.The dependence of radiance with zenith angle is much stronger under clear skies and at the less opaque wavelengths.The lower portion compares the spectral zenith radiance between P-AERI and SBDART at a spectral resolution of 25 cm −1 .Similar plots (not shown) were obtained for the other 3 cases.

Figure 4 :
Figure 4: Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the Dec 10, 2006 case for the time period used in computing the ice crystal surface radiative forcing.The top of the main ice crystal layer varied from about 200 to 250 m above the ground.Above this is another weaker layer, likely ice crystals judging from its depolarization layer which extends close to 500 m.The summary time cross-section shows that this weak upper layer is short lived.

Fig. 4 .
Fig. 4. Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the 10 December 2006 case for the time period used in computing the ice crystal surface radiative forcing.The top of the main ice crystal layer varied from about 200 to 250 m above the ground.Above this is another weaker layer, likely ice crystals judging from its depolarization layer which extends close to 500 m.The summary time cross-section shows that this weak upper layer is short lived.

Figure 5 :
Figure5: Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the Jan 24, 2007 case for the time period used in computing the ice crystal surface radiative forcing.Note that the top of the ice crystal layer varied from about 400 to 700 meters above the ground and the layer consisted of relatively strong and uniform depolarization.

Fig. 5 .
Fig. 5. Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the 24 January 2007 case for the time period used in computing the ice crystal surface radiative forcing.Note that the top of the ice crystal layer varied from about 400 to 700 m above the ground and the layer consisted of relatively strong and uniform depolarization.

Figure 6 .
Figure 6.Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the Feb 6, 2007 case for the time period used in computing the ice crystal surface radiative forcing.During this time period the backscatter and depolarization are very uniform.Here again a wavy pattern is visible at the top of the ice crystal layer.

Fig. 6 .Figure 7 .
Fig. 6.Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the 6 February 2007 case for the time period used in computing the ice crystal surface radiative forcing.During this time period the backscatter and depolarization are very uniform.Here again a wavy pattern is visible at the top of the ice crystal layer.

Fig. 7 .
Fig. 7. Detailed cross-section of the aerosol backscatter cross-section and circular depolarization ratio for the 21 March 2007 case for the time period used in computing the ice crystal surface radiative forcing.During this time period the backscatter and depolarization are very uniform.

Figure 8 :
Figure8: Vertical profiles for the Dec. 10, 2006 case averaged over the ice crystal event times 13:00 to 13:10 UT. a) particle backscatter cross-section per volume (m -1 sr -1 ), b) particle extinction cross-section per volume (m -1 ), c) particle linear depolarization and d) particle optical depth, obtained by averaging the 2.5 s measurements from the AHSRL over the time period given above.The dotted lines in a) and c) are one standard deviation away from the mean calculated from the 2.5 s measurements.

Fig. 8 .
Fig. 8. Vertical profiles for the 10 December 2006 case averaged over the ice crystal event times 13:00 to 13:10 UT.(a) particle backscatter cross-section per volume (m −1 sr −1 ), (b) particle extinction cross-section per volume (m −1 ), (c) particle linear depolarization and (d) particle optical depth, obtained by averaging the 2.5-s measurements from the AHSRL over the time period given above.The dotted lines in (a) and (c) are one standard deviation away from the mean calculated from the 2.5 s measurements.

Fig. 13 .
Fig. 13.Radiosonde profiles extending to 2000 m for the 10 December 2006 case for up to 4 times bracketing the ice crystal event.The four panels show the (a) air temperature, (b) relative humidity, (c) wind speed and (d) wind direction profiles.Note that the wind profile was missing for 2 of the times.The solid half circles are the surface observations taken at the hour of the radiosonde launch.
the north side of Slidre Fiord which extends about 22 km eastward from Eureka Sound into the Fosheim Peninsula.The Fiord itself is about 4 km wide and Eureka is about 9 km from its entrance.Eureka lies at the mouth of a broad valley rising gently to the north.About 10 km to the northwest is a ridge roughly 600 m high while ranging from 5 to 10 km to the northeast is another ridge, named Blacktop Ridge, which reaches heights of about 800 m.Both ridges run roughly north-south for about 15 km.
on Canada since 1953.Currently the hourly observations are missing at 00:00 and 01:00 local time (Eastern Standard Time) because of limited personnel.Here we are particularly interested in extracting those hourly surface observations that report ice crystals in the absence of any other types of precipitation or weather.

Table 2 .
Number of ice crystal events based on hourly surface observations at Eureka given annually for 2000 to 2006 and monthly for 2006.

Table 3 .
Number of cases of cloudy, clear and ice crystal conditions based on hourly lidar observations taken in January, February and December of 2006.The presence of any clouds or ice crystals within an hourly period is counted as clouds or ice crystals regardless of the fraction of time in the hourly that they occurred.The percentage sum of cloudy and clear cases equals 100% for each month.In the lower sub-table the cases with ice crystals are broken down into conditions with clouds, clear skies and no precipitation.Note that roughly half the time the lidar reports clouds occurring with ice crystals.

Table 4 .
Downward thermal infrared (5.56-20 µm except where noted) irradiance at the surface in W m