Cirrus clouds are ice clouds that exist at temperatures below the homogeneous freezing threshold of about 235 K. The required temperatures and humidity conditions are found in the upper troposphere and sometimes in the lowermost stratosphere, leading to an average global coverage of at least 20 %. Cirrus clouds form in ice supersaturated regions in the presence of suitable nuclei, such as solid or liquid aerosol, or via the freezing of (supercooled) liquid droplets in ascending air masses .

The impact of cirrus on the radiative energy budget of the atmosphere can be differentiated into direct and indirect effects. The direct effect is absorption and re-emission of thermal (long-wave) radiation and scattering of solar (short-wave) radiation. Indirect or secondary effects arise from the redistribution of water vapour and through heterogeneous physical (e.g. trapping and release; ) and chemical reactions with radiatively active compounds, such as ozone-depleting substances e.g..

Cirrus occurrence and radiative properties are prone to anthropogenic influence through the introduction of ice-nucleating particles and precursors , including aircraft emissions , and via the evolution of atmospheric humidity as a feedback to changing climate as a whole. It is known that the microphysical properties of cirrus, such as the particle number concentration, size distribution and shape, depend on the nuclei and the environmental conditions during nucleation, such as updraft speeds e.g..

Cirrus clouds are more abundant and thicker in the tropics and at midlatitudes, but there is also considerable coverage in the high-latitude and polar regions. In these areas, the low elevation of the sun and the complete absence of sunlight during polar nights reduce the short-wave effect . In addition, the Arctic surface climate is currently undergoing a much more rapid warming transition than the rest of the globe, known as Arctic amplification, which also changes the spatial and temporal patterns of the surface albedo as sea ice and continental ice retreat . Therefore, the question of how Arctic cirrus clouds impact (will impact) the radiation budget now (and in the future) has not yet been answered satisfactorily. Furthermore, the direction, extent and circumstances under which the cirrus feed back to the current warming are unknown.

Although cirrus clouds are regularly observed from space with active e.g. and passive e.g. sensors, in situ measurements are needed for the closure of the physical appearance of cirrus ice crystals and their bulk radiative properties . There have been some in situ studies on Arctic cirrus clouds in the past, including the following: the Polar Stratospheric Aerosol Experiment (POLSTAR) missions in 1997 and 1998 , the European Polar Stratospheric Cloud and Lee Wave Experiment (EUPLEX) in January–February 2003 and the ENVISAT validation experiments in October 2002 and March 2003, which all produced measurements of the ice water content (IWC) up to 68∘ N summary in, and the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) in 1999–2000, which produced measurements of the IWC and particle size distribution . The mentioned missions accumulate to a total of 15 flights and significantly contributed to our current picture of cirrus in the Arctic. Nevertheless, these are few studies compared with the much more comprehensive observations in the tropics and at midlatitudes e.g..

An earlier study by used spaceborne radar, lidar and radiance measurements to derive the radiative impact at the top of the atmosphere (TOA) of differently shaped IWC profiles including global variations. Although we also examine this effect for the inhomogeneous vertical IWC distributions in our measurements, our study focuses on high-latitude cirrus, and we demonstrate the radiative effects, including the heating rate, in the entire atmospheric column. This also constitutes a link to cloud dynamics modelling studies that investigate the interaction of heating rate profiles with ice supersaturation, vertical velocity, and particle nucleation and growth . present directly measured heating rates in thin tropical cirrus and highlight the importance of further observations to improve the understanding of the radiative effects of cirrus.

The present work introduces a novel dataset of in situ IWC measurements carried out during the Polar Stratosphere in a Changing Climate (POLSTRACC) campaign employing HALO, the German High Altitude and Long Range Research Aircraft . The campaign was based in Kiruna (67.5∘ N, 20.3∘ E), Sweden, and spanned the boreal winter–spring season from 17 December 2015 to 18 March 2016 with 18 science flights and 156 flight hours. The campaign accommodated a number of different science goals, including trace gas composition in the upper troposphere and lower stratosphere , gravity waves and polar stratospheric clouds . Therefore, the flight strategy often needed to avoid extended sections in cirrus clouds. Nevertheless, a total of 7.2 h or 5600 km inside cirrus during 44 individual events was recorded.

The flights included several climb or descent profiles through cirrus clouds, predominantly located just below the thermal tropopause. In this study, we take advantage of these IWC profiles and embed them in the corresponding atmospheric column of temperature and trace gases from the European Centre for Medium-Range Weather Forecasts (ECMWF) model analysis. We investigate the location of cirrus in relation to the local tropopause and the characteristic of a tropopause inversion layer (TIL) above. In a second step, this enables radiative transfer calculations that help understand the effect of these high-latitude ice clouds on the atmospheric radiation budget as a whole and on the heating rate profile inside the cloud layer in detail. This constitutes the link between realistic profiles (as they are directly measured) and their impact on local dynamics and the radiative energy budget.

The paper is laid out as follows: Sect.  contains the experimental acquisition of the in situ IWC data; an overview and the statistical characteristics of sampled Arctic cirrus clouds are given in Sect. ; Sect.  breaks the observations down to the detailed profiles; Sect.  elaborates on the radiative transfer calculations; Sect.  contains a discussion of the findings and conclusions drawn from the material; and Sect.  closes with a short outlook.

The IWC is a derived quantity from our bulk measurements of the total atmospheric water content (TWC), which consists of the gaseous (vapour) phase and condensed phases (liquid and ice). In the present closed-path principle, a sample airflow was led into the aircraft cabin and heated, in order to evaporate all water content. The TWC measurement was then performed with the WAter vapoR ANalyzer (WARAN) laser hygrometer. This instrument derives the concentration of water vapour in the sample flow by using the absorption of the 1.37 µm line from an indium–gallium–arsenide (InGaAs) tunable diode laser (TDL) in a closed measurement cell . The sensor is the commercial WVSS-II system (SpectraSensors Inc.) with a modified stainless-steel inlet line, and it is connected to a forward-facing trace gas inlet (TGI) with a 9.55 mm diameter at 31.9 cm distance from the fuselage of HALO. The measurement range is 50–10000 ppmv, with a precision of 5 % or 50 ppmv, whichever is greater. Values are sampled at a frequency of 0.3–0.4 Hz.

Although the instrument design and data treatment compensate for drifts due to changing environmental conditions, such as temperature and pressure , slow degradation requires repeated calibration within months. A calibration for the POLSTRACC campaign was performed afterwards on 28 July 2016 using a gauged dew point mirror (MBW 373-LX). The result is a linear correction function: 1H2Ocalibrated=a+b⋅H2OWARAN, where H2OWARAN denotes the output water vapour mixing ratio from WARAN, and H2Ocorrected is the calibrated mixing ratio (both in ppmv). The calibration coefficients a and b are determined piecewise: 2a=3.756ppmv,b=1.045forH2OWARAN≤70ppmv;a=12.619ppmv,b=0.944forH2OWARAN>70ppmv.

In cirrus clouds that contain only ice, the ice water content (IWC) can be calculated by subtracting the gas-phase water content (GWC) from the measurement. Due to the lack of an independent GWC measurement in clouds during the POLSTRACC campaign, the saturation mixing ratio is used as a first-order approximation, calculated from the Clausius–Clapeyron equation, as in . In reality, the relative humidity with respect to ice (RHi) inside cirrus mostly assumes values between 80 and 120 %. Stronger sub- and particularly supersaturations, up to the homogeneous freezing threshold (about 150–165 %), are possible wherever particle growth rates are too slow to mitigate the excess relative humidity . Such pronounced supersaturation has an increased probability at temperatures below 200 K, where cirrus clouds are often characterised by initially low particle number concentrations . High supersaturation is strongly correlated with high updraft speeds, which eventually trigger homogeneous nucleation with a rapid increase in the particle number concentration . Updraft speeds for the selected cases in the sections below reach rather high values of up to about 0.5–1 ms-1. This increases the expected quasi-steady-state relative humidity in the relevant temperature range to up to 120–130 %. note that relaxation times towards this state are concurrently reduced at higher updrafts from a few to tens of minutes; thus, supersaturation depends strongly on the timing of the observation within the cirrus life cycle. This natural climatological variability propagates into the uncertainty of the calculated IWC, but the effect is strongly mitigated by the particle sampling characteristics explained in the following.

Sub-isokinetic sampling due to a lower inlet flow velocity (around 3 ms-1) compared with the speed of airflow around the fuselage (typically around 230–240 ms-1) leads to an inertial enhancement of particles in the inlet flow. Therefore, a correcting “enhancement factor” (EF) needs to be applied to the IWC measurement. Following the rationale in , the EF for typical high-latitude cirrus, where significant contributions to the IWC are contained in particles with radii larger than 20 µm , can be calculated as 3EF=uau0 with the known airspeed ua and inlet velocity u0 . This approximation is independent of the precise particle size distribution and uses values of about 70 to 80 for the POLSTRACC measurements. Such high EF values effectively lower the detection limit of the corrected IWC to below 1 ppmv. The error in calculating the EF is ± 5 %, leading to a precision of 10 % for the IWC measurements. With this enhancement of the IWC over the GWC, if we were to assume a hypothetical supersaturation of RHi = 160 % instead of 100 %, the IWC in the cases below would be reduced by a factor of 0.002–0.03 on 25 January 2016 (and by a factor of 0.04–0.14 on 9 March 2016). This is the uncertainty associated with the saturation assumption made above, ranging from negligible within the instrument errors to about 14 % in case of maximum supersaturation.

Another major uncertainty in the airborne IWC measurements arises from further enhancement or depletion of particles of different sizes in the flow around the aircraft before they reach the sampling position. studied this effect in detail, including data from the Midlatitude Cirrus (ML-CIRRUS) HALO mission , and found particular ramifications for cabin instruments with inlet positions at the fuselage. found that the IWC from WARAN was mostly systematically higher than the IWC from the Fast In-situ Stratospheric Hygrometer FISH,, a Lyman-α hygrometer that samples total water in a similar way but at a different position. This comparison is repeated with the POLSTRACC data in Fig. . Here, a systematic deviation is not obvious, as the correlation between both instruments is well centred on the 1:1 line and there are no pronounced off-centre branches. More than 50 % of the measurements coincide within a factor of 2 above 1 ppmv. In a linear regression, the coefficient of determination R2=0.73 is smaller compared with that reported by , but the intercept y0=-0.040 is closer to zero and the slope m=0.952 closer to identity. This enhanced comparability is attributed to improved calibration and a precise synchronisation of data streams. It also shows that the different positions of both inlets, which are about 3 m apart along the length of the fuselage, have a less pronounced impact on the observable IWC than previously thought. Nevertheless, we need to clarify that the IWC measurements from both instruments through roof-mounted TGIs are strongly affected by the enrichment or loss of ice particles due to their inertia in the flow around the aircraft's fuselage. could quantify this effect for the HALO aircraft by comparing the IWC and particle size distribution measurements at the much less affected wing probe position, where IWC deviations of up to 1 order of magnitude were observed at a significant frequency. This flaw cannot be easily overcome for the POLSTRACC data without knowledge about the particle size distribution. Therefore, in the study of the impact of the observed IWC distributions on radiation in Sect. , we address the effects of underestimated IWC sampling. There, we test the sensitivity of our results to an underestimated IWC by re-evaluating all calculations with IWC profiles multiplied by a factor of 5, which corresponds to the average deviation seen for the FISH instrument in the worst particle size regime above 25 µm .

The POLSTRACC mission accommodated several scientific objectives that made use of the high-ceiling-altitude, long-range and heavy-payload capabilities of the HALO research aircraft . For the observation of trace gas composition and gravity waves, the flight strategy often needed to avoid extended in-cloud sections. Therefore, the dataset represents a relatively random sampling of cirrus at the middle to high latitudes. Nevertheless, the 18 science flights spanning 156 flight hours yielded an accumulated residence time of 7.2 h inside cirrus clouds during 44 individual events, as calculated from the number of WARAN IWC measurements at temperatures below 235 K.

Figure  shows the IWC measurements as coloured sections of the POLSTRACC flight tracks. It can be seen that there are many shorter sections around the landing sites during the campaign, including the following: Oberpfaffenhofen, Germany (EDMO); Kiruna, Sweden (ESNQ); and Kangerlussuaq, Greenland (BGSF). Cloud layers in these sections were often crossed during approaches and departures. Some more extended sections in mid-flight are found intermittently. The colour coding shows the range of temperatures during these cirrus encounters. The minimum temperature recording during an IWC event was 195.8 K.

With respect to the IWC as an accessible bulk property, previous studies have set up climatologies that relate the IWC to temperature and other properties, such as effective radius and number concentration e.g., and have broken them down to different latitudes on Earth and different cloud formation pathways. and later collected the range of airborne cirrus measurements from different campaigns as a function of latitude and temperature. In Fig. , the POLSTRACC measurements are added to this scheme found in . The broad latitudinal range of the POLSTRACC measurements can be seen. Although many domains have been covered by previous missions, Fig.  shows that the new data also extend into gaps that were not filled before. In particular, this concerns the lower temperatures below 210 K down to 198 K in the midlatitudes between 45 and 60∘ N. A 4 h fraction of the campaign data was sampled north of 60∘ N, and a significant extension to existing data could be made at 65∘ N over the whole temperature range. Measurements north of 70∘ N are generally sparse, and POLSTRACC can extend the coverage slightly for cold cirrus below 200 K. However, coverage is still sparse with respect to temperature and in the latitudinal range between 70∘ N and the pole.

Since the creation of the original figure by , the POLSTRACC observations represent the only addition of airborne in situ data at northern polar latitudes. Further observations not included in this figure comprise, for example, the previously mentioned ML-CIRRUS campaign at midlatitudes .

This study focuses on cirrus near the tropopause. We examine the distribution of cirrus sampling along different tropopause-related vertical coordinates, which helps to assess the role of the geometrical location and extent, as well as temperature, in the cloud radiative effect, which at the tropopause becomes especially relevant in view of thermal stratification. Furthermore, different formation pathways and ice particle properties are expected for tropospheric and stratospheric cirrus. The following discussion is limited to observations north of 60∘ N. Figure a displays the cirrus observations using two vertical scales: the abscissa indicates the potential temperature difference dTP of the observation location to the 2 PVU (potential vorticity unit, 1PVU=10-6Km2kg-1s-1) isosurface of potential vorticity (often denoted as dynamical tropopause), which is typically located at a potential temperature of 300–320 K (but a further ±10 K is possible). The ordinate axis indicates the squared Brunt–Väisälä frequency (N2) as a measure of the static stability of the atmospheric thermal stratification. The colour coding indicates the temperature as in Fig. . Close to the tropopause, roughly linear correlations between dTP and N2 can be observed, connecting the free, statically less stable troposphere with N2<2×10-4 s-2 to the stratosphere with N2>4×10-4 s-2. This correlation is not compact but spans a corridor with a width of 7 to 10 K. The lower-right branch belongs to January mid-winter measurements, whereas the upper-left branch stems from the March late-winter measurements. This may be interpreted as an increasing stabilisation of the tropopause region within 2 months due to the steady descending motion in the stratosphere . The blue colours highlight the occurrence of cold cirrus at T≲200 K above the dynamical tropopause. In most cases, these cirrus clouds on the stratospheric side have no continuation in the troposphere. They build up a lower branch of polar stratospheric ice clouds (PSCs) and do not rely on a tropospheric water vapour supply. Ice nucleation, particle size and habit as well as chemical constitution differ significantly from the tropospheric tropopause cirrus, and they play a crucial role in heterogeneous chemistry in the lowermost stratosphere, especially impacting the budgets of ozone-depleting substances. Figure b shows the distribution of measurements with respect to N2. Two major accumulation regions can be identified: one at 2×10-4 to 3×10-4 s-2 and another at 4×10-4 to 5×10-4 s-2. The former can be attributed to tropospheric cirrus extending up until the thermal tropopause according to the definition from the World Meterological Organization, WMO; and the latter to stratospheric cirrus with no or only a weak connection to the troposphere.

The distribution of cirrus observations can also be related directly to potential vorticity (PV), as seen in the bars in Fig. c. As already indicated above, a PV of 2 PVU is usually chosen as the dynamical tropopause and follows other tropopause definitions adequately in areas of weak horizontal wind shear, although it is located systematically below the thermal tropopause . For the extratropics a value of 3.5 PVU is often more consistent with the thermal tropopause . In Fig. , the low counts in the 0 to 0.5 PVU range may suggest a low cloud occurrence in the mid-troposphere. This is not necessarily the case and may be a result of the artificial limitation of the data to temperatures below 235 K, which has been applied in order to exclude false hits from mixed-phase clouds. Therefore, certainly more clouds were encountered there, composed of pure ice, mixed phases or liquid water. The highest cirrus occurrence is found below the 2 PVU isosurface with a maximum at 1–1.5 PVU, which is unambiguously located in the troposphere, where the available water vapour content is still at least 1 order of magnitude above stratospheric values. Higher up, cirrus sampling steadily decreases towards a rather uniform distribution above the 3.5 PVU isosurface. This reflects a random distribution of cirrus observations in the stratosphere, where PV varies strongly in the vertical direction and no single level is preferred for cirrus occurrence. Among the generally low IWC sampling frequency in the stratosphere, the apparent rise between 6 and 8 PVU is most likely due to the overall distribution of flight time in the stratosphere during the campaign, as can be seen by the black line in Fig. c.

The following sections (Sects.  and ) focus on one subclass of IWC measurements that represent complete vertical profiles of cirrus clouds at the tropopause, seen as the line-forming dots of data in Fig. a.

In order to investigate the radiative impact of cirrus at the tropopause on the profile of temperature and stability, it is necessary to include the complete profile of the respective cloud in the analysis. The POLSTRACC IWC dataset has been filtered to this end, and a selection of eight events was found that displayed the required profiles. These events are listed in Table . There are some caveats towards the use of the respective cloud profiles. First, very low IWC values at or below about 10 ppmv are below the associated water vapour concentrations. Although thin cirrus clouds are common in the Arctic e.g., this complicates the distinction of the precise cloud impact in face of the surrounding atmosphere. Second, some profiles are interrupted by short periods of horizontal flight, extending the time between the cloud bottom and top crossing to well above 10 min. This exacerbates the uncertainty with respect to how well the measurements actually correspond to a real vertical cloud profile. Third, the midlatitude profiles south of 60∘ N (Germany, Denmark and southern Sweden) are of lesser interest for this study.

Following these criteria, the profiles on 25 January and 9 March 2016 are the best candidates for the present study. The January profile still includes two horizontal sections with an accumulated duration of 18 min, which equals about 230 km in distance.

Focusing on these two profiles, Fig. a and c display respective time–altitude cross sections (curtains). The cirrus profiles are marked by the thicker section of the shown flight paths in orange. The background shading displays the squared Brunt–Väisälä frequency N2 from the ECMWF reanalysis as an indicator of the vertical static stability of the atmosphere. The troposphere and stratosphere can be clearly distinguished by dark blue colours and greenish/yellow tones respectively. Tropospheric enhancements with N2 up to 4.0 s-2 in the boundary layer and at 5 km altitude are identifiable. The strongest N2 enhancement is found directly above the thermal tropopause (cyan line, representing the WMO definition), marking a pronounced tropopause inversion layer in both cases, despite very different tropopause altitudes. The extended vertical profiles of the reanalysis data show different patterns in both profiles: on 25 January, the stratospheric profile features a fine-scaled structure of static stability that trigger multiple thermal tropopause identifications (cyan lines); in contrast, on 9 March, one broad stratospheric inversion above 13 km can be seen. Both of these observations are not uncommon in regions influenced by orographic gravity waves and in the vicinity of the jet. Missing N2 data at low altitudes are due to the orographic cutoff of the reanalysis data above terrain in Scandinavia or Greenland.

Figure b and d contain the in situ IWC measurements from the WARAN instrument, where the time series are aligned with the curtain plots above. On 25 January, the measurements show a considerable variability in the horizontal flight sections, reflecting the overall patchy atmospheric structure. In contrast, the descent towards the end of the profile is characterised by an IWC below 4 ppmv before an enhanced IWC is again observed at the cloud bottom below 8.4 km. On 9 March, the profile exhibits an almost symmetrical double-peaked structure of the IWC distribution inside the cloud. In both cases, the IWC varies frequently by up to 1 order of magnitude. From these slant profiles, we derive vertical IWC profiles for the later sections just by stacking the measurements vertically, neglecting any horizontal inhomogeneity in the clouds. Horizontal flight path sections are condensed into one mean IWC value. Although this no longer represents the actual whole (but unknown) three-dimensional IWC distribution inside the cloud, we still deem these artificial one-dimensional profiles to be realistic. Especially in the critical 25 January case, a very similar profile would result if we only considered the last section of the flight path that contains the majority of the vertical coverage.

We note that possible lower-level clouds might exist below the observed cirrus or also along ground-reaching flight paths. Beyond possible visual identification, such clouds were excluded from sampling (via inlet flow cutoff) during the POLSTRACC campaign to avoid impairment of the targeted measurements of low concentrations of water vapour and cloud water in the upper troposphere and above.

Radiative transfer calculations focus on the ice clouds that have been probed on 25 January and 9 March 2016. Both days show significant differences in the shape of the IWC(z) and in the geometrical thickness of the ice cloud (Fig. ). In particular, the IWC(z) profiles reveal vertical fine structures.

This section describes the radiative transfer model, its input and treated radiation parameters. It is shown how the specific properties of the measured IWC(z) profiles affect irradiances, the radiative forcing and heating rate profiles. The radiative effects of the detected IWC fine structures are discussed via comparison to the results that would emerge for IWC profiles with vertically coarser resolutions. The mentioned uncertainty of the IWC measurements, which is likely an underestimation, is addressed by repeated calculations using the IWC profiles multiplied by 5.

Our radiative transfer simulations for selected profiles of the IWC measured during the POLSTRACC campaign indicate that an ice cloud of extended geometrical thickness and a specific vertically asymmetric fine structure of the IWC change the radiation budget at the TOA compared with the results of a vertically constant IWC profile of equal optical thickness. Effects are significant and, for our cases, in the order of a few watts per square metre (Wm-2). Fine structures in geometrically thin ice clouds that have a more symmetric IWC profile do not lead to a noticeable modification of the TOA radiation budget when compared to the case of a vertically constant IWC. This is consistent with the results of , who performed a statistical analysis of cirrus IWC profiles and their radiative effects from active and passive spaceborne remote sensing products. Similar to our above result, they concluded that the LW effect of a certain profile (at the TOA) is modulated by an “effective radiative layer” with respect to a constant IWC profile, which reflects internal inhomogeneity in the vertical IWC distribution. We show that the uncertainty arising from a possible coarser vertical resolution is reduced to below 20 % for the inhomogeneous IWC profile if layers are thinner than about 500 m. The study by is dominated by low and midlatitude cirrus properties with a higher IWP and optical thickness. In contrast, our analyses are focused on thinner (still opaque) Arctic cirrus clouds that exist in darkness or at high solar zenith angles such that their net radiative forcing may be of alternating sign.

A significant impact on net irradiances at the top and the bottom of the atmosphere is found to originate from the albedo of typical Arctic surface types, such as the open ocean or snow-covered regions, which also implies effects on the radiative forcing. Depending on the solar zenith angle, effects are strong for Fnet,TOA and mostly strong or moderate for Fnet,BOA but weak for Fnet,BOA under conditions of fresh fallen snow. Age and pollution of snow seem important factors in this context. For example, the transition from negative to positive values of the TOA radiation budget, equivalent to a change from a loss to a gain in radiative energy, occurs at greater solar zenith angles over the ocean than, for example, over aged snow (αsnow=0.60). Furthermore, over aged snow, the transition is shifted towards a higher solar zenith angle when compared with results for a cloud-free atmosphere. This means that the presence of cirrus leads to an enhanced gain in radiative energy in the Arctic atmosphere in most cases during autumn to spring as well as over bright surfaces. Therefore, in scenarios with an overall negative trend in surface albedos in the Arctic, the positive radiative forcing effect of cirrus is expected to decrease. This decrease is relevant in view of the Arctic amplification e.g., as it constitutes a possible counteracting effect. Notwithstanding, the (direct) impact on surface net irradiances and, therefore, on surface temperature is much less pronounced.

Instantaneous heating rate profiles of the atmosphere are mainly affected by fine structures of the IWC within the cloud layer and remain negligible below and above the cloud. The effect of different surface albedo values on instantaneous heating rate profiles is only very weak. Depending on the profile of IWC or optical thickness, the heating rate profile can contribute to a strengthening or weakening of a given structure in the temperature profile, especially concerning the cold point of the tropopause, as well as the strength of the above inversion layer. The strong tropospheric lapse rate in some of our observed cloud profiles induces a stabilising radiative effect of the cloud in most of the tropospheric heating rate profile inside the cloud layer, with an overall higher (LW) heating rate at the cloud top (warming) than at the cloud bottom (less warming or cooling). However, first of all, a heating rate profile will trigger cloud internal processes through evaporation or latent heat release that drive vertical motion, modify the temperature gradient and change the microphysical properties of the ice particles.

Concerning a probable underestimation of the IWC in the measurements, we derived that a potentially higher true IWC profile would lead, as can be expected, to a higher warming impact of the cirrus on the TOA radiation budget and would dampen the effect of different surface albedo values. At the same time, the heating rate profiles would be more dominated by the LW contribution due to the strongly enhanced optical thickness. Still, we conclude that our original calculations are of value because many thinner cirrus clouds exist in the Arctic (and were observed) to which our results would apply accordingly, beyond the two cases that we analysed.

Some aspects of Arctic ice clouds have not been treated in this work, such as the effects of the modification of size and number density by specific cloud formation regimes. Likewise, the effects of different ice crystal habits or habit compositions on SW and LW irradiances, radiative forcings and heating rates were not in the focus of this study. Based on measurements of a subtropical cirrus, studies such as and have shown that irradiances are significantly sensitive to different ice crystal shapes in the SW and LW, depending on solar zenith angle, location above or below the cloud, cirrus optical thickness, and the spectral region within the SW and LW range. For certain constellations, maximum changes in irradiances even reach 26 % in the SW and up to 70 % in the LW. The effects of specific habits (i.e. hexagonal columns, plates, rosettes, aggregates and spheres) are intercompared. As mentioned, radiative transfer calculations in this study have been carried out under the assumption of a general habit mixture (GHM) due to the lack of in situ measurements of ice crystal habits during the POLSTRACC campaign. It cannot be ruled out that different habit mixes occur in the Arctic, in which pristine crystals (e.g. hexagonal plates or columns) play a more dominant role. Nevertheless, we choose the GHM because the assumption that an ice cloud consists of one habit type only also seems unrealistic.

The flight and measurement strategies pose additional sources of uncertainty. Slant pseudo-vertical IWC profiles or, even more so, those with intermediate level flight sections extend horizontally over several 10 to 100 km and likely do not adequately account for lateral inhomogeneity within the cirrus cloud field. This uncertainty might be reduced with higher vertical speed, at the cost of reduced vertical resolution or sensor accuracy, or better quantified by repeated sampling in different directions or co-located satellite observations. Similar studies would also greatly benefit from information on potential lower cloud coverage below the cirrus.

Recent and current observational activities will provide additional data on cloud microphysical properties to better represent real Arctic cirrus in future work, aiming for a closure of the physics of particle-scale processes with the radiation field. In this respect, the in situ measurements of the recent CIRRUS-HL (Cirrus in High Latitudes) mission are very promising. Further useful contributions are expected to come from ongoing and planned campaign activities.