The melt season of the Arctic sea-ice shows substantial interannual and spatial variability and is lengthening due to ongoing warming both by earlier spring melt onset and later fall refreeze. Studies highlight the role of surface Cloud Radiative Effect (CRE) in initiating the spring melt onset, with significant implications for sea-ice extent and variability during the summer months . The surface CRE is characterized by a balance between a shortwave cooling effect and a longwave warming effect. Clouds absorb part of the longwave radiation emitted by the sea-ice surface and re-emit it downward, resulting in surface longwave warming relative to clear skies. The LongWave Cloud Radiative Effect (LWCRE) reaches up to 80 Wm-2 locally at the surface with spatial and seasonal variability due to specific surface and atmospheric conditions . Conversely, clouds reflect incoming solar shortwave radiation back to space, leading to surface shortwave cooling. During spring, the surface CRE is dominated by cloud surface longwave warming, as cloud surface shortwave cooling is weak at that time of the year over sea ice because incoming solar radiation is half of that in July (Fig. ) and the surface albedo is still high.

The onset of the Arctic sea-ice melt season occurs around April–May depending on the definition of melt onset , commonly based either on the first surface temperature threshold crossing or on the first satellite-detected wet snow/liquid water signal from passive microwave observations. It coincides with a seasonal increase in cloud cover, referred to here as the Spring Cloud Onset, that has been observed in ground-based measurements , satellite observations and simulated by global climate models (). The increase in cloud cover over sea ice in spring is primarily driven by an increase in low altitude liquid-containing clouds . On the one hand, the presence of liquid droplets enhances cloud longwave emissivity . On the other hand, lower-altitude clouds tend to be warmer than higher-altitude clouds. Both of these processes serve to amplify the downwelling radiative flux at the surface . Consequently, the spring increase in low altitude liquid-containing clouds enhances cloud-induced surface radiative warming .

Two broad mechanisms may drive low liquid cloud formation in spring over the Arctic sea ice: changes in large-scale meridional transport of heat and moisture, and changes in temperature associated with the rapid seasonal increase in solar radiation. From the transport perspective, low-level clouds over sea ice form through the condensation of relatively warm and wet air masses transported from the mid-latitudes over the colder Arctic sea ice . Building on that mechanism, a hypothesis is that the spring cloud onset would be connected to an increasing amount of moisture advected over the sea-ice in spring. However, argue that the poleward moisture transport increases only after the observed cloud onset, potentially challenging the validity of that hypothesis. Modern reanalysis confirms the late timing of the spring poleward moisture flux increase , though its connection to the spring cloud onset has not been explored.

proposed an alternative mechanism in which the spring cloud onset is controlled primarily by the temperature dependence of cloud phase partitioning associated with the Wegener–Bergeron–Findeisen (WBF) process . In this framework, the low temperatures prevailing in winter (below ∼-13 °C) favor efficient ice production at the expense of liquid droplets, depleting atmospheric moisture and inhibiting liquid-containing clouds from forming. As spring progresses and the lower troposphere warms, the efficiency of the WBF process decreases, allowing liquid-containing clouds to form more readily and persist. Importantly, the spring warming over sea ice is largely attributable to increasing incoming solar radiation. Estimates of the Arctic energy budget show that moist static energy convergence from the mid-latitudes decreases into spring, while the positive spring tendency of moist static energy stored over the sea-ice aligns with the seasonal increase in solar radiation (; Appendix ). This suggests that the springtime temperature rise relevant for WBF weakening is predominantly forced by solar radiation, rather than driven by increasing large-scale energy transport. Studies have since demonstrated the dependency of cloud phase on temperature across different regions (e.g., ), primarily in the context of improving the representation of mixed-phase clouds in climate models. However, there remains a need to examine this relationship specifically within the Arctic spring, where rapid temperature changes driven by the transition from polar night to polar day could have a decisive impact on cloud phase partitioning. Aerosol availability can also modulate Arctic cloud phase partitioning, since both liquid droplets and ice crystals form on aerosol particles. Liquid cloud formation depends on the concentration and properties of cloud condensation nuclei (CCN), whereas ice formation in the heterogeneous regime (i.e., for temperatures warmer than ∼-38 °C) depends on ice-nucleating particles (INP) . Because long-range transport and surface conditions influence the concentration and type of CCN and INP in the Arctic troposphere (e.g., ), aerosol variability represents an additional control on cloud phase over sea ice beyond temperature alone . In turn, low-level cloud properties can be influenced by boundary-layer processes . For example, cloud-top longwave radiative cooling can drive turbulence and mixing in the boundary layer, which sustains low-level Arctic clouds by supplying moisture to the cloud layer . With these contrasting perspectives, the mechanisms underlying the spring cloud onset remain an open question, and the two historical hypotheses continue to be cited as plausible explanations .

A key reason for the remaining question lies in the limitations of available datasets for studying Arctic cloud evolution in spring. Field campaigns such as SHEBA and MOSAiC are restricted in both space and time. These campaigns, while invaluable, offer snapshots that may not fully capture the spatio-temporal variability and trends across the entire Arctic region. The Central Arctic, particularly during spring, is difficult to access due to high sea-ice cover, as noted in the experience of the Art of Melt campaign in 2023 . Passive remote sensing aboard satellites faces reliability issues in the Arctic environment. The combination of solar radiation and extensive ice cover introduces biases in atmospheric observations, leading to potential inaccuracies in data interpretation . Lastly, models often struggle with the complex physical processes and feedback mechanisms that govern cloud physics in the Arctic .

Analyzing recent multi-annual cloud observations from satellite active remote sensing offers an improved understanding of the spring cloud onset. Active remote sensing techniques, such as cloud radar and lidar, are valuable tools for cloud detection above both the sea-ice and open water surface in all seasons and offer continuous observation of the whole tropospheric column. However, there are some limitations. The lidar signal can be attenuated by liquid clouds and cloud beyond liquid layers can be missed. showed that if not considered, these limitations can introduce biases in the cloud surface radiative warming of -54 Wm-2 locally. The space-based lidar onboard CALIPSO provides opportunities to give insights into the Arctic spring cloud onset by examining 13 years of observed cloud profiles over all surface types of the Arctic, at instantaneous high spatial and temporal resolution. By leveraging this extensive dataset, we can reassess the two hypotheses proposed to explain the spring cloud onset.

We first describe (Sect. ) the different datasets used in this study. In Sect. , we present 13 years of space-based observations, analyzing the evolution of cloud properties during the spring cloud onset over Arctic sea ice. We then examine the two main hypotheses proposed to explain this phenomenon: Sects.  and  explore the mechanism suggested by , while Sect.  investigates the hypothesis put forward by .

As demonstrated in Fig. 1, the mean evolution of low cloud cover (summarized in Fig. 4a) shows an increase of +37 % between early April and early May, marking the spring cloud onset. This increase suggests the need of a significant moisture source to (i) maintain “close to saturation” conditions while the Arctic atmosphere can contain an increasing amount of water vapor as spring temperatures rise and (ii) support the formation of more frequent liquid-containing layers.

During early spring, cold and dry conditions over sea ice limit the total atmospheric water vapor, which totals only 2.1×1013 kg in March and 2.9×1013 kg in April within the region outlined in Fig. 4. Moisture advected over the Arctic sea-ice (Fig. 4c) does not demonstrate an obvious seasonal increase related to the timing of the spring cloud onset, consistent with and , and the daily advected moisture mass is estimated at around 0.3×1013 kg in March and 0.32×1013 kg in April. Therefore, on a daily basis, a mass that represents 14 % in March and 11 % in April of the total atmospheric water vapor present above the sea ice is advected into that domain. We investigated the sensitivity of present results on the convergence moisture flux by considering segment of the contour (Fig.  in Appendix J): the Atlantic sector, the Pacific Sector, the Canadian archipelago sector and the Siberian sector. Although the Pacific sector highlights an increase in May and the Atlantic sector highlights a decrease of that poleward moisture flux, none of the sector highlights an increase in early April. Overall, we do not see an increase of the moisture transport. either in relative or absolute terms, that would affect the timing of the spring cloud onset, but it is likely that the amount of moisture transported is enough to support “close to saturation” state from March to May over sea-ice. Moreover, since the convergence moisture flux is approximately constant over time (Fig. 4c) but the moisture carrying capacity of the atmosphere increases with seasonal warming (Fig. 4b), this suggests that there could be less excess moisture available to support a “close to saturation” state in April relative to March. This implies that another process likely inhibits the development of optically thick clouds in March, despite sufficient moisture supply.

To examine the relationship between the convergence moisture flux and the low cloud cover over the Arctic sea-ice, we examine two events from 2015, one strong Arctic moisture intrusion before the spring cloud onset and one moisture depletion event during the spring cloud onset. During 2015, early March experiences a convergence moisture flux anomaly of +2.3×1012 kgd-1 (+80 % relative to the long-term mean on 5 March). Looking at synoptic maps and specific humidity maps (Fig.  in Appendix F), we assess that cyclonic conditions east of Greenland cause a strong moisture intrusion over the North Atlantic that reaches the Laptev Sea on 10 March. The increase of low cloud cover (50 % instead of the long-term mean of 35 % on 7 March) is localized at the site of the moisture intrusion, therefore the increase is not homogeneous over the Arctic sea-ice covered region. Once the moisture intrusion is passed, the low cloud cover goes back to near average values around 30 % on 18 March. Later in the middle of the spring cloud onset, around 25 April, a blocking event, characterized by a high-pressure system centered north of Greenland and spread across much of the central Arctic prevents more moisture from advecting into the Central Arctic, also visible in the small decrease of total water vapor a week later (Fig. ). Following a small drop in low cloud cover on 20 April, the blocking period is otherwise marked by a sustained increase of low cloud cover, rising from 45 % on 18 April to 80 % by 1 May. Despite these two moisture transport events that serve to counteract the transition, the spring cloud onset is still observed in 2015, with low cloud cover increasing from 35 % in March (+1 % above the long-term monthly mean evolution) to 74 % in May (+3 % above the long-term monthly mean evolution). However, during this year the timing of the spring cloud onset is shifted a week earlier as it occurs between 1 April and 1 May.

The MOSAiC humidity observations from radiosondes highlight quasi permanent saturation w.r.t ice (Fig. 5a) between March and mid-May, with 96 % of profiles containing at least one level that is saturated w.r.t ice above 700 m altitude, which is consistent with past observations . Independent cloud observations made by the ground-based lidar during MOSAiC (Fig. 5b) confirm that ice particles are frequently observed. Using a threshold for cloud detection of SR>3, atmospheric ice layers are observed in 31 % of the lidar profiles, and 94 % of the days see at least one ice detection over the day, highlighting their highly frequent existence in the atmosphere. Although ice saturation is not a sufficient condition for ice formation, which in the heterogeneous regime additionally depends on the availability of INPs, we observe that a large proportion of saturation w.r.t ice occurrence coincide with ice detections from the lidar (60 %). Overall, Fig. 5 shows that the cloud ice detection from the ground-based lidar observations agrees well with the saturation w.r.t ice from the radiosonde profiles. The figure highlights also that, during MOSAiC, there was almost always enough moisture to favor ice particle formation somewhere in the lowest 3.2 km of the atmosphere, with the exception of a one-week period around 25 April when the atmosphere was particularly dry and cloud free.

Examining the thermodynamic conditions, periods with saturation w.r.t liquid were observed to closely align with warmer temperatures. Figure 5a mainly highlights saturation w.r.t liquid after 15 April (85 % of the occurrences of saturation w.r.t liquid), after which time the 800 m temperatures remained around -10 ± 5 °C until mid-May (Fig. 5c). The increase of temperature on 15 April is well documented and is linked to an important moisture intrusion from the mid-latitudes and a general shift in circulation patterns. Figure 5a shows that the atmosphere prior to that event is already often saturated w.r.t ice, and the apparent efficiency of ice processes and colder temperatures during this period may have limited the occurrence of saturation w.r.t liquid. The main occurrences of saturation w.r.t liquid during this period were in late-March, when temperatures periodically increased to -10 °C at 1 km altitude (Fig. 5c).

Figure 6 demonstrates that while only 2 % of atmospheric layers below 3.2 km contain liquid at -25 °C, liquid-containing clouds occur in 12 % of atmospheric layers at -13 °C. This steep increase of the liquid-containing cloud percentage occurs from about -25 °C to about -13 °C, with a small decrease in liquid-containing cloud occurrence at warmer temperatures. Liquid-containing clouds become more frequent than ice-only clouds at about -20 °C. This result is similar to, but slightly colder than past in-situ and ground-based observations of Arctic clouds showing that clouds containing liquid water become more prevalent than those that do not at temperatures above -15 °C. . Considering unclassified clouds as ice clouds , the sum of ice cloud and unclassified cloud frequencies is almost independent of the temperature, 4 % at -30 °C, 3 % at -20 °C and 5 % at -10 °C. We further observe the increased frequency of unclassified cloud layers to be larger than ice cloud layers around -10 °C (3 % unclassified layers against 2 % ice at -10 °C), which might be a signature of more mixed-phase clouds.

Within clouds, observations from the MOSAiC lidar and from CALIPSO-GOCCP over the Arctic sea-ice covered region agree that three distinct temperature regimes exist for the ice phase ratio (Fig. 7a). First, there is a plateau between -42 to -30 °C with the dominance of ice clouds (ice phase ratio >90 %). Then, a transition occurs between -30 and -13 °C where the ice phase ratio decreases steeply from ∼90 % to ∼20 %. The last temperature regime ranges from -13 to 0 °C, and is dominated by liquid-containing clouds with a ice phase ratio <20 %.

Despite the general agreement between each observational system, the MOSAiC lidar sees the icy plateau at a higher ice phase ratio than CALIPSO (∼100 % against 90 %), the liquid plateau at a lower ice phase ratio (10 % against 20 %) and a shape of the transition between the two plateaus that is slightly different, mainly due to irregularities in the MOSAiC curve.

Figure 7b shows the temperature distributions of all atmospheric layers below 3.2 km including clear-sky layers, contrary to Fig. 7a which was for cloudy layers only. Figure 7b highlights an increase of atmospheric layer temperature from -19.7 ± 5.7 °C in March to -13.2 ± 5.1 °C in May. This shift in temperature range coincides with a decrease of the ice phase ratio (Fig. 7a), from generally ice-cloud dominated to liquid-containing cloud dominated in May. We recall that this change in ice phase ratio is mainly due to the steep increase of liquid relative frequency between -20 and -10 °C, while the ice occurrence is poorly dependent on temperature between -40 to 0 °C (Fig. 6)

This study shows the seasonal increase of low clouds over the Arctic sea-ice between March and May as seen by 13 years of space-based lidar observations and examines mechanisms leading to this increase. Over most of the Arctic, Fig. 1 shows that the low cloud cover increases from 34 % to 71 % between the first half of April and the first half of May, without strong regional patterns. In particular, the spring cloud onset does not show earlier timing or stronger amplitude when looking at the edge of the sea-ice. This seasonal increase is strongly connected to the increase of liquid-containing optically thick clouds below 1 km altitude (Figs. 2 and 3). However, ice particles are ubiquitous in spring close to the surface either in the form of thin ice particle layers or within Arctic mixed-phase clouds later in May. Figure 4 shows that no clear temporal correlation exists between the spring cloud onset and the seasonal increase of convergence moisture flux above the sea-ice, although individual moisture intrusions from lower latitudes and Arctic atmospheric blockings do appear to influence the low cloud cover over the Arctic sea-ice covered region (Fig. 4). Furthermore, we assess that the magnitude of the moisture flux entering over the Arctic sea-ice in March and April (around 3.1×1012 kgd-1) is likely sufficient to support a continuous “close to saturation” state over the period by supplying the additional water vapor needed as rising temperatures increase the atmosphere's capacity to hold moisture during this period. The daily advected moisture mass corresponds to 11 % to 14 % of the total water vapor mass over the sea-ice, supporting both the maintenance of near-saturation and the seasonal increase in atmospheric water vapor. In addition, MOSAiC in-situ measurements (Fig. 5) show that 95 % of atmospheric soundings contain layers that are at least saturated with respect to ice, while the ground-based lidar detects atmospheric ice particles below 3.2 km in 94 % of observed days between March and May 2020. These results suggest that water vapor transport is not a limiting factor for the spring cloud onset and that early spring ice cloud production processes, especially at temperatures below -15 °C, might deplete atmospheric moisture and hinder liquid-containing cloud formation. A broader statistical analysis of space-based and ground-based lidar data presented in Figs. 6 and 7 reinforces the temperature dependency of cloud phase. Figure 6 shows the steep increase in liquid-containing layers between -20 and -10 °C while ice particles are observed at all temperatures below 0 °C. Examining the phase ratio within clouds, this rapid increase of liquid-containing cloud occurrences indicates the predominance of liquid-containing clouds over ice-only clouds for temperatures between -15 to 0 °C. As ice is present at all temperatures and liquid is infrequent below -20 °C, the phase ratio within clouds highlights the dominance of ice clouds over liquid-containing clouds for temperatures lower than -20 °C. Additionally, we show that the seasonal variation of lower troposphere temperatures between March and May is consistent with a transition from ice dominant clouds to liquid dominant clouds. The difference of temperature in the lower troposphere between March and May is great enough that March temperatures statistically favor more ice production at the expense of liquid droplets and May temperatures allow for relatively more liquid droplet formation. Considering all these points, it appears that the increase of cloudiness around mid-April across the Arctic sea-ice region is connected to the seasonal increase of temperature, which modifies the balance of cloud phase occurrence, with cloud formation supported by large but steady moisture advection from the mid-latitudes. The predominantly solar radiation origin of the spring warming further reduces the role of atmospheric meridional transport in controlling the spring cloud onset (Appendix ). Overall, this study supports the hypothesis that the spring cloud onset is primarily set by solar radiation constraints, with atmospheric transport mainly modulating day-to-day and interannual variability (See Figs.  and  in Appendix I).

Three elements following our analysis should be noted. First, this study establishes a hierarchy of mechanisms. The spring warming, induced by the increase of solar radiation, sets the existence of the pan-Arctic transition toward more liquid-containing low clouds between March and May, while atmospheric dynamics may largely control day-to-day short-term variability superimposed on this seasonal evolution. Building on existing Arctic air-mass transformation case-study literature , future work could extend these insights by leveraging multi-year, pan-Arctic satellite datasets together with trajectory-based diagnostics to quantify synoptic variability and interannual anomalies. Second, MOSAiC in-situ measurements show that the atmosphere above the Arctic sea-ice is almost always saturated at least w.r.t ice during most of spring, suggesting that water vapor is not a limiting factor for the spring cloud onset. However, a statistical approach for analyzing water vapor saturation over the full 13 years of cloud observations presented here is not possible due to the lack of suitable observations. We found that while ERA5 performs well overall, it fails to capture most of the saturation, making it unsuitable for this specific analysis. Finally, within the temperature range of about -40 to 0 °C, liquid droplets and ice crystals can coexist, and we acknowledge that additional processes influence cloud phase partitioning beyond temperature alone . Strong vertical motions can induce adiabatic warming or cooling, while the availability of ice-nucleating particles (INPs) versus cloud condensation nuclei (CCN) also plays a critical role in determining cloud phase partitioning , although previous studies showed no temporal correlation between the spring cloud onset and a seasonal increase of any type of aerosols . Beyond that study, further investigation into the relative contributions of these mechanisms could help explain both the timing of the spring cloud onset and its interannual variability. Nevertheless, although the processes driving the projected springtime warming remain debated , spring temperatures are expected to increase by 5 °C by the end of the century . Our results therefore motivate assessing whether the timing of the spring cloud onset may shift earlier in spring in a warmer Arctic, potentially contributing to earlier sea-ice melt onset and a lengthening of the melt season as already observed in recent decades . Supporting this interpretation, our short interannual variability analysis suggests that warmer March conditions are associated with increased March cloudiness (see Fig. ). Current efforts to bridge cloud space-based observations such as the new EarthCARE mission to obtain a longer dataset, from 2008 to the present, seem of high importance in that perspective.