Figure 4(a) shows the solar climate at the top of the atmosphere (TOA) over the four villages in the Yukon Flats, Arctic Village on the slope of the Brooks Range

and for comparison Fairbanks (64.8378N, 147.7164W) calculated for the cold season (October 1 to April 30). Even Circle barely has any insolation around winter solstice.

Figure 4(b) shows exemplarily the shortwave downward radiation at 2-m and 10-m height as observed in Beaver, Chalkyitsik and Circle towards the end of the cold season. At all sites, incoming solar radiation was lower when a low pressure system went over the region. Slight temporal offsets occurred due to the different locations. As expected, on average, near-surface solar radiation was higher in the farther south than farther northern locations (Figure 1, Figure 4(a)).

The comparison of the near-surface shortwave downward radiation and that at the TOA revealed the following. At all sites, the total amount of solar radiation absorbed between the TOA and 10-m above the ground level (AGL) increased with increasing insolation (compare Figure 4(a) and Figure 4(b)). Even during times without frontal passages, notable absorption of solar radiation occurred between the TOA and the surface sites. On average over all available data, the near-surface shortwave downward radiation was 75.2, 38.6 and 66.2 W∙m⁻² lower than at the TOA for Beaver, Chalkyitsik and Circle, respectively. This means at these sites, on average, near-surface shortwave downward radiation was about 27%, 43% and 24% of the respective values at the TOA (Figure 4(c)).

Incoming solar radiation at 10-m and 2-m heights correlated about 90%, 95% and 99% at Beaver, Circle and Chalkyitsik, respectively. Typically, at all sites, incoming solar radiation at 10-m was notably higher than at 2-m height above ground level (Figure 4(d)). Even during frontal passages differences occurred. On average over all available observations, differences were 30.3, 28.1 and 24.7 W∙m⁻² at Beaver, Circle and Chalkyitsik, respectively. The percentage of solar radiation absorbed between 10-m and 2-m decreased with increasing solar radiation (Figure 4(d)).

Together these observations suggest that notable amounts of absorbing gases, particles and potentially water droplets and/or ice existed between 10-m and 2-m height. These findings can be explained by local emissions of pollutants and water vapor that accumulated under SBIs (Figure 3, Figure 5, Figure 6).

The gradient between the 2-m air temperatures at Arctic Village and Ft. Yukon served as a proxy for the “vertical” temperature gradient over the Yukon Flats. Comparing the hourly meteorological conditions at Ft. Yukon and Arctic Village revealed the following. During the October to April 2017/18 cold season, mean 10-m wind speeds and 2-m temperatures were 1.67 m∙s⁻¹, −14.8˚C and 1.62 m∙s⁻¹, −16.4˚C at the sites in 138 m and 638 m above sea-level (ASL), respectively. Mean sea-level pressure was about 1019.43 hPa. In March and April, 10-m wind speeds increased. Such increase in March wind speed was also found for Fairbanks [24] [25] [26].

During inversions over the Yukon Flats, the mean temperature gradient was 1.07∙(100 m)⁻¹, and −1.11∙(100 m)⁻¹ otherwise. On average over all data, the temperature gradient was −0.63 K∙(100 m)⁻¹, i.e. close to the mean environmental lapse rate. During the times without insolation, the inversions between 638 m and 138 m were the strongest reaching up to 3.76 K∙(100 m)⁻¹ (Figure 5). On cold season average, the temperature gradient was 1.07 K∙(100 m⁻¹) and −1.11 K∙(100 m⁻¹) during times with and without inversions, respectively. The strongest decrease of temperature with height was −3.66 K∙(100 m⁻¹), which hints at conditions with nearly height-constant density.

The frequency and duration of inversions over the Yukon Flats increased as insolation decreased towards winter solstice and as time without insolation continued thereafter (cf. Figure 4, Figure 5). After the Sun raised above the horizon again, inversion duration and frequency decreased (cf. Figure 4, Figure 6). Typically, the strongest and longest inversions coincided with no or only weak insolation at the TOA (Figures 4-6). On daily average, the total duration of inversions between 638 m and 138 m was 3.8 h.

The 10-m to 2-m temperature-gradient data revealed that low SBIs developed daily over the villages in the Yukon Flats through the night and lasted into late morning (e.g. Figure 6). The duration of SBIs increased as insolation decreased. The opposite was true when the Sun raised again above the horizon and insolation increased (cf. Figure 4, Figure 6). Note that this pattern also was found for the time after and before summer solstice [11].

Daily mean durations of inversions were 9.43 h (Beaver), 10.60 h (Chalkyitsik) and 13.33 h (Circle). During SBIs, the daily mean temperature gradients were 5.35∙(100 m)⁻¹, 5.53∙(100 m)⁻¹ and 7.19 K∙(100 m)⁻¹ for Beaver, Chalkyitsik, and Circle, respectively. On average, the strength and length of these SBIs exceeded the strength and length of inversions found over the Yukon Flats.

The temperature gradients between Arctic Village and Ft. Yukon indicated temperature inversions, isothermal and no inversion conditions for about 21%, 2% and 77% of the data, respectively (Figure 5(a)). Obviously, in the Yukon Flats, the frequency for SBI was higher than the 68.5% and 74.9% reported for Fairbanks during the November to February cold seasons of 1957-2008 [5] and 1999-2009 [17], respectively. This finding can be explained by the farther north location and hence lower insolation and greater radiation loss in the Yukon Flats than at Fairbanks during the cold season.

Over the Yukon Flats, about 2%, 8% and 11% of the observations showed an increase of temperature with height under negative, no and positive vertical wind shear conditions. Isothermal conditions occurred for 2% of the observations. In about 21%, 19% and 36% of the data, temperature decreased with height under negative, no and positive wind shear conditions. Vertical wind shear was negative, zero and positive 39%, 28% and 33% of the time, respectively. The negative shear indicates the impact of topography like channeling of the wind field.

Analysis of the cloud ceiling data from Arctic Village and Ft. Yukon indicated cloud-free conditions for about 57% of the time. Arctic Village is located 500 m higher than Ft. Yukon. Normalizing to height ASL revealed that the percentage of cloud ceilings above 838 ASL showed quite different behavior, while those between 638 and 838 m agreed well (Figure 7). When looking at ceilings with respect to height above ground level (AGL), the percentage of ceilings above 700 m AGL showed similar decrease at both villages. This finding can be related to

large-scale systems and lifting/descending of the air mass over/from the mountain range.

At Ft. Yukon, ceilings below 100 m occurred nearly three times as often than at Arctic Village, while ceilings between 100 and 200 m had about the same percentage of occurrence (Figure 7). Such low level haze layers may occur when relatively warmer, moist air is lifted over relatively colder air. At Arctic Village, ceilings above 20 m and below 700 m AGL seemed to be governed by local topographic forcing.

Under weak synoptic scale forcing, local classical mesoscale mountain-valley wind circulation systems and downslope winds developed (Figure 1). According to the observations, cold air drained from the slopes of the Brooks Range, the White Mountain Range and the Ogilvie Range and built a cold air pool in the valley. Despite the low insolation (cf. Figure 4), reduced heat loss during twilight and increased heat loss at night were sufficient to produce differential heating between the upper and lower part of the valley floor. Thus, along the line of the Porcupine- and Yukon rivers a mountain-valley flow established (Figure 1) that turned its direction after onset of twilight and after twilight ended (Figure 8).

Various large lakes exist close to Beaver and Chalkyitsik. All rivers and lakes were frozen and snow covered. The Yukon is more than 3.5 km wide at Circle, 3 km wide at Ft. Yukon, 6 km at Beaver; the Porcupine is about 60 m at Chalkyitsik and about 800 m before it splits into its upper and lower mouth at Ft. Yukon. Both rivers meander strongly and have many sloughs and bogs. Thus, the high albedo of the snow-covered frozen waters and riverbanks as compared to the low albedo of the boreal forest contributed to radiative cooling. Consequently, the cold air mass got channeled along the Porcupine to its mouth and from there downstream along the Yukon (Figure 1).

Under weak large-scale forcing, mountain-valley wind systems formed parallel to the Ogilvie Range, south slope of the Brooks Range and the north slope of the White Mountain Range (Figure 1). Figure 8(a) exemplarily shows the change in wind direction in the diurnal course during times with weak large-scale

forcing. Figure 8(b) exemplarily illustrates that such mountain-valley circulations occurred at various times during the cold season.

Close to the outlet of the Yukon from the Yukon Flat, as well as between Beaver and Ft. Yukon, and between Ft. Yukon and Circle, the slope of the White Mountain Range is exposed broadly to the southwest, east-east north-east and east-south east, respectively. Thus, before and after the total lack of insolation (Figure 4) differences in surface heating may cause this mountain-valley wind system under weak synoptic forcing conditions. For the mountain-valley wind system along the Ogilvie Range, the earlier onset no insolation and later return of insolation at the northern than southern locations may cause the small temperature, density and pressure differences that drive the system.

Arctic Village is the only observational site located on a slope. On average, the 2-m temperatures at Arctic Village were −5.5 K lower and 5.3 K higher than at Ft. Yukon for 77% and 21% of the time, respectively. During inversions, a cold air pool formed on the valley floor. The limited vertical extend and strong SBI (Figure 5) restricted the vertical motion.

During night, air close to slopes cools faster than air at the same height over valley [27]. The cold air drained down the slopes. This air slightly differed in density and/or temperature from that in the valley. Thus, these two air masses of different characteristics formed a cold surface front wedge. Herein the colder/drier air was pushed under the comparatively warmer/moister air, resulting in a SBI. Recall that at Beaver, Chalkyitsik and Circle inversions existed for a couple of hours each day (e.g. Figure 5).

Obviously, the duration of the SBI and the constant wind just above) with a direction parallel along the mountain range/valley boundary (Figure 1) depended on distance from the mountains. The distances to the mountains in N, E, S, W directions are about 18, 76, 16, and 17 km at Beaver, 25, 1.6, 0, 128 km at Chalkyitsik, 29, 5, 15, and 11 km at Circle, and 48, 64, 51 and 74 km at Ft. Yukon, respectively. A greater distance from the mountains means a larger pool and reservoir of cold surface air as compared to close to the White Mountain and Ogilvie ranges.

There were also cases of cold air downslope-flow inversions that occurred without the topography channeling the winds along the valley surface as well as classic SBI due to radiative cooling.

The mountain-valley circulations were perpendicular to downslope flow (Figure 1). Thus, concurrent existence of slope winds and mountain-valley winds may initiate vorticity [27]. Such vorticity generation due to slope winds from the Alaska Range and mountain-valley winds was found to be one of the pre-requisites for ABL funnel clouds in the Tanana Flats Valley during summer [28].

In the Tanana Flats valley near Fairbanks, winter SBI were 16 m on average [6]. The three times higher frequency of ceilings below 20 m at Ft. Yukon than Arctic Village (Figure 7) suggests that the cold air pool on the bottom of the Yukon Flats valley most likely had only several tenth of meter in vertical extension as well.

The reanalysis revealed five distinct situations of downslope cold air flow and cold air pooling. In the following, we discuss exemplarily the five days from April 1 to April 5, 2018, which are representative of all the meteorological situations when downslope drainage and cold air pooling created SBIs (Figure 9). The 2-m temperature reanalysis data showed a strong influence of the Chandalar River inlet into the Yukon Flats Valley. It acts as a channel for the downslope cold air flow into the Yukon Flats off the southern slope of the Brooks Range. The cold air moved towards Ft. Yukon. Therefore, Ft. Yukon typically had colder conditions than the surrounding villages. Note that e Chandalar River has no connection to an outside air shed.

In the case of an SBI, the maximum difference between the measured 10-m and 2-m temperature indicates the proximity to the meteorological cause of the SBI during the cold season. Figure 10 shows the situation where cold air drainage from the Brooks Range occurred through the Chandalar River canyon toward Ft. Yukon. In this case, the SBI occurred first in Chalkyitsik. Here the greatest difference between the 10-m and 2-m temperatures was the largest of the three villages. The maximum difference observed in Chalkyitsik during the cold season was 10 K.

The SBI occurred on average an hour later in Circle with a slightly lower temperature difference between the 10-m and 2-m temperatures due to mixing with the ambient air that was replaced by the cold air drainage. Once reaching Beaver another hour later, the temperature difference was 5 K. This means that the SBI strength decreased with increasing distance from the origin of the cold air drainage. In this type of event, the 10-m wind speed was initially very weak (<2 m∙s⁻¹) when the cold air arrived at a village. In Chalkyitsik and Circle, the wind speed was about 3.5 m∙s⁻¹ when the SBI ended around noon (Figure 9). On the contrary, at Beaver, the SBI ended about an hour later at similar 10-m wind speeds as observed for Chalkyitsik and Circle.

Figure 11 shows the case when cold air moved down the Chandalar River watershed and led to cold air pooling. The pattern with respect to temperature difference (Figure 9) and the difference in timing when the SBI was seen at the village, was similar to the event of April 1.

However, the initial wind speed was less than for April 1. This type of event led to an earlier onset of the SBI that was created by the drainage flow with a longer duration than for April 1. As the flow stopped, a cold pool remained and prolonged the duration of the SBI (Figure 9). The SBI finally dissipated during the day due to solar heating and a change in the overall wind direction in the Yukon Flats.

April 3, 2018 (Figure 12) is an example where cold air drainage occurred from all mountain ranges (Brooks Range, Ogilvie Range, White Mountain Range). Thus, the relatively warmer air in the Yukon Flats was lifted as a whole. The large scale near-surface winds were weak from the North. The SBI dissipated when near-surface winds increased during the day (Figure 9).

April 4, 2018 (Figure 13) represents the case where cold pooling occurred right at the slope of the White Mountain Range. At the beginning, cold air drained from the Brooks Range into the Yukon Flats. The onset was seen at the same time in Chalkyitsik and Beaver. Once the drainage flow stopped, the SBI remained due the cold pool. However, the cold pool occurred more to the southwest of Ft. Yukon than on April 2 when the wind came from the northwest. On April 4, however, winds were sporadic and from north easterly directions except for Circle where they were westerly right from the Yukon Flats.

April 5 (Figure 14) is similar to the cases with drainage from the Brooks Range and formation of a cold pool. However, during this event temperatures decreased more than 10 K (Figure 9). The large-scale near-surface wind came

from east-northeasterly directions. However, the cold pool had its center with the lowest temperatures between Ft. Yukon, Chalkyitsik and Circle in the early morning (Figure 14). In the next three hours, the cold pool moved towards the Steven Village area, while warming notably due to solar heating. Due to the

extremely low temperature of the cold pool, the SBI lasted longer into the day than for the other cases with a lasting cold pool after the drainage stopped. Relatively warmer air moved along the Ogilvie Range northward around the cold pool and terminated the SBIs with Circle being the first, Chalkyitsik next followed by Beaver.

The reanalysis revealed that during the cold season under weak large scale forcing, the Brooks Range governs the duration of the SBIs and the mesoscale dynamics in the Yukon Flats. The direction of the large-scale flow governs which of the circulation patterns establishes and hence the duration of the SBIs.