Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the ...aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg.sup.-1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface.
This article presents a comprehensive analysis of the foehn episode which
occurred over Svalbard on 30–31 May 2017. This episode is well documented by
multiplatform measurements carried out during ...the Arctic CLoud
Observations Using airborne measurements during polar Day (ACLOUD) and Physical feedbacks of Arctic PBL, Sea ice, Cloud And
AerosoL (PASCAL) campaigns.
Both orographic wind modification and foehn warming are considered here. The
latter is found to be primarily produced by the isentropic drawdown, which
is evident from observations and mesoscale numerical modeling. The
structure of the observed foehn warming was in many aspects very similar to
that for foehns over the Antarctic Peninsula. In particular, it is found
that the warming was proportional to the height of the mountain ridges and
propagated far downstream. Also, a strong spatial heterogeneity of the foehn
warming was observed with a clear cold footprint associated with gap flows
along the mountain valleys and fjords. On the downstream side, a shallow
stably stratified boundary layer below a well-mixed layer formed over the
snow-covered land and cold open water. The foehn warming downwind of
Svalbard strengthened the north–south horizontal temperature gradient across
the ice edge near the northern tip of Svalbard. This suggests that the
associated baroclinicity might have strengthened the observed northern tip
jet. A positive daytime radiative budget on the surface, increased by the
foehn clearance, along with the downward sensible heat flux provoked
accelerated snowmelt in the mountain valleys in Ny-Ålesund and Adventdalen,
which suggests a potentially large effect of the frequently observed
Svalbard foehns on the snow cover and the glacier heat and mass balance.
In the paper, we present the results of an analysis of ceilometer measurements at North Pole (NP) drifting station 37, 39, and 40. The frequencies of the total cloud amount (in tenth) = TCA and the ...cloud base heights (CBHs) are calculated for the period of the polar night. A comparison of the cloud-cover score according to the ceilometer data with the visual observation data showed good agreement. However, the value of the correlation coefficient depends on the interpretation of the ceilometer data. In general, a bimodal distribution of the cloud-cover score with the highest frequency of clear sky and overcast clouds are characteristic for indicated stations. The analysis of the frequency of the CBH showed that the most characteristic CBHs are below 600 m. In November, cloud heights in the range of 1000–2000 m are also observed, while their frequency decreases during the winter. Cloudiness during the polar night is characterized by a strong positive radiative forcing, which has a warming effect on the surface temperature of ice and air. However, cloud radiative forcing, as well as the cloudiness frequency and its effect on the temperature regime, varies significantly from station to station. These differences may become the subject of further research on the interrelation of cloud characteristics with other processes in the Arctic climate system. These statistical estimates significantly supplement the available data on the cloud cover of the Central Arctic.
In this study, we evaluated the climatology and interannual variability of marine cold-air outbreaks (MCAOs) in the Russian Arctic marginal seas (from the Barents to Chukchi seas). We used a simple ...index for identifying MCAOs based on the vertical potential temperature gradient between the sea surface and the 800 hPa level. We calculated the index using 6-hourly Era-Interim data for the 1979-2018 period. Given the index, we evaluated spatial and temporal variability of weak, medium, and strong MCAOs frequency as well as their dependence on sea-ice concentration using non-parametric tests. The most intense MCAOs were found in the Barents and Kara seas. The annual cycle maximum for the western Russian Arctic (WRA) were found in wintertime, while it was revealed in mid-late autumn for the eastern Russian Arctic (ERA). In the WRA, we found a statistically significant decrease in amount of strong MCAOs in winter and late autumn and a general strengthening of MCAOs in spring. Meanwhile, over the ERA region, increase of moderate and weak cold-air intrusions during October and November was revealed.
An analytical model describing the evolution of a convective atmospheric boundary layer in marine cold-air outbreaks in the Arctic is presented. The novelty of the model is a detailed description of ...the baroclinicity associated with the boundary-layer growth and heating. Ekman friction is also taken into account. Thereby, the model describes the evolution of mixed-layer wind components over the ocean. It is shown theoretically that baroclinicity leads either to deceleration or to acceleration of the flow over the ocean, which depends on the direction of the large-scale flow relative to the orientation of the ice edge. Acceleration of the flow leads to a formation of a low-level jet strongly affecting the surface fluxes of heat and momentum. Baroclinicity and the magnitude of the low-level jet are strongest close to the ice edge being proportional to the ocean-ice temperature difference and decays further downwind. Horizontal decay of the low-level jet strength is governed by the airmass transformation length scale which is estimated to be in the order of 500-1000 km for typical cold-air outbreaks. The model solutions are shown to be in good agreement with aircraft observations over the Fram Strait and results of a numerical nonhydrostatic model.
Abstract
One of the main challenges in the boundary-layer turbulence research consists in advancing turbulence theory towards describing turbulent regimes over inhomogeneous and irregular surface ...which is often the case in nature. Novel observational techniques are required to sample turbulence statistics in such conditions. To that aim, a light-weight payload for atmospheric turbulence measurements is designed and used on a vertical take-off and landing (VTOL) fixed-wing unmanned aerial vehicle (UAV). Such an observational platform complements traditional mast observations and makes it possible to sample turbulence statistics over remote and inhomogeneous landscapes. The advantages of the VTOL scheme consist in: i) minimizing the risk of the payload damage during landing; ii) less strict requirements to the surface conditions in the landing area. The turbulence payload consists of a miniature seven-hole probe, inertial navigation system and a fine-wire resistance thermometer. The seven-hole probe and the fine-wire temperature sensor were designed and calibrated as part of this study. In addition, the payload also includes a slow-response temperature-humidity sensor, relatively fast humidity sensor as well as an infrared surface temperature sensor. For the onboard data-logging, a computer based on Raspberry Pi 4 is used. The whole system has been successfully tested at the Tsimlyansk research station, Russia.
The growing relevance of the use of unmanned aerial vehicles (UAVs) for studying turbulence in the atmospheric boundary layer (ABL) is associated with the need to obtain new observational data at ...different heights in the boundary layer, as well as over heterogeneous landscapes. Such new data is needed for developing turbulence models for spatially inhomogeneous and unsteady conditions. This paper describes the new Tsimlyanin UAV and its meteorological payload, developed specifically for studying the turbulent structure of the ABL. Tsimlyanin differs from similar vehicles by its hybrid scheme, which combines the possibility of vertical take-off and landing and horizontal flight in airplane mode. The meteorological payload includes specially designed measuring devices, namely, a multihole air pressure probe and a low-inertia resistance thermometer. Also, analog-to-digital converters for several sensors, as well as the onboard data-acquisition system, are newly developed. This paper presents the results of measurements carried out using the UAV during test flights in Tsimlyansk in August 2020. A good agreement between UAV observations with respect to vertical profiles of air temperature, wind speed and direction, dispersion of vertical speed with the data of observations of other complexes (acoustic anemometers, an automatic meteorological station, sodars, and a temperature profiler) was obtained. The results demonstrate the high potential of this UAV for studying turbulence in the ABL.
The effect of clouds, wind speed and long-wave radiative balance on the surface and near-surface air temperature in the Arctic during polar night is presented. The most pronounced bimodality in ...frequency distributions of the cloud fraction corresponding to cloudy and clear-sky situations is found for the stations NP-35 (2007-2008), NP-37 (2009-2010) and NP-38 (2010-2011). A strong impact of the presence or absence of clouds on the air-surface temperature difference is shown. For clear-sky situations nonmonotonic dependency of near-surface air temperature on wind speed is found.
Abstract
A fixed-wing unmanned aerial vehicle “Tsimlyanin” having a hybrid vertical take-off and landing (VTOL) scheme with a compact meteorological payload is designed for an application to the ...atmospheric boundary layer (ABL) studies. The main purpose of such an observational platform is to supplement traditional mast observations by sampling horizontal and vertical inhomogeneities of mean variables and turbulent statistics. Such observations are needed to advance turbulence theory especially for the ABL over inhomogeneous landscapes and also in non-stationary conditions. The VTOL scheme is chosen as it provides a safer take-off and landing and is less demanding to the landing conditions. The designed turbulence payload consists of a seven-hole probe, inertial navigation system and a fast-response platinum-wire temperature sensor. It is complemented with the static pressure sensor, a slow-response temperature and humidity sensor and an infrared surface temperature sensor. The data acquisition and logging is performed by an on-board computer. The designed system has been successfully tested in the summer campaign in 2020 in Tsimlyansk, Russia.
A review of the current state of research in the field of numerical modelling and forecasting of cold-air outbreaks over the ocean at high latitudes and associated mesoscale circulations is ...presented. It is shown that the most relevant tasks are as follows: (1) the improvement of predictability and the adequacy of reproduction of polar mesocyclones, (2) a more adequate representation of the marginal sea-ice zone in the numerical models, and (3) solving problems of the parametrization and explicit reproduction of organized convection and orographic jets in numerical atmosphere models. It is demonstrated that these tasks only can be accomplished as a result of a comprehensive development of different components of the climatic system models and technology of the numerical weather prediction (NWP). One of the most promising approaches to overcome the identified problems is to develop and use methods of satellite remote sensing of the atmosphere and underlying surface in NWP technology. The high potential of analyzing the satellite multisensor data for quantifying parameters of different-scale atmospheric circulations is demonstrated using the example of cold-air outbreaks over the seas of the Far East.
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