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The microwave radiometers HATPRO (Humidity and Temperature Profiler) and MiRAC-P (Microwave Radiometer for Arctic Clouds - Passive) continuously measured radiation emitted from the atmosphere throughout the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition on board the research vessel Polarstern. From the measured brightness temperatures, we have retrieved atmospheric variables using statistical methods in a temporal resolution of 1 s covering October 2019 to October 2020. The integrated water vapour (IWV) is derived individually from both radiometers. In addition, we present the liquid water path (LWP), temperature and absolute humidity profiles from HATPRO. To prove the quality and to estimate uncertainty, the data sets are compared to radiosonde measurements from Polarstern. The comparison shows an extremely good agreement for IWV, with standard deviations of 0.08–0.19 kg m−2 (0.39–1.47 kg m−2) in dry (moist) situations. The derived profiles of temperature and humidity denote uncertainties of 0.7–1.8 K and 0.6–0.45 gm−3 in 0–2 km altitude. Measurement(s) radio wave radiation • far-infrared radiation Technology Type(s) microwave radiometer Factor Type(s) brightness temperature Sample Characteristic - Environment planetary atmosphere Sample Characteristic - Location Arctic Ocean [ABSTRACT FROM AUTHOR]

Within the transpolar drifting expedition MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate), the Global Navigation Satellite System (GNSS) was used among other techniques to monitor variations in atmospheric water vapor. Based on 15 months of continuously tracked GNSS data including GPS, GLONASS and Galileo, epoch-wise coordinates and hourly zenith total delays (ZTDs) were determined using a kinematic precise point positioning (PPP) approach. The derived ZTD values agree to 1.1 ± 0.2 mm (root mean square (rms) of the differences 10.2 mm) with the numerical weather data of ECMWF's latest reanalysis, ERA5, computed for the derived ship's locations. This level of agreement is also confirmed by comparing the on-board estimates with ZTDs derived for terrestrial GNSS stations in Bremerhaven and Ny-Ålesund and for the radio telescopes observing very long baseline interferometry in Ny-Ålesund. Preliminary estimates of integrated water vapor derived from frequently launched radiosondes are used to assess the GNSS-derived integrated water vapor estimates. The overall difference of 0.08 ± 0.04 kg m -2 (rms of the differences 1.47 kg m -2) demonstrates a good agreement between GNSS and radiosonde data. Finally, the water vapor variations associated with two warm-air intrusion events in April 2020 are assessed. [ABSTRACT FROM AUTHOR]

Water vapor is an important component in the water and energy cycle of the Arctic. Especially in light of Arctic amplification, changes in water vapor are of high interest but are difficult to observe due to the data sparsity of the region. The ACLOUD/PASCAL campaigns performed in May/June 2017 in the Arctic North Atlantic sector offers the opportunity to investigate the quality of various satellite and reanalysis products. Compared to reference measurements at R/V Polarstern frozen into the ice (around 82 ∘ N, 10 ∘ E) and at Ny-Ålesund, the integrated water vapor (IWV) from Infrared Atmospheric Sounding Interferometer (IASI) L2PPFv6 shows the best performance among all satellite products. Using all radiosonde stations within the region indicates some differences that might relate to different radiosonde types used. Atmospheric river events can cause rapid IWV changes by more than a factor of 2 in the Arctic. Despite the relatively dense sampling by polar-orbiting satellites, daily means can deviate by up to 50 % due to strong spatio-temporal IWV variability. For monthly mean values, this weather-induced variability cancels out, but systematic differences dominate, which particularly appear over different surface types, e.g., ocean and sea ice. In the data-sparse central Arctic north of 84 ∘ N, strong differences of 30 % in IWV monthly means between satellite products occur in the month of June, which likely result from the difficulties in considering the complex and changing surface characteristics of the melting ice within the retrieval algorithms. There is hope that the detailed surface characterization performed as part of the recently finished Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) will foster the improvement of future retrieval algorithms. [ABSTRACT FROM AUTHOR]

A cloud particle sensor (CPS) sonde is an observing system attached with a radiosonde sensor to observe the vertical structure of cloud properties. The signals obtained from CPS sondes are related to the phase, size, and number of cloud particles. The system offers economic advantages including human resource and simple operation costs compared with aircraft measurements and land-/satellite-based remote sensing. However, because CPS systems are limited for data downlink to land stations, the observed information should be appropriately corrected. We launched approximately 40 CPS sondes in the Arctic region between 2018 and 2020 and use these data sets to develop correction methods that exclude unreliable data, estimate the effective cloud water droplet radius, and determine a correction factor for the total cloud particle count. We apply this method to data obtained in October 2019 over the Arctic Ocean and March 2020 at Ny-Alesund, Svalbard, Norway to compare with a particle counter onboard a tethered balloon and liquid water content retrieved by a microwave radiometer. The estimated total particle count and liquid water content from the CPS sondes generally agree with those data, which exemplifies the promising advantages of this approach to retrieve quantitative and meaningful information on the vertical distribution of cloud microphysics. [ABSTRACT FROM AUTHOR]

Water vapor is an important component in the water and energy cycle of the Arctic. Especially in the light of Arctic amplification, changes of water vapor are of high interest but are difficult to observe due to the data sparsity of the region. The ACLOUD/PASCAL campaign performed in May/June 2017 in the Arctic North Atlantic sector offers the opportunity to investigate the quality of various satellite and reanalysis products. Compared to reference measurements at R/V Polarstern frozen into the ice (around 82° N, 10° E) and at Ny-Ålesund, the Integrated Water Vapor (IWV) from IASI shows the best performance among all satellite products. Using all radiosonde stations within the region indicates some differences that might relate to different radiosonde types used. Though the region is well sampled by polar orbiting satellites daily means can deviate by up to 50 % due to strong spatio-temporal IWV variability associated with atmospheric river events. For monthly mean values, this weather induced variability cancels out but systematic differences dominate which particularly appear over different surface types, e.g. ocean, sea ice. In the data sparse central Arctic above 84° N, strong differences of 30 % in IWV monthly means between satellite products occur in the month of June which likely results from the difficulties to consider the complex and changing surface characteristics of the melting ice within the retrieval algorithms. There is hope that the detailed surface characterization performed as part of the recently finished Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) will foster the improvement of future retrieval algorithms. [ABSTRACT FROM AUTHOR]

The occurrence of events with increased and decreased integrated water vapor (IWV) at the Arctic site Ny-Ålesund, their relation to cloud properties, and the surface cloud radiative effect (CRE) is investigated. For this study, we used almost 2.5 years (from June 2016 to October 2018) of ground-based cloud observations processed with the Cloudnet algorithm, IWV from a microwave radiometer (MWR), long-term radiosonde observations, and backward trajectories FLEXTRA. Moist and dry anomalies were found to be associated with North Atlantic flows and air transport within the Arctic region, respectively. The amount of water vapor is often correlated to cloud occurrence, presence of cloud liquid water, and liquid water path (LWP) and ice water path (IWP). In turn, changes in the cloud properties cause differences in surface CRE. During dry anomalies, in autumn, winter, and spring, the mean net surface CRE was lower by 2–37 W m -2 with respect to normal conditions, while in summer the cloud-related surface cooling was reduced by 49 W m -2. In contrast, under moist conditions in summer the mean net surface CRE becomes more negative by 25 W m -2 , while in other seasons the mean net surface CRE was increased by 5–37 W m -2. Trends in the occurrence of dry and moist anomalies were analyzed based on a 25-year radiosonde database. Dry anomalies have become less frequent, with rates for different seasons ranging from -12.8 % per decade to -4 % per decade, while the occurrence of moist events has increased at rates from 2.8 % per decade to 6.4 % per decade. [ABSTRACT FROM AUTHOR]

Our study shows what we know (in)consistently about the vertically integrated water vapor (IWV) trends in the Arctic and what their uncertainties are. For this, we analyze the IWV trends in most modern global reanalyses (CFSR, ERAI, JRA55, MERRA2) over the period covering 1979-2016. The presented monthly spatial trends highlight important differences from month to month and from region to region. Our results indicate the overall positive IWV trend averaged over the central Arctic (>70˚N), but also highlight the heterogeneous spatiotemporal patterns of the derived trends. Although the across-reanalyses intercomparison suggests an agreement in an Arctic becoming wetter in almost all regions, IWV trends were statistically significant only over specific regions for specific months in all reanalyses. Furthermore, the reanalyses substantially disagree on the regional trend magnitudes. We quantify the differences in the monthly IWV trends and identify the regions and months of greatest difference between the reanalyses. We further discuss the interannual and decadal IWV variations, which underlie the trend. In concert with the reanalyses, we assess IWV data derived from radiosondes at 35 stations north of 65˚N and from GPS data at the Arctic station Ny-Ålesund. The reanalyses exhibit in general a remarkable agreement with the station data. We find interesting region-specific annual characteristics in the trends, which are consistent across the data sets. Importantly, our results also demonstrate the large uncertainty in the calculated trends. [ABSTRACT FROM AUTHOR]

The role of atmospheric moisture within the Arctic climate system is raising increasing interest. However, with sparse surface and radiosonde measurements mainly limited to land areas, satellite observations provide the main information on moisture distribution across the Arctic. While a wealth of satellite measurements from different instruments exist, in particular for integrated water vapor (IWV), the special conditions in the Arctic hamper accurate observations. For example, methods based on solar radiation cannot be applied during polar night and the highly variable emissivity of snow and ice hinders microwave retrievals. With few measurements available, reanalysis is less constrained by observations increasing the influence of the underlying numerical model. Therefore, it is no surprise that the Arctic belongs to those regions that show strongest relative discrepancies between different long-term, global IWV data sets, i.e. satellite and reanalysis products (GEWEX water vapor assessment (G-VAP); WCRP Report 16/2017).In this contribution, we provide a systematic assessment of different reanalysis, i.e. ERA-Interim, ERA5, MERRA2, CFSR, JRA55, and satellite products in the Arctic. Satellite products include operational retrievals using among others Microwave Integrated Retrieval System (MIRS), Infrared Atmospheric Sounding Interferometer (IASI), Moderate-resolution Imaging Spectroradiometer (MODIS), and Global Ozone Monitoring Experiment (GOME-2). As reference, we make use of the comprehensive set of measurements from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) and the Physical feedbacks of Arctic planetary boundary level Sea ice, Cloud and AerosoL (PASCAL) campaigns which took place near Svalbard from May 23 to June 26, 2017. Thus, we focus on the Nordic Seas including the marginal sea ice zone and the neighboring land areas for the two months May and June in 2017. In addition to campaign observations by ground-based remote sensing, aircraft and radiosondes, all operational radiosonde soundings and surface-based IWV from the GNSS network are used. We investigate the specific advantages and disadvantages of the different data sources in order to finally answer the question to which degree we can reliably describe Arctic IWV patterns on the monthly scale. [ABSTRACT FROM AUTHOR]