Marion Maturilli

Characteristics of Temperature and Humidity Inversions and Low-Level Jets over Svalbard Fjords in Spring

Air temperature and specific humidity inversions and low-level jets were studied over two Svalbard fjords, Isfjorden and Kongsfjorden, applying three tethersonde systems. Tethersonde operation practices notably affected observations on inversion and jet properties. The inversion strength and depth were strongly affected by weather conditions at the 850 hPa level. Strong inversions were deep with a highly elevated base, and the strongest ones occurred in warm air mass. Unexpectedly, downward longwave radiation measured at the sounding site did not correlate with the inversion properties. Temperature inversions had lower base and top heights than humidity inversions, the former due to surface cooling and the latter due to adiabatic cooling with height. Most low-level jets were related to katabatic winds. Over the ice-covered Kongsfjorden, jets were lifted above a cold-air pool on the fjord; the jet core was located highest when the snow surface was coldest. At the ice-free Isfjorden, jets were located lower.

A comparison of the two Arctic atmospheric winter states observed during N‐ICE2015 and SHEBA

Winter time atmospheric observations from the 2015 Norwegian young sea-ICE campaign (N-ICE2015) are compared with data from the 1997-1998 Surface Heat Budget of the Arctic (SHEBA) campaign. Both datasets have a bimodal distribution of the net longwave radiative flux for January-February, with modal values of -40 W m-2 and 0 W m-2. These values correspond to the radiatively clear and opaquely cloudy states, respectively, and are likely to be representative of the wider Arctic. The new N-ICE2015 observations demonstrate that the two winter states operate in the Atlantic sector of the Arctic and regions of thin sea ice. We compare the N-ICE2015 and SHEBA data with ERA-Interim and output from the coupled Arctic regional climate model HIRHAM-NAOSIM. ERA-Interim simulates two Arctic winter states well and captures the timing of transitions from one state to the other, despite underestimating the cloud liquid water path. HIRHAM-NAOSIM has more cloud liquid water compared with ERA-Interim, but simulates the two states poorly. Our results demonstrate that models must simulate realistic synoptic forcing and temperature profiles to accurately capture the two Arctic winter states, and not only the presence of mixed-phase clouds. Using ERA-Interim, we find a positive trend in the number of opaquely cloudy days in the western Atlantic sector of the Arctic, and a strong correlation with the mean winter temperature over much of the Arctic Basin. Hence, the two Arctic winter states are important for understanding inter-annual variability in the Arctic. The N-ICE2015 dataset will help improve our understanding of these relationships.

Impact of radiosonde observations on forecasting summertime Arctic cyclone formation

The impact of Arctic radiosonde observations on the forecasting of the 2012 early August Arctic cyclone AC12—the “strongest” since records began—has been investigated using an observing system experiment (OSE). An atmospheric ensemble reanalysis (ALERA2) was used as the control experiment (CTL) to reproduce the development of the Arctic cyclone and surrounding large-scale atmospheric fields. The OSE applies the same reanalysis as the CTL except for the exclusion of radiosonde observations from the German icebreaker Polarstern, which cruised near Svalbard during mid-July to early August 2012. Comparison of the two reanalyses revealed a difference in the upper tropospheric circulation over northern mid-Eurasia, just before the Arctic cyclone developed, in the form of a stronger tropopause polar vortex in the CTL. This indicated that the upper tropospheric field in the CTL had greater potential for baroclinic instability over mid-Eurasia. Ensemble predictions were then conducted using the two reanalyses as initial values at which the tropopause polar vortex approached northern mid-Eurasia. The CTL prediction reproduced the formation of the Arctic cyclone, but the OSE shows a significantly weaker one. These results indicate that the improved reproduction of upper tropospheric circulation in the Arctic region due to additional radiosonde observations from a mobile platform was indispensable for the prediction of AC12. In particular, observations being acquired far from the Arctic cyclone affect the prediction of the cyclone via the upper tropospheric circulation in the atmospheric west wind drift.

Climatology of Arctic polar stratospheric clouds as measured by lidar in Ny‐Ålesund, Spitsbergen (79°N, 12°E)

Polar stratospheric cloud (PSC) measurements have been taken by means of a lidar system at the German Koldewey Station in Ny-Alesund, Spitsbergen [79°N, 12 °E], since 1988. Here we present the first PSC climatology for the high Arctic built on Ny-Alesund lidar data collected from winter 1995/1996 to 2003/2004, thus avoiding the effects of earlierbackground aerosol enhancement by large volcanic eruptions. As in a similar study performed on the Antarctic McMurdo PSC dataset, a numerical code has been applied to distinguish PSCs based on their vertical structure displayed on the lidar profile. Two cloud categories showing low or high variability of backscattering ratio with altitude are individuated andaddressed as Large and Small Scale Variation PSCs (LSV and SSV, respectively). It is possible to make reliable assumptions concerning the relationship between the obtained PSCcategories and the conditions under which they are likely to form. Ny-Alesund is typically situated in the centre of the northern polar vortex, where the majority of PSC observations can be linked to the synoptic temperature field. The present study not only provides a general description of PSCs occurring at Ny-Alesund, but it also focuses on the temporal and spatialvariability of cloud types observed under both LSV and SSV categories. Finally, the comparison with the McMurdo climatology provides an overview of long term inter-hemispheric differences in PSC appearance as measured by ground based lidars.

Additional Arctic observations improve weather and sea-ice forecasts for the Northern Sea Route.

During ice-free periods, the Northern Sea Route (NSR) could be an attractive shipping route. The decline in Arctic sea-ice extent, however, could be associated with an increase in the frequency of the causes of severe weather phenomena, and high wind-driven waves and the advection of sea ice could make ship navigation along the NSR difficult. Accurate forecasts of weather and sea ice are desirable for safe navigation, but large uncertainties exist in current forecasts, partly owing to the sparse observational network over the Arctic Ocean. Here, we show that the incorporation of additional Arctic observations improves the initial analysis and enhances the skill of weather and sea-ice forecasts, the application of which has socioeconomic benefits. Comparison of 63-member ensemble atmospheric forecasts, using different initial data sets, revealed that additional Arctic radiosonde observations were useful for predicting a persistent strong wind event. The sea-ice forecast, initialised by the wind fields that included the effects of the observations, skilfully predicted rapid wind-driven sea-ice advection along the NSR.

Estimate of the Arctic Convective Boundary Layer Height from Lidar Observations: A Case Study

A new automated small size lidar system (microlidar or MULID) has been developed and employed to perform aerosol measurements since March 2010 at Ny Alesund (, ), Svalbard. The lidar observations have been used to estimate the PBL height by using the gradient method based on abrupt changes in the vertical aerosol profile and monitor its temporal evolution. The scope of the present study is to compare several approaches to estimate the PBL height, by using lidar observations, meteorological measurements by radio soundings, and a zero-order one-dimensional model based on a parameterization of the turbulent kinetic energy budget within the mixing layer, under the assumptions of horizontal homogeneity, and neglecting radiation and latent heat effects. A case study is presented here for a convective PBL, observed in June 2010 in order to verify whether the Gradient Method can be applied to lidar measurements in the Arctic region to obtain the PBL height. The results obtained are in good agreement with the PBL height estimated by the analysis of thermodynamic measurements obtained from radio sounding and with the model.

Vertical thermodynamic structure of the troposphere during the Norwegian young sea ICE expedition (N‐ICE2015)

The Norwegian young sea ICE (N-ICE2015) expedition was designed to investigate the atmosphere-snow-ice-ocean interactions in the young and thin sea ice regime north of Svalbard. Radiosondes were launched twice daily during the expedition from January to June 2015. Here we use these upper air measurements to study the multiple cyclonic events observed during N-ICE2015 with respect to changes in the vertical thermodynamic structure, moisture content, and boundary layer characteristics. We provide statistics of temperature inversion characteristics, static stability, and boundary layer extent. During winter, when radiative cooling is most effective, we find the strongest impact of synoptic cyclones. Changes to thermodynamic characteristics of the boundary layer are associated with transitions between the radiatively “clear” and “opaque” atmospheric states. In spring, radiative fluxes warm the surface leading to lifted temperature inversions and a statically unstable boundary layer. Further, we compare the N-ICE2015 static stability distributions to corresponding profiles from ERA-Interim reanalysis, from the closest land station in the Arctic North Atlantic sector, Ny-Alesund, and to soundings from the SHEBA expedition (1997/1998). We find similar stability characteristics for N-ICE2015 and SHEBA throughout the troposphere, despite differences in location, sea ice thickness, and snow cover. For Ny-Alesund, we observe similar characteristics above 1000 m, while the topography and ice-free fjord surrounding Ny-Alesund generate great differences below. The long-term radiosonde record (1993–2014) from Ny-Alesund indicates that during the N-ICE2015 spring period, temperatures were close to the climatological mean, while the lowest 3000 m were 1–3∘C warmer than the climatology during winter.

Remote sensing of aerosols in the Arctic for an evaluation of global climate model simulations.

In this study Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua retrievals of aerosol optical thickness (AOT) at 555 nm are compared to Sun photometer measurements from Svalbard for a period of 9 years. For the 642 daily coincident measurements that were obtained, MODIS AOT generally varies within the predicted uncertainty of the retrieval over ocean (ΔAOT = ±0.03 ± 0.05 · AOT). The results from the remote sensing have been used to examine the accuracy in estimates of aerosol optical properties in the Arctic, generated by global climate models and from in situ measurements at the Zeppelin station, Svalbard. AOT simulated with the Norwegian Earth System Model/Community Atmosphere Model version 4 Oslo global climate model does not reproduce the observed seasonal variability of the Arctic aerosol. The model overestimates clear-sky AOT by nearly a factor of 2 for the background summer season, while tending to underestimate the values in the spring season. Furthermore, large differences in all-sky AOT of up to 1 order of magnitude are found for the Coupled Model Intercomparison Project phase 5 model ensemble for the spring and summer seasons. Large differences between satellite/ground-based remote sensing of AOT and AOT estimated from dry and humidified scattering coefficients are found for the subarctic marine boundary layer in summer. KEY POINTS Remote sensing of AOT is very useful in validation of climate models.

Improved forecasts of winter weather extremes over midlatitudes with extra Arctic observations

Recent cold winter extremes over Eurasia and North America have been considered to be a consequence of a warming Arctic. More accurate weather forecasts are required to reduce human and socioeconomic damages associated with severe winters. However, the sparse observing network over the Arctic brings errors in initializing a weather prediction model, which might impact accuracy of prediction results at midlatitudes. Here we show that additional Arctic radiosonde observations from the Norwegian young sea ICE cruise project 2015 drifting ice camps and existing land stations during winter improved forecast skill and reduced uncertainties of weather extremes at midlatitudes of the Northern Hemisphere. For two winter storms over East Asia and North America in February 2015, ensemble forecast experiments were performed with initial conditions taken from an ensemble atmospheric reanalysis in which the observation data were assimilated. The observations reduced errors in initial conditions in the upper troposphere over the Arctic region, yielding more precise prediction of the locations and strengths of upper troughs and surface synoptic disturbances. Errors and uncertainties of predicted upper troughs at midlatitudes would be brought with upper level high potential vorticity (PV) intruding southward from the observed Arctic region. This is because the PV contained a ‘‘signal’’ of the additional Arctic observations as it moved along an isentropic surface. This suggests that a coordinated sustainable Arctic observing network would be effective not only for regional weather services but also for reducing weather risks in locations distant from the Arctic.

Multilevel Cloud Structures over Svalbard

The presented picture of the month is a superposition of space-borne lidar observations and high-resolution temperature fields of the ECMWF integrated forecast system (IFS). It displays complex tropospheric and stratospheric clouds in the Arctic winter 2015/16. Near the end of December 2015, the unusual northeastward propagation of warm and humid subtropical air masses as far north as 80°N lifted the tropopause by more than 3 km in 24 h and cooled the stratosphere on a large scale. A widespread formation of thick cirrus clouds near the tropopause and of synoptic-scale polar stratospheric clouds (PSCs) occurred as the temperature dropped below the thresholds for the existence of cloud particles. Additionally, mountain waves were excited by the strong flow at the western edge of the ridge across Svalbard, leading to the formation of mesoscale ice PSCs. The most recent IFS cycle using a horizontal resolution of 8 km globally reproduces the large-scale and mesoscale flow features and leads to a remarkable agreement with the wave structure revealed by the space-borne observations.