Erko Jakobson

Exploring Arctic Transpolar Drift During Dramatic Sea Ice Retreat

The Arctic is undergoing significant environmental changes due to climate warming. The most evident signal of this warming is the shrinking and thinning of the ice cover of the Arctic Ocean. If the warming continues, as global climate models predict, the Arctic Ocean will change from a perennially ice-covered to a seasonally ice-free ocean. Estimates as to when this will occur vary from the 2030s to the end of this century. One reason for this huge uncertainty is the lack of systematic observations describing the state, variability, and changes in the Arctic Ocean.

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.

Column water vapour: an intertechnique comparison of estimation methods in Estonia

Despite different techniques for the estimation of column integrated water vapour (precipitable water, PW) no method has yet been identified as the most accurate or the reference one. In this work we report intercomparisons between four PW estimation methods - radiosonde, Aerosol Robotic Network (AERONET), Global Positioning System (GPS), and High Resolu- tion Limited Area Model (HIRLAM). Two intensive observation periods at Toravere, Estonia, were used: 22 June−6 November 2008 and 9−12 August 2010. During the longer campaign, only observations by GPS, AERONET, and HIRLAM were performed. An agreement with average difference less than 2.2% among all three methods was established. However, compared to HIRLAM and GPS, the AERONET method overestimated PW by 5-9% at PW 25 mm. In addition, the consistency test applied indicated that previously reported uncertainty in AERONET-measured PW is too high. During the shorter but more complex campaign, data obtained with all four methods were available. Although the average differences between PW from radiosonde and three other methods were < 5%, the discrepancy between single measurements reached 33%. Relatively low temporal and spatial resolution of the HIRLAM grid as well as launching sparseness of radiosondes caused higher scatter from the other methods. The study suggests that besides radiosonde, as a traditional meteorological tool, the most reliable PW estimation can be made by GPS.

Climatological Modelled and Measured AOD in Baltic Sea Region

This study is based on AOD values from long-term re-analysis of atmospheric composition and air quality performed with SILAM model in Finnish Meteorological Institute. This study uses two spatial scales: global (1.44° Resolution, ERA-Interim re-analysis meteo data) and Northern Europe (0.1°, BaltAn65+ meteo). The emission information is compiled from the MACCITY and EDGAR anthropogenic, GEIA lightning and aircraft, MACCity-ACCMIP biomass-burning, and MEGAN biogenic emission inventories. The emission of sea salt and wind-blown dust is computed with embedded SILAM modules. Comparison of AOD from global run for 2008–2014 with 13 Aeronet stations in the Baltic Sea region (54 to 63° N and 8 to 38° E) show underestimation of station-wise average AOD-s by factor of 1.5–2.6, whereas the predicted and measured values are well correlated: linear correlation coefficients based on hourly values in different stations range from 0.46 to 0.85 (average 0.59). Nordic run made for only year 2010 show underestimation by factor of 1.6–4.1 with linear coefficients ranging from 0.33 to 0.73. Thus, the underestimation was a bit lower in the global run. A reason of underestimation may be missing local ground dust emissions and long-term realistic fire emissions that are only available until 2008 Granier et al. (Clim Change 109:163, 2011). Also, AOD measurements made with sun photometer like it is done in Aeronet stations tend to give higher AOD values than actinometric measurements do. The analysis based on longer time series (since 1990) is in progress.

Vertical profiles of atmospheric variables based on tethersonde soundings from Ny-Ålesund

Soundings were carried out on daily basis by using Vaisala tethersonde system DigiCORA TT12 with maximum altitude of 2000 m. The instrumentation consisted of 7 m3 helium filled balloon for lifting sondes, an electrical winch, three sondes suspended on the tether line below the balloon at approximately 20 m vertical intervals, and a ground station. Limited by the wind speed, however, soundings were not possible to perform every day. Each profile in dataset includes ascending and descending profiles of variables. Dataset presents raw data where no averaging over the sondes and heights is done. Yet, data is checked manually for errors. Some distinct obviously erroneous signals and spike values were removed from the data. More about the methods is described in ReadMe text file.