Timo Palo

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.

Evidence of Very Shallow Summertime Katabatic Flows in Dronning Maud Land, Antarctica

AbstractThe three-axis “Latan-3” Doppler sodar was operated near the Finnish Antarctic station Aboa in Dronning Maud Land (73.04°S, 13.40°W) in the austral summer of 2010/11. The measuring site is located at a practically flat, slightly sloped (about 1%) surface of the glacier. The sodar was operated in multiple-frequency parallel mode with 20–800-m sounding range, 20-m vertical resolution, and 10-s temporal resolution. To reveal the wind and temperature profiles below the sounding range as well as turbulent fluxes at 2 and 10 m, the data from a 10-m meteorological mast were used. During the measurements, the atmospheric boundary layer was within the sounding range of the sodar most of the time. Despite a large variety of observed sodar echo patterns and wind speed profiles, several cases of clear steady katabatic flows were observed. Practically all of them were easterly, whereas the uphill direction is southern. The thickness of the katabatic flow varied from a few tens to several hundreds of meters; th...

An intercomparison and validation of satellite-based surface radiative energy flux estimates over the Arctic

Accurate determination of radiative energy fluxes over the Arctic is of crucial importance for understanding atmosphere-surface interactions, melt and refreezing cycles of the snow and ice cover, and the role of the Arctic in the global energy budget. Satellite-based estimates can provide comprehensive spatiotemporal coverage, but the accuracy and comparability of the existing data sets must be ascertained to facilitate their use. Here we compare radiative flux estimates from Clouds and the Earth’s Radiant Energy System (CERES) Synoptic 1-degree (SYN1deg)/Energy Balanced and Filled, Global Energy and Water Cycle Experiment (GEWEX) surface energy budget, and our own experimental FluxNet / Satellite Application Facility on Climate Monitoring cLoud, Albedo and RAdiation (CLARA) data against in situ observations over Arctic sea ice and the Greenland Ice Sheet during summer of 2007. In general, CERES SYN1deg flux estimates agree best with in situ measurements, although with two particular limitations: (1) over sea ice the upwelling shortwave flux in CERES SYN1deg appears to be underestimated because of an underestimated surface albedo and (2) the CERES SYN1deg upwelling longwave flux over sea ice saturates during midsummer. The Advanced Very High Resolution Radiometer-based GEWEX and FluxNet-CLARA flux estimates generally show a larger range in retrieval errors relative to CERES, with contrasting tendencies relative to each other. The largest source of retrieval error in the FluxNet-CLARA downwelling shortwave flux is shown to be an overestimated cloud optical thickness. The results illustrate that satellite-based flux estimates over the Arctic are not yet homogeneous and that further efforts are necessary to investigate the differences in the surface and cloud properties which lead to disagreements in flux retrievals.

Surface Albedo of the inner Arctic: Validation of the Climate-SAF satellite Albedo Product with in-situ observations

This paper describes a validation study performed by comparing the AVHRR-based Climate-SAF Surface Albedo Product (SAL) to ground truth observations over Greenland (Greenland Climate Network) and the ice-covered Arctic Ocean (Tara floating ice station) over Arctic summer 2007. The AVHRR dataset consists of 2755 overpasses. The SAL algorithm derives the surface broadband albedo from AVHRR channels 1 and 2 using an atmospheric correction, temporal sampling of an observation geometry to smooth out anisotropy effects, and a narrow-to-broadband conversion algorithm. At the Summit and DYE-2 stations on the Greenland ice sheet, instantaneous SAL RMSE is 0.073, weekly RMSE is 0.053. The heterogeneous surface conditions at satellite pixel scale over the stations near the Greenland west coast increase RMSE to over 0.12. Over Tara, the instantaneous SAL RMSE is 0.069, and 0.045 for weekly means. Considering various sources of uncertainty for both satellite retrievals and in situ observations, we conclude that SAL agrees with in situ observations within their limits of accuracy and spatial representativeness.

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.