Juha Karhu

Dehydration and sedimentation of ice particles in the Arctic stratospheric vortex

Balloon borne frost-point hygrometers and backscatter sondes were launched at Sodankyla, Finland in January and February of 1996. These instruments measure water vapor and the backscatter ratio of light due to polar stratospheric clouds in the Arctic stratospheric vortex. Here we report the results of a hygrometer sonde and a backscatter sonde launched within 3.5 hours of each other on January 22/23. Together these soundings show a strong loss of water vapor due to the formation of ice clouds as a result of record cold temperatures in the Arctic stratosphere. The separation of the upper edge of the layer showing water vapor loss and the upper edge of the PSC layer indicates sedimentation of the ice particle layer, possibly leading to a permanent dehydration in the upper part of the layer exhibiting water vapor loss.

Polar stratospheric cloud threshold temperatures in the 1995-1996 arctic vortex

Balloon-borne backscattersondes have been used to study the relationship between particle scattering and ambient temperature near the vertical edge of arctic polar stratospheric clouds (PSCs) as well as to delineate the cloud type occurrence probability as a function of temperature. The observed typical threshold temperatures as a function of altitude are about1°K warmer than the temperature TSTS expected for rapid growth of supercooled ternary solution aerosols. A more descriptive analysis shows that the threshold temperatures occur over a definable range of temperatures and tend to cluster near, but somewhat warmer than, TSTS. Considering the experimental and theoretical uncertainties, this difference may not be significant. The probability of type Ib PSC occurrence shows a dramatic increase at TSTS±1°K, while for type Ia PSCs the probability is roughly constant at 10% for temperatures below the formation point of nitric acid trihydrate (TNAT).

SNORTEX (Snow Reflectance Transition Experiment): Remote sensing measurement of the dynamic properties of the boreal snow-forest in support to climate and weather forecast: Report of IOP-2008

Large discrepancies are observed between snow albedo in Numerical Weather Prediction (NWP) models and from satellite observations in the case of high vegetation. Knowledge of the Bidirectional Reflectance Distribution Function (BRDF) of snow-forest system is required to solve the problem. The 3-years SNORTEX (Snow Reflectance Transition Experiment) campaign acquires from 2008 in situ measurements of snow and forest properties in support to the development of modelling tools and to validate coarse resolution satellite products (POLDER, MODIS, MERIS, METOP). The measurement scheme and some first example results are presented from the Intensive Observing Period (IOP) of 2008, which can be decomposed into airborne and ground operations. Multi-temporal BRDF at a metric resolution were acquired from OSIRIS (airPOLDER) onboard a helicopter and from ground with FigiFiGo spectrogoniometer. The same helicopter embarked a pair of UV sensors, pyranometers and a wide-optics camera. Ground component includes exhaustive snow measurements.

Evolution of the Arctic stratospheric aerosol mixing ratio measured with balloon-borne aerosol backscatter sondes for years 1988–2000

Balloon-borne aerosol backscatter measurements were made at 12 Arctic stations as part of a polar stratospheric cloud study. The record starts in 1988, which is well before the eruption of Mount Pinatubo in the beginning of June 1991, and continues to 2000. These measurements provide absolutely calibrated in situ detection of atmospheric aerosols with simultaneous measurements of pressure, temperature, relative humidity, and O3 partial pressure. The instrument is also capable of operating during cloudy conditions, which may be considered as an advantage compared with lidar measurements. Even though backscatter soundings represent the state of the atmosphere at the sounding time and site, we demonstrate here that with a limited, homogeneous set of measurements it is possible to effectively study the time development of atmospheric aerosol loading. The initial aim of the study has been to define the general features of aerosol distribution in the Arctic winter troposphere and stratosphere and then to document the perturbation in the lower stratospheric aerosols caused by the eruption of Mount Pinatubo and, in addition, to infer the background state of lower stratospheric aerosol loading during the pre- and post-Pinatubo conditions. Our measurements suggest that the e-folding time for the decaying volcanic aerosol intrusion was ∼0.7 year and the full recovery of the Arctic lower stratosphere from the Mount Pinatubo perturbation was roughly 5 years.

Ozone and Spectroradiometric UV Changes in the Past 20 Years over High Latitudes

Abstract This study analyzes changes in solar ultraviolet (UV) irradiances at 305 and 325 nm at selected sites located at high latitudes of both hemispheres. Site selection was restricted to the availability of the most complete UV spectroradiometric datasets of the past twenty years (1990–2011). The results show that over northern high latitudes, between 55° and 70°N, UV irradiances at 305 nm decreased significantly by 3.9% per decade, whereas UV irradiance at 325 nm remained stable with no significant long-term change. Over southern high latitudes (55°–70°S), UV irradiances did not show any significant long-term changes at either 305 or 325 nm. Changes in solar UV irradiances are discussed in the context of long-term ozone and other atmospheric parameters affecting UV variability at ground level.

Out-of-Range Stray Light Characterization of Single-Monochromator Brewer Spectrophotometers

ABSTRACT Stray light in single-monochromator Brewer instruments increases the uncertainty of solar ultraviolet spectral irradiance measurements and ozone retrievals. To study how spectral irradiance within and outside the measurement ranges of the instruments affects stray light, two Brewer MKII instruments were characterized for the level of in- and out-of-range stray light at multiple laser wavelengths. In addition, several solar-blind filters utilized in single-monochromator Brewers to limit out-of-range stray light were characterized for spectral and spatial transmittances. Finally, the measurement results were used to simulate the effect of stray light and stray light correction on spectral irradiance and ozone measurements at different wavelength regions. The effect of stray light from wavelengths above 340 nm was found to be negligible compared with other sources of uncertainty. On the other hand, contributions from wavelengths between 325 and 340 nm can form a significant portion of the overall stray light of the instrument, with 325 nm being the upper limit of the nominal measurement range of the instrument.

25 years of spectral UV measurements at Sodankylä

At Sodankyla (67°N), spectra of solar ultraviolet radiation (UVR) have been measured with a Brewer spectroradiometer since 1990. The time series is one of the longest in the European Arctic region. In this work, the time series 1990-2014 was homogenized, and the data were corrected with respect to known error sources using laboratory characterizations and theoretical approaches. Methods for cosine correction, temperature correction and determination of long-term changes in spectral responsivity were applied. Bad measurements were identified by using various quality assurance tools including comparisons with reconstructed UV dose rates, synchronous broadband UV dose rates, global radiation and clear sky model calculations. We calculated daily maximum UV indices from the spectral time series. The daily maxima reached on average a value of 5 in midsummer, whereas the maximum UV index value of 6 was measured only twice: in 2011 and in 2013. We calculated the relative spectral changes in measured UV irradiance...

Two decades of spectral UV measurements at Sodankylä

A Brewer spectrophotometer MK II measures UV irradiances since 1990 at Sodankyla (67.4°N, 26.6°E), Finland. In this work, the observed spectral features of the time series 1990-2011 are studied. The data have been corrected for temperature, cosine, noise spike and the wavelength-shift. The long-term spectral responsivity is scaled to the irradiance scale of the Aalto University, and the quality of the measurements has been assessed during intercomparison campaigns. In order to study long-term changes in the spectral UV irradiance, a linear regression line is fitted to the monthly mean irradiances from April to August. No statistically significant changes are observed for any months for the 22-year period. However, one single year can have special features. In 2011, UV index 6 was measured for the first time, and the ratio of UV-B irradiance at 305 nm to UV-A irradiance at 324 nm exceeded the 1990-2010 average by 16%.