A new formula is proposed for estimating the long-wave radiation from clear skies. The formula depends on screen-level air temperature and water vapour pressure. The formula has been extensively tested using long-wave measurements covering a large range of environmental temperatures (-40 to +40 "C), and by using radiosonde profiles and an accurate radiative-transfer model (Lowtran-7). It is shown that the new formula out-performs five other widely used formulas.
A quantitative analysis of the properties of several Mt Ruapehu, New Zealand, ash plumes has been performed using multispectral satellite data from the AVHRR-2 and ATSR-2 instruments. The analysis includes: identification of the plume from background clouds using the 'reverse' absorption effect in the thermal channels; modelling and retrieval of particle sizes; determination of the plume height from cloud shadows, stereoscopy and meteorological data; and estimates of the mass of fine particles (radii less than 10 pm). A new spectral technique for identifying opaque, silica-rich ash clouds is demonstrated by utilizing the near-infrared (1.6 pm) and visible (0.67 pm) channels of the ATSR-2, and the optical properties of a simple volcanic cloud are presented for use in radiative transfer studies. It is found that the Ruapehu eruption cloud contained silica-rich ash particles with radii generally less than a few micrometres. The distribution of fine particles is monomodal with a dominant mode peak of about 3 p m radius. Mass loadings of fine particles are found to be in the range = I to ~7 mg m-3, and are consistent with estimates of mass loadings of volcanic clouds from eruptions of other volcanoes. The height of the plume top, derived from radiosonde data and plume-top temperatures in the opaque regions, was found to be between 7.5 and 8.5 km, while the plume thickness was estimated to be between 1.5 and 3 km. Cloud height derived from ATSR-2 stereoscopy on a different plume gave heights in the range 5 to 8 km.The results of this study provide important information on the optical properties of nascent volcanic eruption plumes. This information may prove useful in determining the potential effects of volcanic clouds on local climate, and in assessing any hazard to aviation. t The time taken for ash particles to fall from 20 to 5 km can be up to a month for spherical particles with radii x.5 pm, but several times longer for some other shapes (Wilson and Huang 1979).* Our analysis will show that this height estimate is in error and that the plume height was certainly no greater than 10 km.
, Eyjafjallajökull volcano in Iceland erupted a large amount of fine grained ash. Dispersion models and satellite data were used to identify the location of the ash cloud, but accurate quantitative forecasts of the concentrations could not be made. By using multispectral satellite measurements from the Spin Enhanced Visible and Infrared Imager (SEVIRI), it is shown that quantitative estimates of the mass loadings (g m À2 ) can be made with a detection limit $0.2 g m À2 , every 15 minutes. These data represent the most comprehensive coverage, in space and time, of the Eyjafjallajökull ash and its movement. A new ash concentration chart is proposed that removes the ambiguity inherent in assigning high concentrations to highly negative brightness temperature differences. Validation of satellite ash retrievals against measurements from aircraft, ground-based lidars, and air quality data is presented. The results show a mean bias of À47 mg m À3 and standard deviation of AE154 mg m À3 . The satellite ash retrievals are sufficiently accurate for use with dispersion models to constrain ash concentration forecasts. Concentrations in the dense parts of the dispersing ash cloud occasionally exceeded 4 mg m À3 ($3% of ash-affected pixels), and ash clouds with concentrations >2 mg m À3 covered large parts of European airspace on several occasions ($50% of ash-affected pixels). The statistical analysis leads naturally to a logarithmic scale for assigning ash concentration limits. We suggest that a better approach is to utilize a dosage and illustrate this using a simple model of ash deposition on jet engines.
An 80,000 km 2 stratospheric volcanic cloud formed from the 26 February 2000 eruption of Hekla (63.98° N, 19.70° W). POAM-III profiles showed the cloud was 9-12 km asl. During 3 days this cloud drifted north. Three remote sensing algo rithms (TOMS S0 2 , MODIS & TOVS 7.3 urn IR and MODIS 8.6 urn IR) estimat ed -0.2 Tg S0 2 . Sulfate aerosol in the cloud was 0.003-0.008 Tg, from MODIS IR data. MODIS and AVHRR show that cloud particles were ice. The ice mass peaked at -1 Tg -10 hours after eruption onset. A -0.1 Tg mass of ash was detected in the early plume. Repetitive TOVS data showed a decrease of S0 2 in the cloud from 0.2 Tg to below TOVS detection (i.e.O.Ol Tg) in -3.5 days. The stratospheric height of the cloud may result from a large release of magmatic water vapor early (1819 UT on 26 February) leading to the ice-rich volcanic cloud. The optical depth of the cloud peaked early on 27 February and faded with time, apparently as ice fell out. A research aircraft encounter with the top of the cloud at 0514 UT on 28 February, 35 hours after eruption onset, provided validation of algorithms. The aircraft's instruments measured -0.5-1 ppmv S0 2 and -35-70 ppb sulfate aerosol in the cloud, 10-30% lower than concentrations from retrievals a few hours later. Different S0 2 algorithms illuminate environmental variables which affect the qual ity of results. Overall this is the most robust data set ever analyzed from the first few days of stratospheric residence of a volcanic cloud.
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