Incident and in situ irradiance in Lakes Cadagno and Lucerne: A comparisonof methods and models

Incident surface irradiance of photosynthetically available radiation (PAR) , and ultraviolet radiation (UV-A and UV-B) was monitored during the GAP Wo rkshop (8-15 September 1999) at the field stations Kastanienbaum at Lake Lu cerne (434 m a.s.l.) and Piora at Lake Cadagno (1923 m a.s.l.), Switzerland , with broad band sensors (LiCOR and Macam) and multifilter spectral radiom eters (Smithsonian UV-B radiometer SR18, Satlantic and Biospherical Profile rs). Calculations of clear sky spectral irradiance were performed using com puter implementations of atmospheric radiative transfer models STAR and FAS TRT as well as a simple two-stream model. Clear sky conditions prevailed at Lucerne so that detailed comparisons could be made between measured irradi ance and model predictions. Good correspondence was found for the measureme nts and predictions. The best agreement was with the STAR model. Spectral m easurements were consistent with measurements of the broad band UV-A and UV -B sensors after accounting for the manufacturer specified spectral respons e. The effective center wavelength of the broad band UV-B sensor is 320 nm, despite the fact that the maximum sensitivity of the sensor is at 310 nm. A daily UV index was estimated from the midday SR18 data. An analysis of th e UV-A and UV-B data at the two sites of different altitude indicates that the so called altitude effect (AE) is variable during the course of the day and usually higher for UV-B than for UV-A. Lakes Lucerne and Cadagno have moderate transparency to UV and PAR with 50% of 320 nm irradiance penetrati ng to 1 m in Lake Lucerne and 0.3 m in Lake Cadagno.