Trends in Precipitable Water Over Southern Greece

The precipitable water (PW) content was estimated over southern Greece for three atmospheric layers, using 28-year twice daily radiosonde measurements of temperature, humidity and atmospheric pressure. PW demonstrates considerable variation at the monthly and inter-annual timescales as well as statistically significant upward trends at the annual level and for JJA and SON. The relationship between PW and surface climate was also found to vary by season and surface climate variable. Disaggregation of seasonal PW trends by synoptic type revealed that seasonal trends may be attributed to increases in moisture in a limited number of air mass types such that maritime air masses appear to be becoming moister while their continental counterparts are drying. Despite fundamental changes to the intrinsic PW properties for some synoptic categories, the significance of these for determining the overall trend in seasonal PW levels is shown to be related to their weighted contribution as determined by changing air mass frequency. Consequently, the climatological effects of a moistening or drying air mass may be offset by a declining frequency. It is also shown that increasing PW trends cannot be attributed unequivocally to air mass warming as significant increases in PW have occurred in conjunction with falling temperatures in some seasons and air mass types. Accordingly, factors other than the commonly assumed global warming related increase in atmospheric moisture via the link between temperature and saturation pressure must be considered if the mechanisms responsible for PW trends over southern Greece are to be fully understood.