A case study of natural variability of water vapor content in the Baltic Sea region

Water vapor is the most essential component of the Earth's atmosphere. It is contributing about 60 % of the natural greenhouse effect, being the resource for precipitation in the lower troposphere and playing a critical role in many chemical reactions. Therefore, its quantity must be known precisely to understand, associate and forecast meteorological processes. On the other hand, temporal as well as spatial variability of water vapor occur such a fine scales, that resolving it adequately presuppose observing systems with high sampling resolution in space and time. Regular radiosondes with 12 h or 24 h sampling interval are not sufficient for detecting fast changes neither in the humidity profiles nor in the water vapor total content. During three days (10th-12th August 2010) total of 24 radiosoundings with interval 3 h were made in Toravere, Estonia (58°15' N, 26°27' E), using GRAW DFM-06 radiosondes. Column-integrated water vapor content, known as precipitable water, varied during the campaign from 24 mm to 36 mm. The temporal variation of specific humidity was surprisingly uniform, up to 2 g/kg within any layer in the profile below 6 km. It is noteworthy, as the average values varied even one magnitude - from 12 g/kg at the ground level to 1 g/kg at 6000 m. These changes in the humidity content in the whole profile can be explained only with exchanges of the air masses. In addition to the radiosondes data, NCEP-CFSR vertical profile data of specific humidity and temperature for the Baltic Sea region (here defined as region 52° - 68° N, 12° - 32° E) was used with temporal and spatial resolution of 6 h and 0.5 degrees, respectively. For the overlapping period, NCEP-CFSR followed the measured profiles reasonably well, giving us some justice to use this model for the whole period and region. The region average of precipitable water was 22.8 mm, though local extreme values varied through the summer even one magnitude - from 4.5 mm to 51 mm. The average precipitable water value was at 12 UTC averagely 0.4 mm higher than at 00 UTC, and regional values varied even more than 2 mm in both directions. However, monthly mean diurnal cycle patterns were very different form each other, indicating that the diurnal cycle of precipitable water is highly random. Regular diurnal cycle of precipitable water is driven by diurnal cycles of radiation and evaporation processes in the atmosphere and on the underlying surface, as well as by local air circulation. Still, the regular diurnal cycle is negligible, as fast variations in the precipitable water, caused by changes in the air masses, can be even two magnitudes larger than the regular cycle could be. The summer mean diurnal variation in Tõravere in the specific humidity profile fade out (Δ < 0.1 g/kg) above 5500 m, while in the temperature vertical profile it fade out (Δ < 0.5 °C) above 1500 m. This shows that while the vertical changes in water vapor and temperature are closely coupled, humidity variations reach much higher. At the ground level, the monthly mean specific humidity value was higher at 00 UTC, but at levels from 1000 m to 2500 m at 12 UTC.