Long-term trends of global maximum atmospheric mixed layer heights derived from radiosonde measurements

The height of the atmospheric mixed layer is a critical parameter controlling the vertical dispersion of pollutants. Here we calculate daily maximum mixed layer height (MMLH) using operational radiosonde and surface meteorological measurements made at 219 carefully selected WMO weather stations and analyze their long-term trends from 1973 to 2018. We found that 74 stations showed significant increases in MMLH, whereas 48 sites showed negative trends. Positive trends are mainly found in Central US, Europe, Africa, East and Southeast Asia and East Australia. Stations over the coastal US, India and West Australia generally exhibit negative trends. The trends can be attributed to changes in vertical temperature gradient del theta(v) between 950 and 700 hPa and diurnal temperature range (DTR). del theta(v) in general decreased and caused positive MMLH trends over themajority of the regions. DTRdecreases globally, causing negativeMMLHtrends (corresponding to decreased DTR), and plays a more important role over India and Central Asia.