Temperatures, polar mesosphere summer echoes, and noctilucent clouds over Spitsbergen (78°N)

Simultaneous measurements of temperatures, polar mesosphere summer echoes (PMSE), and noctilucent clouds (NLC) took place in the polar cap (78°N) during the Rocketborne Observations in the Middle Atmosphere campaign (ROMA) in summer 2001. PMSE were observed practically permanently from the beginning and prior to the ROMA campaign (mid‐July) until mid‐August and disappeared by the end of August. PMSE occur between 81 and 92 km but have maximum occurrence rates between 83 and 89 km (up to 80% per day). PMSE are compared with temperatures from falling spheres, with frost point temperatures (T f ), and with degrees of saturation (S) using water vapor mixing ratios from models. PMSE occur nearly exclusively at altitudes with supersaturation, but the reverse is not true. Temperatures within PMSE layers can be up to 20–25 K below T f . There is no correlation between the radar echo power and the magnitude of S. Around the mesopause we frequently find S values significantly larger than 1 (up to 13000) at altitudes with the confirmed absence of PMSE. This could indicate that the actual water vapor mixing ratio is substantially smaller than our assumed model values in line with the “freeze‐drying effect.” The mean of all NLC peak altitudes is 83.6 km (variability: ±1.1 km), and the occurrence rate is 77% in the main summer season. Most of the time the lower edges of PMSE and NLC are colocated within a few hundred meters or less, which can be explained by a rapid evaporation of ice particles. This close agreement also indicates a rather homogeneous horizontal distribution of ice particles at scales below a few kilometers. The seasonal and height variation of PMSE nicely agrees with the time/height range of S > 1 and confirms the overwhelming importance of low enough temperatures for the existence of PMSE. The variation of NLC also agrees with the seasonal variation of S but covers only the lower height range with supersaturation in line with the different sensitivity of NLC and PMSE on particle radius. From the combined observations of PMSE and NLC, mean occurrence rates of neutral air turbulence of larger than ∼50% are deduced. The experimental results at Spitsbergen confirm the standard scenario of PMSE and NLC, namely that particles start to nucleate around the mesopause, and grow and sediment until they reach “warm” atmospheric regions around 82 km where they quickly evaporate. Small ice particles can affect the plasma leading to PMSE, whereas they need to grow to radii larger than approximately 20 nm to be seen by lidar.