Diurnal variations of humidity and ice water content in the tropical upper troposphere

Abstract

Abstract.Observational results of diurnal variations of humidity from Odin-SMR and AURA-MLS, and cloud ice mass from Odin-SMR and CloudSat are presented for the first time. Comparisons show that the retrievals of humidity and cloud ice from these two satellite combinations are in good agreement. The retrieved data are combined from four almost evenly distributed times of the day allowing mean values, amplitudes and phases of the diurnal variations around 200 hPa to be estimated. This analysis is applied to six climatologically distinct regions, five located in the tropics and one over the subtropical northern Pacific Ocean. The strongest diurnal cycles are found over tropical land regions, where the amplitude is ∼7% RHi for humidity and ∼50% for ice mass. The greatest ice mass for these regions is found during the afternoon, and the humidity maximum is observed to lag this peak by ∼6 h. Over tropical ocean regions the variations are smaller and the maxima in both ice mass and humidity are found during the early morning. Observed results are compared with output from three climate models (ECHAM, EC-EARTH and CAM3). Direct measurementmodel comparisons were not possible because the measured and modelled cloud ice masses represent different quantities. To make a meaningful comparison, the amount of snow had to be estimated from diagnostic parameters of the models. There is a high probability that the models underestimate the average ice mass (outside the 1-σ uncertainty). The models also show clear deficiencies when it comes to amplitude and phase of the regional variations, but to varying degrees.

Overview and sample applications of SMILES and Odin-SMR retrievals of upper tropospheric humidity and cloud ice mass

“…The measurements of SMILES drifted in local time, and full sampling of seasonally averaged diurnal cycles can be achieved. This fact was used to improve on results from Eriksson et al (2010), where SMR was combined with Aura MLS and CloudSat to obtain a rough sampling of the diurnal cycle of humidity and ice mass for a number of regions. The main results from Eriksson et al (2010) were confirmed; amplitude and phase of the diurnal cycles agree in general between the two studies.…”

“…They reported the relative deviation from the local mean pIWP 12.5 km . The selection of Eriksson et al (2010). Black markers are global climate model results, only covering "cloud ice", where * is CAM5, × is ECHAM6 and + is ECEarth3 (all uncoupled, 2007ECEarth3 (all uncoupled, -2008 regions is similar to the one here, but in MI2013 the cycles have hourly time resolution.…”

“…The second objective is simply to demonstrate that the SMILES and Odin-SMR retrievals are of interest for various applications. Emphasis is given to regional diurnal cycles of humidity and cloud ice mass, where some results from Eriksson et al (2010) are assessed and updated. The impact of the ENSO (El Niño-Southern Oscillation) state on upper tropospheric humidity and ice mass is also considered.…”

“…The regions considered (Fig. 8) are taken from Eriksson et al (2010, to allow comparison with that study. To compensate for ENSO influences and the fact that ER2010 considered a slightly higher layer than the one selected for the SMILES retrievals, the mean of the ER2010 results is adjusted to match the ones derived for SMILES.…”

“…Data from different sensors have been combined to obtain better sampling of diurnal cycles (e.g. Eriksson et al, 2010), but this approach is very sensitive to differences in systematic errors of the sensors included. SMILES differs in this regard as it flew on the International Space Station (ISS).…”