Observational Constraints on the Lapse Rate Feedback Using GPS Radio Occultations

The lapse rate defines the rate of change of temperature with increasing altitude that regulates the amount of outgoing longwave radiation. Thus, precise and accurate knowledge of the vertical variability of temperature is key to accurately quantifying the top-of-the-atmosphere (TOA) radiative budget, which is central to climate change research. To-date, climate models predict a decrease in the lapse rate, which suggest larger temperature variations in the upper troposphere (UT). However, measurements of the rate of temperature change with altitude are very sensitive to small observational errors, making it difficult to establish whether the models agree with observations. Current research shows that models and reanalyses data sets disagree both on the magnitude and the sign of the lapse rate feedback, which could introduce errors in the estimation of climate sensitivity. Global Positioning System Radio Occultation (GPS RO) measurements provide high quality temperature observations with an accuracy of < 0.5 K and a vertical resolution of ~ 100–200 m. We will exploit these measurements in order to observationally constrain models’ and reanalyses results. We will present the time series of tropospheric temperature using GPS RO data sets from 2006 onwards, and will compare our results with different sources such as the European Center for Medium Range Weather Forecasts (ECMWF), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), and the Atmospheric Infrared Sounder (AIRS). We will carefully quantify the statistical differences among the series to identify and document biases among the data sets. Finally, we will correlate the aforementioned temperature series with surface temperature climatologies in order to estimate the variability of the UT temperature in response to surface warming, dT/dT_s, which is directly related to the lapse rate feedback. Inter–comparing the dT/dT_s among the different data sets will provide us with an additional constraint on the lapse rate feedback. The critical role of the up–coming Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC–2) mission in late 2016, in characterizing the tropical climatology and the associated climate feedbacks, will be discussed.