Abstract. The basic structure parameters of lower tropospheric inversions (LTIs) have been derived from 10 years (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) of high vertical resolution (∼50 m) radiosonde observations over 56 United States stations. Seasonal and longitudinal variability of these parameters are presented and the formation mechanisms of LTI are also discussed. It is found that LTI seems to be a common feature over the continental United States. The LTI occurrence rates (defined as the fraction of measurements with LTI, which is calculated from the number of LTI cases divided by the number of measurements of the whole 10 years) at these 56 stations vary from 3.7% to 14.5%; the averaged base heights of LTI have a range of 3-5 km above mean sea level (a.m.s.l.); the averaged thicknesses and temperature jump ranges from 420-465 m and 1.9-2.2 K, respectively. These parameters have an obvious seasonal variation. In winter, all the occurrence rates, thicknesses and temperature jumps of LTI have much larger values than those in summer. LTI occurrence rate shows an obvious west-east increasing trend in all 4 seasons. Detailed analyses reveal that dynamical instability induced by strong zonal wind shear is responsible for LTI in winter, spring and autumn; the frontal system tends to generate LTI in summer. Since the higher occurrence rate, larger temperature jump and larger thickness of LTI occur in winter, we believe strong zonal wind shear plays a more important role in the formation of LTI.

Owing to changes in temperature with time in active tectonic setting, for example, melt migration and magma-chamber formation, vast volumes of crustal rock will develop extensive microcracks in response to thermal loading (Johnson et al., 2021), which has a significant influence on their physical and mechanical properties. The damage horizon might occur at depths of 5 km geothermal fields or at greater depths where thermal gradients can reach >120°C/km (Marini & Manzella, 2005). Since granites are considered the target formation for deep geothermal energy, understanding and controlling the mechanical behavior of granites under thermal loading are critical for geothermal exploration from hot dry rock (Aghahosseini & Breyer, 2020;Salimzadeh et al., 2018). Thermally induced cracking has been identified as the cause of the reduced strength and enhanced permeability of rock at high temperatures (Nasseri et al., 2009). Furthermore, the reduction of friction coefficient (Du et al., 2022) at elevated temperatures and pore fluid pressures may potentially promote fault destabilization

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