  The best-characterized exoplanets to date are planets on close-in transiting
orbits around their host stars. The high level of irradiation and transiting
geometry of these objects make them ideal targets for atmospheric
investigations. However, the modest apertures of many current telescopes allow
mostly low resolution spectra to be observed for transiting planets, failing to
extract key physical and chemical properties of their atmospheres. Ground-based
30-meter class telescopes will set the stage for a substantial leap in our
understanding of exoplanet atmospheres. We outline a two-pronged survey,
recently submitted as a Key Science Project (KSP) for the US ELTs, which would
yield unprecedented insight into the atmospheres of close-in exoplanets via
combined observations with the GMT and TMT. (1) The first opportunity involves
measuring the global-scale atmospheric circulation and planetary rotation for a
sample of 40 hot Jupiters to glean insight into the unique radiative forcing
regime governing highly-irradiated, tidally-locked giant planets. (2) The
second opportunity involves measuring atmospheric mass-loss and extracting
atmospheric composition and abundance ratio information for $\sim60$
sub-Neptunes and super-Earths (including candidate disintegrating planets) to
constrain their formation and evolution histories. These efforts would be made
possible by the unparalleled combination of high spectral resolution
instrumentation and large aperture size of the ELTs. This survey would enable
the first statistical study of atmospheric circulation in extrasolar giant
planets, and would provide detections of trace gases and measurements of
atmospheric escape in small-planet atmospheres, far exceeding the reach of
\textit{JWST}.

Earth and Planetary Astrophysics