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We present a novel retrieval technique that attempts to model phase curveobservations of exoplanets more realistically and reliably, which we call the2.5-dimension (2.5-D) approach. In our 2.5-D approach we retrieve the verticaltemperature profile and mean gaseous abundance of a planet at all longitudesand latitudes \textbf{simultaneously}, assuming that the temperature orcomposition, $x$, at a particular longitude and latitude $(\Lambda,\Phi)$ isgiven by $x(\Lambda,\Phi) = \bar{x} + (x(\Lambda,0) - \bar{x})\cos^n\Phi$,where $\bar{x}$ is the mean of the morning and evening terminator values of$x(\Lambda,0)$, and $n$ is an assumed coefficient. We compare our new 2.5-Dscheme with the more traditional 1-D approach, which assumes the sametemperature profile and gaseous abundances at all points on the visible disc ofa planet for each individual phase observation, using a set of synthetic phasecurves generated from a GCM-based simulation. We find that our 2.5-D model fitsthese data more realistically than the 1-D approach, confining the hotterregions of the planet more closely to the dayside. We then apply both models toWASP-43b phase curve observations of HST/WFC3 and Spitzer/IRAC. We find thatthe dayside of WASP-43b is apparently much hotter than the nightside and showthat this could be explained by the presence of a thick cloud on the nightsidewith a cloud top at pressure $< 0.2$ bar. We further show that while the molefraction of water vapour is reasonably well constrained to$(1-10)\times10^{-4}$, the abundance of CO is very difficult to constrain withthese data since it is degenerate with temperature and prone to possiblesystematic radiometric differences between the HST/WFC3 and Spitzer/IRACobservations. Hence, it is difficult to reliably constrain C/O.

2.5-D retrieval of atmospheric properties from exoplanet phase curves: Application to WASP-43b observations