As is well known $\sum e^{-\lambda_{k}t}\approx(4\pi t)^{dim(M)/2}\sum a_{j}t^{j}$, where $a_{j}$ are geometric properties of manifold M.
Moreover, the arbitrary order coefficients don't have closed form (though some progress in "Combinatorics of heat kernel coefficients").
However, say $\widetilde{g}=e^{f}g$ where g=Euc. or Sph. metric, then the curvature terms are simpler in dimension 2. Are the $a_{j}$ coefficients known explicitly for such surfaces?