= Solution
Let $\delta_T$ be the thickness of the <thermal boundary layer> immediately below $z=h$, let $\nu$ be the <kinematic viscosity>, let $\kappa$ be the <thermal diffusivity>, and let $\operatorname{Ra}_c$ be the critical local <Rayleigh number>. The density contrast driving the boundary layer follows from the <density anomaly of water>:
$$
\Delta\rho
=\rho(T_h)-\rho(T_w)
=\rho_m\alpha
\left[(T_w-T_m)^2-(T_h-T_m)^2\right].
$$
A <Local Rayleigh-number closure> sets
$$
\frac{g\Delta\rho\,\delta_T^3}
{\rho_m\nu\kappa}
=\operatorname{Ra}_c,
$$
and therefore
$$
\delta_T=
\left[
\frac{\operatorname{Ra}_c\nu\kappa}
{g\alpha\{(T_w-T_m)^2-(T_h-T_m)^2\}}
\right]^{1/3}.
$$
By <Fourier's law>, the upward <heat flux> through this layer is
$$
\boxed{
F_c=k(T_w-T_h)
\left[
\frac{g\alpha\{(T_w-T_m)^2-(T_h-T_m)^2\}}
{\operatorname{Ra}_c\nu\kappa}
\right]^{1/3}},
$$
where $k$ is the water's <thermal conductivity>. This expression applies while the quantity inside braces is positive.
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