  We present time-resolved near-infrared spectroscopy of two L5 dwarfs, 2MASS
J18212815+1414010 and 2MASS J15074759-1627386, observed with the Wide Field
Camera 3 instrument on the Hubble Space Telescope (HST). We study the
wavelength dependence of rotation-modulated flux variations between 1.1 $\mu$m
and 1.7 $\mu$m. We find that the water absorption bands of the two L5 dwarfs at
1.15 $\mu$m and 1.4 $\mu$m vary at similar amplitudes as the adjacent
continuum. This differs from the results of previous HST observations of L/T
transition dwarfs, in which the water absorption at 1.4 $\mu$m displays
variations of about half of the amplitude at other wavelengths. We find that
the relative amplitude of flux variability out of the water band with respect
to that in the water band shows a increasing trend from the L5 dwarfs toward
the early T dwarfs. We utilize the models of Saumon & Marley (2008) and find
that the observed variability of the L5 dwarfs can be explained by the presence
of spatially varying high-altitude haze layers above the condensate clouds.
Therefore, our observations show that the heterogeneity of haze layers - the
driver of the variability - must be located at very low pressures, where even
the water opacity is negligible. In the near future, the rotational spectral
mapping technique could be utilized for other atomic and molecular species to
probe different pressure levels in the atmospheres of brown dwarfs and
exoplanets and uncover both horizontal and vertical cloud structures.
