  White dwarfs offer a unique opportunity to search nearby stellar systems for
signs of life, but the habitable zone around these stars is still poorly
understood. Since white dwarfs are compact stars with low luminosity, any
planets in their habitable zone should be tidally locked, like planets around
M-dwarfs. Unlike planets around M-dwarfs, however, habitable white dwarf
planets have to rotate very rapidly, with orbital periods ranging from hours to
several days. Here we use the ExoCAM Global Climate Model (GCM) to investigate
the inner edge of the habitable zone (HZ) around white dwarfs. Our simulations
show habitable planets with ultrashort orbital periods ($P\lesssim$1 day) enter
a ``bat rotation" regime, which differs from typical atmospheric circulation
regimes around M dwarfs. Bat rotators feature mean equatorial subrotation and a
displacement of the surface's hottest regions from the equator towards the
midlatitudes. We qualitatively explain the onset of bat rotation using shallow
water theory. The resulting circulation shifts increase dayside cloud cover and
decrease stratospheric water vapor, expanding the white dwarf habitable zone by
$\sim$50\% compared to estimates based on 1D models. The James Webb Space
Telescope (JWST) should be able to quickly characterize bat rotators around
nearby white dwarfs thanks to their distinct thermal phase curves. Our work
underlines that tidally locked planets on ultrashort orbits may exhibit unique
atmospheric dynamics, and guides future habitability studies of white dwarf
systems.
