should RasterCount be a parameter within mem_drv.Create?  something like:

    dest = mem_drv.Create('', x_size, y_size, 1, gdal.GDT_Float32)
    

EDIT: oh, i see it [here](http://jgomezdans.github.io/gdal_notes/reprojection.html) - i didn't see the destination raster as a parameter to ReprojectImage at first - hopefully the above change will help. i was originally thinking of re-interpolating the array, then writing it to the dest raster band -

    band = dest.GetRasterBand(1)
    res = SomeReInterpolationFunction(**kwargs) # if this were a custom function to interpolate an array
    band.WriteArray(res)

though it doesn't appear to be needed in this case.