A brightness temperature difference (BTD) technique is used to constrain the ice to snow autoconversion threshold, a tunable parameter of a bulk microphysical cloud scheme in a regional meteorological model. The technique based on a contrasted absorption property of ice crystals at two wavelengths within the atmospheric infrared window is applied to 3-hourly Meteosat Second Generation (MSG) observations in the 8.7- and 10.8-μm bands over southern Brazil. The cirrus coverage presents a diurnal cycle associated with tropical deep convection peaking at 1800 local solar time (LST). A similar signal is obtained from the regional model when the ice to snow autoconversion threshold is reevaluated using the BTD data. The improved diurnal cycle furthermore better captures the observed upper-tropospheric humidity (UTH) maximum that lags the cirrus cover by 12 hours.