A Double Bright Band Originating From Atmospheric Instability Based on Ka-Band Radar Observation at Wuhan in China

Using observation from a Ka-band cloud radar at a midlatitude station, we present a double melting layer (ML) bright band phenomenon in stratiform precipitation, which is found to originate from atmospheric instability rather than warm front passage in earlier reports. In radar observations, a preexisting bright band experiences a descent and then rises back to its initial height after a little while over an hour. During the two transitions, a weak and a strong double bright band occurs, respectively. When the double bright band arises, hydrometeors undergo incomplete melting in the upper ML and subsequently refreeze before entering the lower ML. Reanalysis and radiosonde data show that there is no significant change in the zonal and meridional winds, but a strong updraft is induced due to the occurrence of atmospheric instability. Analysis indicated that the instability is caused by intense atmospheric convergence; in the meantime, there are lots of upward Doppler velocities for hydrometeors in the radar measurement, confirming the occurrence of convection due to the instability. Hence, the expansion effect of updrafts causes cooling, and then, the temperature gradually recovers from the high to low levels as the updrafts weaken and ultimately cease, leading to the thermal inversion layer associated with the double bright band.