"drop rapidly" is correct and usable in written English.
It is typically used to describe a sudden decrease in something. For example, "The temperature dropped rapidly during the storm."
Temperatures continue to drop rapidly until about December.
Mark Braley, publishing analyst at Deutsche Bank in London, expects the Standard's sales to drop rapidly.
As devices become familiar, prices drop rapidly (as has happened with DVD players, for example).
The motor gets out of steps and the precision will drop rapidly.
August is winter in Botswana and though the days are warm, the temperature can drop rapidly at night.
Insects lay their eggs just below the water level, and if levels drop rapidly it can dry them out.
On top of that, the cost of wind and solar continues to drop rapidly as technology advances.
As the upper panel of Exhibit III shows, reserve-production ratios would drop rapidly under all scenarios.
While NRBC are frequently found in umbilical cord blood after fetal distress, NRBC counts drop rapidly after birth.
Under cyclic equibiaxial loading the modulus of the annealed specimen was found to drop rapidly with increasing numbers of cycles.
C.R.T.'s will undoubtedly hang on for special functions and devoted users, but the number sold is expected to drop rapidly in coming years.
