dynamics of short rate(nonlinearity in drift)

let's talk the drift only, coz u didn't mention the diffusion term.

different studies find different results of relationships between the mean-reverting speed and the short rate dynamics. For example, Ait-sahalia(1996) and Conley et al.(1997)find evidence of strong mean reversion when r(t) is below the 2% normal range [U-shaped function of mena reversion], while others like Stanton(1997)argues no evidence of strong mean reverting at low level interest rates, among which Ahn and Gao(1999) defined a quadratic nonlinear model of the drift term[dr(t)=a(b-r(t))r(t)+...], which assumes the mean reverting speed increases linearly with the short rate.

for detailed descriptions of the nonlinearity of interest rate term structure model, please go through the references list above. i hope it could help.