A model for simulation of laser produced plasmas that couples absorbed energy, plasma dynamics and radiation pressure was developed. The unusual experimental result of spectral narrowing of reflected laser pulses was attributed to deceleration of the critical (reflective) electron density surface due to the radiation pressure. A mechanism different from the Doppler effect that can cause spectral shifts was identified; a plasma reflectivity that changes rapidly in time can act as a frequency filter when the reflected laser pulse is chirped. The plasma expansion length and temperature that were calculated with the model was used to estimate the X-ray emission from the plasma with a simple scheme involving Kirchhoffs radiation law and Planck's black body radiation law.