Sensitivity of WRF Model Output to Planetary Boundary Layer Height Variation over North Africa

The first intention of this work was the modification of the Planetary Boundary Layer (PBL) Yonsei University scheme (YSU) in order to improve the estimation of Planetary Boundary Layer Height (PBLH) over Sahara desert during daytime. It is anticipated that the problem concerns the estimation of surface layer temperature. The surface boundary layer temperature, in the Revised YSU version (RYSU), is increased by (TSK-TT)/d when the skin temperature TSK > TT. It is found that when the parameter TT is 40o C the optimal value of the parameter d is around 3.5. Evaluation of WRF model forecast output when implemented with RYSU shows better performance for PBL atmospheric variables. The other intention of this work is to study the impact of using the WRF model implemented with RYSU scheme relative to that using YSU scheme. It is shown that the difference between the two models run is significant even far from the Sahara desert where the modification is applied. It is found that very small fast disturbances propagating out of a dipole vortex where the surface boundary layer temperature is modified excite specific locations forming local dipole vorticies. Out of these dipole vortices gravity waves propagate. Development that cause significant local variation in weather elements occur where the interaction evolve. It is found that deep convection areas are one of the possible specific locations where interaction can arises. One can argue that since very small propagating disturbances are part of our chaotic atmosphere, the side effect that resembles the Butterfly effect could be numerical rather than physical outcome.