Many students confuse the LCL (Lifted Condensation Level) with the CCL (Convective Condensation Level). They often ask "why are the LCL and CCL at different levels in the troposphere? What about the rising process makes them different?" The primary difference has to do with the surface temperature. A LCL occurs when forced lifting occurs. A surface parcel, with its temperature and dewpoint are forced into the vertical by a trigger mechanism such as a front, vort max, dryline bulge, convergence boundary, mountain, and so forth. This air (originally at the surface or lower PBL) cools at the dry adiabatic lapse rate until the temperature equals the dewpoint (temperature lapse rate = 10 degrees C per kilometer, dewpoint lapse rate = 2 degrees C per kilometer (dewpoint lapse rate is the same as the mixing ratio lapse rate.. see laminated skew-T). When the air parcel becomes saturated, the LCL is reached. Now onto the CCL, the CCL is not found by forced lifting, but by rather a warming of the earth's surface. The air does not rise until the surface temperature warms and reaches a critical value with this process. The CCL is generally higher than the LCL because the AIR MUST FIRST WARM before the air can rise to the CCL (remember air warming causes the relative humidity to decrease and the dewpoint depression to increase, because of this, the air must rise to a higher altitude before becoming saturated). The CCL will be higher than the LCL. The LCL and CCL are found by the same process EXCEPT from the CCL the surface temperature must rise to a critical value (called convective temperature) before a surface parcel will begin the ascent in the vertical due to positive buoyancy. Finding the CCL is the same as the process of finding the LCL when air has warmed to the critical convective temperature. Use the CCL for summertime air mass thunderstorms and thermodynamic daytime heating lifting and the LCL for any dynamical lifting (jet streak, vorticity, frontal, convergence uplift). Can forced lifting and the rising of air due to reaching the convective temperature occur at the same time? Yes, in this case the height of the cloud base will be between the theoretical LCL and CCL. Does the LCL and CCL value found on a Skew-T correlate perfectly with the true cloud base of a thunderstorm or cloud deck? Sometimes yes, sometimes no; The character of the PBL with respect to temperature and dewpoint can change rapidly during the day. A sounding and even a forecast sounding can not perfectly portray the boundary layer conditions moment to moment. It takes a skilled forecaster to know how close the real troposphere will mirror the theoretical LCL and CCL Skew-T values. |