Accurately Tracking Cloud Vertical Motions

Summary

The tracking of cloud vertical motions and how these interact with atmospheric moisture and temperature are key for climate simulation and weather prediction. One of the most fundamental and ubiquitous calculations is the calculation of the properties of a cloud that rises vertically through the atmosphere. In fact, this calculation is performed thousands of times per day at weather centers around the world to quantify atmospheric instability and storm potential. It also is calculated many millions of times per day on supercomputers that are forecasting next week’s weather and next century’s climate. Despite the importance of this process, there is no agreement on how it should be calculated.

A recent study by a researcher at Lawrence Berkeley National Laboratory shows that previous methods for calculating these fluxes are flawed, and a new approach was developed. Three of the most common approaches are to use conservation of moist static energy (MSE), conservation of equivalent potential temperature, or conservation of entropy (the last two are actually the same). The new study shows that none of these is the correct choice: their use can lead to temperature errors on the order of 1 K. While 1 K may not sound like a lot, that is the typical buoyancy of a convecting cloud. The correct conservation principle is MSE minus CAPE, where CAPE is the parcel’s convective available potential energy. This quantity is the sum of the parcel buoyancy from the parcel height to its level of neutral buoyancy. The new results will lead to improvements in model methods for simulating atmospheric convection and dynamics.

References

Romps, D. M. 2015. “MSE Minus CAPE is the True Conserved Variable for an Adiabatically Lifted Parcel,” Journal of the Atmospheric Sciences, DOI: 10.1175/JAS-D-15-0054.1.