The Tropical Cyclone Diurnal Cycle in CM1 Using an Ensemble Approach

The tropical cyclone diurnal cycle (TCDC) directly impacts storm structure and intensity. Thus far, the TCDC has been studied using satellite IR data, lightning data, rawinsondes and dropsondes, numerical modeling, a high-resolution global precipitation dataset, and TRMM. The TCDC manifests in different variables, and consistent timing of the diurnal cycle for those variables is found in the literature: the cirrus canopy reaches its maximum areal extent around 18 LT, precipitation peaks around 06 LT, and enhanced convection occurs overnight producing the coldest cloud-tops at around 04 LT. Despite the consistent story told, there are conflicting theories behind what drives the cirrus canopy expansion, whether and how precipitation is tied to the cirrus canopy expansion, whether the diurnal cycle is tied mainly to the outflow layer or is a column-deep phenomenon, and whether and how the observed diurnal pulse (Dunion et. al 2014) is related to the diurnal cycle.

This research will use the axisymmetric version of Cloud Model 1 (CM1) to try and answer these questions. A control run is produced. It is the average of five ensemble members created by applying a random Gaussian noise distribution with a mean of zero and a standard deviation of ten percent to the initial surface latent and sensible heat fields. The last six days of the ensemble mean run are averaged to further remove transients. Fourier harmonic analysis is then conducted to separate out the diurnal cycle. Results shown will focus on comparing the timing of the diurnal cycle in the model to the observed timing of cirrus canopy expansion, overnight convection, precipitation, and intensity changes. This simulation is then used to conduct further experiments to determine the main drivers behind the diurnal cycle as portrayed by CM1.