Estimation of the Variability of Mesoscale Energy Spectra with Three Years of COSMO-DE Analyses

Abstract

Research on the mesoscale kinetic energy spectrum over the past few decades has focused on finding a dynamical mechanism that gives rise to a universal spectral slope. Here we investigate the variability of the spectrum using 3 years of kilometer-resolution analyses from COSMO configured for Germany (COSMO-DE). It is shown that the mesoscale kinetic energy spectrum is highly variable in time but that a minimum in variability is found on scales around 100 km. The high variability found on the small-scale end of the spectrum (around 10 km) is positively correlated with the precipitation rate where convection is a strong source of variance. On the other hand, variability on the large-scale end (around 1000 km) is correlated with the potential vorticity, as expected for geostrophically balanced flows. Accordingly, precipitation at small scales is more highly correlated with divergent kinetic energy, and potential vorticity at large scales is more highly correlated with rotational kinetic energy. The presented findings suggest that the spectral slope and amplitude on the mesoscale range are governed by an ever-changing combination of the upscale and downscale impacts of these large- and small-scale dynamical processes rather than by a universal, intrinsically mesoscale dynamical mechanism.

Comparing Convective Self‐Aggregation in Idealized Models to Observed Moist Static Energy Variability Near the Equator

“…Ultimately, we hope the tool summarized here can be deployed across the emerging hierarchy of global cloud resolving models (Satoh et al, ) to help clarify their intrinsic thermodynamics. Our spectral framework can be generalized to spatially limited domains by choosing a transform insensitive to nonperiodic boundaries, such as the discrete cosine transform (e.g., Denis et al, ; Selz et al, ). It can also be generalized to arbitrary subsets of the domain by choosing a transform retaining localization information, such as the wavelet transform (e.g., Torrence & Compo, ).…”

“…boundaries, such as the discrete cosine transform (e.g.,Denis et al, 2002;Selz et al, 2018). It can also be generalized to arbitrary subsets of the domain by choosing a transform retaining localization information, such as the wavelet transform (e.g.,Torrence & Compo, 1998).…”

Comparison of Methods Accounting for Subgrid-Scale Model Error in Convective-Scale Data Assimilation

“…In addition, a spectral analysis of horizontal and vertical kinetic energy based on the comparison to E_BASE is carried out for short-term forecasts. At scales smaller than 300 km, the horizontal kinetic energy is strongly related to the amount of convective precipitation as shown by Selz et al (2019). One goal of the spectral analysis here is to examine if the improvements in precipitation forecasts correspond to the added kinetic energy.…”

“…Table 4 gives the relative changes (%) of kinetic energy (in spectrum space) of analyses (initial states) in E_P compared to E_BASE for different scales and heights. Similar as Selz et al (2019), we focus on the wavelength range between 14 and 1000 km, and for ease of discussion, the wavelengths are divided into 14-100, 100-300, and 300-1000 km, which may roughly represent convective scale, mesoscale, and synoptic scale, respectively. The spectra of horizontal kinetic energy (KE) are calculated by the sums of energy spectra of divergent (DIV) and rotational (ROT) wind.…”

Explicit Prediction of Continental Convection in a Skillful Variable‐Resolution Global Model

“…There is still substantial variability in the magnitude and the shape of the spectrum even during this 5‐day forecast, especially at the smallest resolved scales; further experiments (Figure , right) representing all seasons show that there can be strong regime‐dependent shifts in the spectral shape, spectral slope, and even total energy of the nested region, which are exacerbated if the region on which the analysis is performed is reduced. This behavior has been found previously by Selz et al (), who found a strong connection between convective precipitation and the variability of the mesoscale kinetic energy spectrum. These results from regional domains are in conflict with our experience with globally uniform fvGFS simulations (Lin et al, ), which typically have very robust global kinetic energy spectra across runs, even at uniform global 3‐km grid cell widths.…”