The spectral composition of fluxes and variances over land and sea out to the mesoscale

Abstract

We discuss the accuracy requirements for measuring mesoscale (roughly horizontal scales >10 km or 5 to 10 times the planetary boundary-layer (PBL) depth) fluxes in the convective PBL, and the ability of current research aircraft to achieve this accuracy. We conclude that aircraft equipped with inertial nagivation systems capable of < 3 km hr −1 navigational accuracy are able to resolve mesoscale fluctuations in velocity, and thus variances and fluxes on the mesoscale. We then discuss measurements of velocity and scalar spectra, and cospectra of vertical velocity with horizontal velocity components and scalars, obtained from long flight legs with the National Center for Atmospheric Research Electra aircraft over the boreal forest of Canada in summer during the BOreal Ecosystem-Atmosphere Study (BOREAS), over the tropical Pacific Ocean from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and over the East China Sea during wintertime cold-air outbreaks from the Air Mass Transformation Experiment (AMTEX). Each of these studies has somewhat different forcings and boundary conditions, so we can compare their consequences on the spectra and cospectra. On average, we found no significant scalar or momentum fluxes for horizontal scales >10 km. We also develop a simple model based on observed thermal structure to explain the phase angle between vertical velocity and the along-wind horizontal velocity as a function of height, which shows good agreement with the observed phase angle in AMTEX.

Properties of a simulated convective boundary layer over inhomogeneous vegetation

“…This is consistent with the result of the LEMs study by Kang and Davis (2009) and is also supported by observations (e.g. Mahrt et al, 1994;LeMone et al, 2002;Kang et al, 2007;Lenschow and Sun, 2007;Taylor et al, 2007). However, this weak mesoscale circulation may not be considered negligible, as stressed by Ek and Mahrt (1991).…”

Using sonic anemometer temperature to measure sensible heat flux in strong winds

“…5b1). Lenschow and Sun (2007) show that w should lag u by a phase angle of around 90 • which suggests that T s in Fig. 5b1 is being affected (or contaminated) by streamwise velocity fluctuations.…”

The definition of ‘truth’ for Numerical Weather Prediction error statistics

“…Fortunately, for many atmospheric conditions (Gage, 1979;Nastrom and Gage, 1985;Lindborg, 1999;Wikle et al, 1999;Cho and Lindborg, 2001;Lindborg and Cho, 2001;Lenschow and Sun, 2007;Riley and Lindborg, 2008) there is a robust scaling of turbulence in the horizontal plane that connects the resolved scales of the NWP models to the subgrid scale turbulence statistics. More work is required to extend these results to other atmospheric conditions, such as the night-time residual layer and stable boundary layers.…”