Effects of Low-Frequency Motions on Momentum and Scalars Exchange: A Case Study from EBEX2000

EBEX was conducted in San Joaquin Valley, California, over a flood-irrigated flat cotton field during the period from July 20 to August 24, 2000. The cotton field with the area of 1.6 km by 0.8 km was almost rectangular shaped and north-to-south orientated. During the experimental period, the canopy heights were about 0.8-0.9 m. The field was flood-irrigated over a period of several days (working north to south), twice during the observation period. The patch-by-patch flood-irrigations created a non-homogeneous soil moisture regime. Additionally, just to the north of the cotton field was a large dry bare field that caused a step dry-to-wet change in the soil moisture.

We choose the data on August 7, 2000 for our study. Due to the patched irrigation working from north to south, there were two drier patches upstream on August 7 to the north of the patch where Site 7 was located. However, the Site 7 patch was still moist on August 7. The fluctuations in the correlation coefficient between u and w also are found to correspond to fluctuations in the latent flux. We chose 6 half-hour runs (2 low Ruw at 12:30 LT and 14:00 LT, 4 adjacent high Ruw at 12:00 LT, 13:00 LT, 13:30 LT, and 14:30 LT) for our analysis. In this study, quadrant and wavelet analysis are used to investigate the effects of low-frequency motions on turbulent fluxes. The run-to-run comparisons of wavelet spectra and cospectra give the spectra density and covariance contribution at different scales, which indicate variations of low-frequency motions and their influence on the vertical motions and exchanges for momentum and scalars. Their flux contributions occur at the low-frequency parts, or the higher frequency through modulation effects. Our wavelet analysis of longer time-series data hint the non-stationary background from the low frequency parts.