Examining the Role of the Land Surface in Modulating the Great Plains Low-Level Jet at the Subseasonal-to-Seasonal Timescale

The warm season in the United States Great Plains (GPs) is frequently characterized by a nocturnal low-level jet (LLJ). LLJs are defined broadly as confined wind corridors found in the lower atmosphere and act as a major mechanism of atmospheric moisture transport leading to extreme precipitation at their terminus. Previous studies have shown that Great Plains low-level jets (GPLLJs) are generated and maintained by an interplay of synoptic atmospheric forcing and local land surface forcing. Although primarily studied at diurnal and synoptic timescales, we have found dry soil moisture anomalies over the southern Great Plains varying at the subseasonal-to-seasonal (S2S) timescale correlated with GPLLJ activity. We hypothesize that S2S dry soil moisture anomalies modulate GPLLJ intensity through warmer near surface temperatures, deepening of the planetary boundary layer, and stronger ageostrophic winds at sunset, all leading to nocturnal supergeostrophic winds through the Blackadar inertial oscillation. To test our hypothesis and disentangle the timescales embedded in GPLLJ variability, we use a set of daily ECMWF Reanalysis v5 (ERA5) variables (e.g., soil moisture, geopotential height, virtual potential temperature) as timeseries inputs to Multivariate Singular Spectrum Analysis (MSSA). MSSA leverages the covariance of the input streams to compute temporal empirical orthogonal functions (T-EOFs). The T-EOFs are then used to reconstruct the original wind speed timeseries with variability from representative timescales (i.e., sub-weekly, weekly, S2S). Our findings show that reconstructing the GPLLJ without the S2S soil moisture variability leads to underestimated wind speeds during dry periods and overestimated during wet. These findings quantitatively show how the land surface plays an important role in modulating GPLLJ intensity which can have major implications for improving S2S predictability of GPLLJ activity and subsequent extreme precipitation in one of the most important socio-economic regions of the world.