Investigating the Role of the Land Surface in Modulating the Great Plains Low-Level Jet
Advisor: Professor Francina Dominguez
Abstract
The warm season in the United States Great Plains (GP) is characterized by frequent nocturnal
low-level jets (LLJs). The GPLLJ serves as a major mechanism of atmospheric moisture transport,
contributing to severe weather and precipitation in the region. Additionally, the nocturnal wind
maxima of GPLLJs are a resource for wind energy production throughout the GP. These
hydrometeorological and socioeconomic implications motivate a better understanding of the
GPLLJ to improve predictability. It is accepted that a combination of atmospheric and land surface
forcing modulates GPLLJ diurnal variability. Previous studies have examined the diurnal variability
of soil moisture associated with GPLLJs, this is the first analysis at the subseasonal timescale.
This thesis is the first documented evidence of a subseasonal timescale embedded within GPLLJ
intensity that covaries with soil moisture over the U.S. Great Plains. I show that, due to the
memory of the land surface, longer time scale variability associated with surface moisture affects
GPLLJ intensity through a chain of land-atmosphere processes. Through a combined
observational and statistical framework, this dissertation addresses the following questions on
subseasonal GPLLJ variability:
1. What are the land and atmosphere conditions, at the diurnal, synoptic/pentad, and
subseasonal timescales, antecedent to extreme GPLLJ events?
2. What physical mechanisms lead to stronger GPLLJ when dry soil moisture anomalies
prevail over the southern Great Plains?
3. Can the temporal scales embedded within GPLLJ variability be disentangled to extract the
subseasonal variability and link to soil moisture variability?
The work starts with the first documented study of antecedent, subseasonal soil moisture
anomalies associated with GPLLJs. We then present mechanistic evidence of how dry soil
moisture anomalies can contribute to stronger GPLLJs. Our findings suggest that subseasonal
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.
We lastly disentangle the different timescales of variability of GPLLJ to extract the land surfacedriven
subseasonal timescale embedded within GPLLJ variability with Multichannel Singular
Spectrum Analysis. Our findings show that reconstructing the GPLLJ without the subseasonal
variability, which is strongly linked to soil moisture, leads to underestimated wind speeds during
dry periods and overestimated during wet. These findings quantitatively demonstrate how the land
surface plays an important role in modulating GPLLJ intensity which can have major implications
for improving subseasonal predictability of GPLLJ activity, and subsequent energy production and
precipitation.