John Wesley Powell, in the nineteenth century, introduced the notion that the 100th meridian divides the North American continent into arid western regions and humid eastern regions. This concept remains firmly fixed in the national imagination. It is reexamined in terms of climate, hydrology, vegetation, land use, settlement, and the agricultural economy. It is shown there is a stark east-west gradient in aridity roughly at the 100th meridian that is well expressed in hydroclimate, soil moisture, and ''potential vegetation.'' The gradient arises from atmospheric circulations and moisture transports. In winter, the arid regions west of the 100th meridian are shielded from Pacific stormrelated precipitation and are too far west to benefit from Atlantic storms. In summer, the southerly flow on the western flank of the North Atlantic subtropical high has a westerly component over the western plains, bringing air from the interior southwest, but it also brings air from the Gulf of Mexico over the eastern plains, generating a west-east moisture transport and precipitation gradient. The aridity gradient is realized in soil moisture and a west-to-east transition from shortgrass to tallgrass prairie. The gradient is sharp in terms of greater fractional coverage of developed land east of the 100th meridian than to the west. Farms are fewer but larger west of the meridian, reflective of lower land productivity. Wheat and corn cultivation preferentially occur west and east of the 100th meridian, respectively. The 100th meridian is a very real arid-humid divide in the physical climate and landscape, and this has exerted a powerful influence on human settlement and agricultural development.

The 100th meridian bisects the Great Plains of the United States and effectively divides the continent into more arid western and less arid eastern halves and is well expressed in terms of vegetation, land hydrology, crops, and the farm economy. Here, it is considered how this arid-humid divide will change in intensity and location during the current century under rising greenhouse gases. It is first shown that state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project generally underestimate the degree of aridity of the United States and simulate an arid-humid divide that is too diffuse. These biases are traced to excessive precipitation and evapotranspiration and inadequate blocking of eastward moisture flux by the Pacific coastal ranges and Rockies. Bias-corrected future projections are developed that modify observationally based measures of aridity by the model-projected fractional changes in aridity. Aridity increases across the United States, and the aridity gradient weakens. The main contributor to the changes is rising potential evapotranspiration, while changes in precipitation working alone increase aridity across the southern and decrease across the northern United States. The ''effective 100th meridian'' moves to the east as the century progresses. In the current farm economy, farm size and percent of county under rangelands increase and percent of cropland under corn decreases as aridity increases. Statistical relations between these quantities and the biascorrected aridity projections suggest that, all else being equal (which it will not be), adjustment to changing environmental conditions would cause farm size and rangeland area to increase across the plains and percent of cropland under corn to decrease in the northern plains as the century advances.

Observations of near-surface vertical wind profiles and vertical momentum fluxes obtained from a Doppler lidar and instrumented towers deployed during VORTEX-SE in the spring of 2017 are analyzed. In particular, departures from the predictions of Monin–Obukhov similarity theory (MOST) are documented on thunderstorm days, both in the warm air masses ahead of storms and within the cool outflow of storms, where MOST assumptions (e.g., horizontal homogeneity and a steady state) are least credible. In these regions, it is found that the nondimensional vertical wind shear near the surface commonly exceeds predictions by MOST. The departures from MOST have implications for the specification of the lower boundary condition in numerical simulations of convective storms. Documenting departures from MOST is a necessary first-step toward improving the lower boundary condition and parameterization of near-surface turbulence (“wall models”) in storm simulations.

New regulations are being issued by the Federal Aviation Administration (FAA) that require three-dimensional hydrometeor-phase diagnosis, including discrimination between freezing rain (FZRA) and freezing drizzle (FZDZ), for all commercial airports in the United States. Herein, a novel hydrometeor-phase algorithm, the Spectral Bin Classifier (SBC), is adapted to meet these requirements. First, the SBC’s Particle Size Distribution (PSD) is upgraded to be variable rather than fixed. This, along with some changes to the logic, allows for drizzle (DZ)/FZDZ to be diagnosed. Second, the SBC is modified to provide a low-altitude (LA), above-ground diagnosis (SBC-LA). Last, necessary changes to account for resolution issues in NWP thermal profiles are presented. Adding a dynamic-PSD capability improves the Probability of Detection (POD) by about 12%, but adapting the algorithm to include DZ/FZDZ worsens the PODs. This is due to potentially-errant reports of rain (RA)/FZRA in environments more conducive to DZ/FZDZ. Assuming a diagnosis of DZ is a hit when RA is observed, and likewise for FZRA/FZDZ, increases the POD by between 35 and 60%. While performance statistics for SBC-LA cannot be computed, about one-third of all RA and DZ soundings herein have an elevated layer of FZRA/FZDZ, underscoring the importance of an above-ground diagnosis for the aviation sector. The comparatively-low vertical resolution of NWP profiles is found to degrade the SBC’s performance. Interpolating to a higher resolution helps mitigate this problem. Several case studies of mixed phases at different airports are presented to highlight the enhanced decision support made possible by the above modifications.