Larry D. Oolman

On the Role of Sloping Terrain in the Forcing of the Great Plains Low-Level Jet

Abstract The summertime Great Plains low-level jet (LLJ) has been the subject of numerous investigations during the past several decades. Characteristics of the LLJ include nighttime development of a pronounced wind maximum of typically 15–20 m s−1 at levels 300–800 m above the surface and a clockwise rotation of the wind maximum during the course of the night. Maximum frequency of occurrence of the LLJ is found in the southern Great Plains. Theories proposed to explain the diurnal wind maximum of the Great Plains LLJ include inertial oscillation of the ageostrophic wind, the diurnal oscillation of the horizontal pressure field associated with heating and cooling of the sloping terrain, and the western boundary current interpretations. A simple equation system and output from the 12-km horizontal resolution Weather Research and Forecasting Nonhydrostatic Mesoscale Model (NAM) for July 2008 are used to provide evidence as to the importance of the Great Plains topography in driving the LLJ. Summertime heati...

Dust transport over Iraq and northwest Iran associated with winter Shamal: A case study

Dynamical processes leading to dust emission over Syria and Iraq, in response to a strong winter Shamal event as well as the subsequent transport of dust over Iraq and northwest Iran, are analyzed on the basis of a case study (22-23 February 2010) using a suite of ground-based and spaceborne remote sensing platforms together with modeling tools. Surface measurements on 22 February show a sharp reduction in horizontal visibility over Iraq occurring shortly after the passage of a cold front (behind which the northwesterly Shamal winds were blowing) and that visibilities could be as low as 1 km on average for 1-2 days in the wake of the front. The impact of the southwesterly Kaus winds blowing ahead (east) of the Shamal winds on dust emission over Iraq is also highlighted. Unlike what is observed over Iraq, low near-surface horizontal visibilities (<1 km) over northwest Iran are observed well after the passage of the cold front on 23 February, generally in the hours following sunrise. Ground-based lidar measurements acquired in Zanjan show that, in the wake of the front, dust from Syria/Iraq was transported in an elevated 1 to 1.5 km thick plume separated from the surface during the night/morning of 23 February. After sunrise, strong turbulence in the developing convective boundary layer led to mixing of the dust into the boundary layer and in turn to a sharp reduction of the horizontal visibility in Zanjan. The timing of the reduction of surface horizontal visibility in other stations over northwest Iran (Tabriz, Qom, and Tehran) is consistent with the downward mixing of dust in the planetary boundary layer just after sunset, as evidenced in Zanjan. This study sheds new light on the processes responsible for dust emission and transport over Iraq and northwest Iran in connection with winter Shamal events. Enhanced knowledge of these processes is key for improving dust forecasts in this region.

Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part I: Observations*

AbstractTwo cases of mountain waves, rotors, and the associated turbulence in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part study using aircraft observations and numerical simulations. In Part I, observations from in situ instruments and high-resolution cloud radar on board the University of Wyoming King Air aircraft are presented and analyzed. Measurements from the radar compose the first direct observations of wave-induced boundary layer separation.The data from these two events show some striking similarities but also significant differences. In both cases, rotors were observed; yet one looks like a classical lee-wave rotor, while the other resembles an atmospheric hydraulic jump with midtropospheric gravity wave breaking aloft. High-resolution (30 × 30 m2) dual-Doppler syntheses of the two-dimensional velocity fields in the vertical plane beneath the aircraft reveal the boundary layer separation, the scale and structure of the attendant rotors, and downsl...

Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part II: Numerical Modeling

AbstractMountain waves and rotors in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part paper. Part I by French et al. delivers a detailed observational account of two rotor events: one displays characteristics of a hydraulic jump and the other displays characteristics of a classic lee-wave rotor. In Part II, presented here, results of high-resolution numerical simulations are conveyed and physical processes involved in the formation and dynamical evolution of these two rotor events are examined.The simulation results reveal that the origin of the observed rotors lies in boundary layer separation, induced by wave perturbations whose amplitudes reach maxima at or near the mountain top. An undular hydraulic jump that gave rise to a rotor in one of these events was found to be triggered by midtropospheric wave breaking and an ensuing strong downslope windstorm. Lee waves spawning rotors developed under conditions favoring wave energy trapping at low levels in differe...

Spring 1996 and 1997 ozonesonde measurements over McMurdo Station, Antarctica

Ozone and temperature profiles were measured with balloon-borne instruments from McMurdo Station, Antarctica (77.85°S, 166.67°E) from late August to late October, on 38 occasions in 1996 and on 26 occasions in 1997. Minimum column ozone values of 147 DU and 141 DU were measured in 1996 and 1997, reduced from initial measurements in late August of 277 DU and 313 DU, respectively. These ozone minima are slightly higher than the lowest seasonal minimum detected in 1993, and are similar to observations of 1994, 1995. The observed recoveries during 1996, and especially 1997, as indicated by increasing temperatures and ozone above 20 km as the polar vortex became unstable, were among the weakest measured since 1986 with low ozone extending into late October. We continue to observe in both 1996 and 1997 almost complete ozone depletion over a layer more than 4 km thick and centered near 17 km, similar to the first such observation over McMurdo in 1995. The maximum observed thickness of this layer has remained relatively constant since 1995.

Ozone measurements over McMurdo Station, Antarctica during spring 1994 and 1995

Ozone and temperature profiles were measured with balloon-borne instruments from McMurdo Station, Antarctica (77.85°S, 166.67°E) from late August to late October, on 39 occasions in 1994 and on 35 occasions in 1995. Minimum column ozone values of 138 DU and 139 DU were measured in 1994 and 1995, reduced from initial measurements in late August of 272 DU and 256 DU, respectively. These minima are higher than the record low measurement from McMurdo taken in October 1993 (130 DU), but lower than all other years since 1986 when ozone measurements were initiated at McMurdo. In 1994 and 1995, as in previous years, the onset of ozone recovery began mid-to-late October above 20 km. Below 14 km ozone concentrations have returned to pre-Pinatubo levels. Between 16–22 km they are near, and in 1995 often lower than, the record low levels set in 1993. In late August of 1995, the total and 12–20 km column ozone were also often observed to be near or lower than that for the same period of all previous years since 1986. By early October 1995, almost complete ozone depletion was experienced uniformly over the altitude region 14.5–19.5 km.

Isobaric Height Perturbations Associated with Mountain Waves Measured by Aircraft during the Terrain-Induced Rotor Experiment

AbstractIt has only been in the last few years that accurate measurement of the horizontal pressure gradient has been possible over complex terrain using an airborne platform. To infer forcing mechanisms for the wind, an independent measure of the height of an isobaric surface is required. Differential GPS analyses have enabled determination of the aircraft height with sufficient accuracy to infer isobaric heights. When coupled with an accurate measurement of static pressure, the horizontal pressure field can be determined. To demonstrate this measurement technique, research flight legs by the University of Wyoming King Air (UWKA) conducted in support of the Terrain-Induced Rotor Experiment (T-REX) in March and April 2006 are examined. UWKA flights conducted on 14 and 25 March and 16 April 2006 encountered strong mountain waves in response to winds directed primarily normal to the Sierra Nevada ridgeline. Winds at flight level showed pronounced variation that suggested topographic influence. The magnitude...