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  • Author or Editor: Kaylee Heimes x
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Kaylee Heimes
,
Troy J. Zaremba
,
Robert M. Rauber
,
Sarah A. Tessendorf
,
Lulin Xue
,
Kyoko Ikeda
,
Bart Geerts
,
Jeffrey French
,
Katja Friedrich
,
Roy M. Rasmussen
,
Melvin L. Kunkel
, and
Derek R. Blestrud

Abstract

In Part II, two classes of vertical motions, fixed (associated with vertically propagating gravity waves tied to flow over topography) and transient (associated primarily with vertical wind shear and conditional instability within passing weather systems), were diagnosed over the Payette River basin of Idaho during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE). This paper compares vertical motions retrieved from airborne Doppler radial velocity measurements with those from a 900-m-resolution model simulation to determine the impact of transient vertical motions on trajectories of ice particles initiated by airborne cloud seeding. An orographic forcing index, developed to compare vertical motion fields retrieved from the radar with the model, showed that fixed vertical motions were well resolved by the model while transient vertical motions were not. Particle trajectories were calculated for 75 cross-sectional pairs, each differing only by the observed and modeled vertical motion field. Wind fields and particle terminal velocities were otherwise identical in both trajectories so that the impact of transient vertical circulations on particle trajectories could be isolated. In 66.7% of flight-leg pairs, the distance traveled by particles in the model and observations differed by less than 5 km with transient features having minimal impact. In 9.3% of the pairs, model and observation trajectories landed within the ideal target seeding elevation range (>2000 m), whereas, in 77.3% of the pairs, both trajectories landed below the ideal target elevation. Particles in the observations and model descended into valleys on the mountains’ lee sides in 94.2% of cases in which particles traveled less than 37 km.

Free access
Troy J. Zaremba
,
Kaylee Heimes
,
Robert M. Rauber
,
Bart Geerts
,
Jeffrey R. French
,
Coltin Grasmick
,
Sarah A. Tessendorf
,
Lulin Xue
,
Katja Friedrich
,
Roy M. Rasmussen
,
Melvin L. Kunkel
, and
Derek R. Blestrud

Abstract

Updrafts in wintertime cloud systems over mountainous regions can be described as fixed, mechanically driven by the terrain under a given ambient wind and stability profile (i.e., vertically propagating gravity waves tied to flow over topography), and transient, associated primarily with vertical wind shear and conditional instability within passing weather systems. This analysis quantifies the magnitude of fixed and transient updraft structures over the Payette River basin sampled during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE). Vertical motions were retrieved from Wyoming Cloud Radar measurements of radial velocity using the algorithm presented in Part I. Transient circulations were removed, and fixed orographic circulations were quantified by averaging vertical circulations along repeated cross sections over the same terrain during the campaign. Fixed orographic vertical circulations had magnitudes of 0.3–0.5 m s−1. These fixed vertical circulations were composed of a background circulation in which transient circulations were embedded. Transient vertical circulations are shown to be associated with transient wave motions, cloud-top generating cells, convection, and turbulence. Representative transient vertical circulations are illustrated, and data from rawinsondes over the Payette River basin are used to infer the relationship of the vertical circulations to shear and instability. Maximum updrafts are shown to exceed 5 m s−1 within Kelvin–Helmholtz waves, 4 m s−1 associated with transient gravity waves, 3 m s−1 in generating cells, 6 m s−1 in elevated convection, 4 m s−1 in surface-based deep convection, 5 m s−1 in boundary layer turbulence, and 9 m s−1 in shear-induced turbulence.

Free access
Troy J. Zaremba
,
Robert M. Rauber
,
Samuel Haimov
,
Bart Geerts
,
Jeffrey R. French
,
Coltin Grasmick
,
Kaylee Heimes
,
Sarah A. Tessendorf
,
Katja Friedrich
,
Lulin Xue
,
Roy M. Rasmussen
,
Melvin L. Kunkel
, and
Derek R. Blestrud

Abstract

Vertical motions over the complex terrain of Idaho’s Payette River basin were observed by the Wyoming Cloud Radar (WCR) during 23 flights of the Wyoming King Air during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) field campaign. The WCR measured radial velocity Vr , which includes the reflectivity-weighted terminal velocity of hydrometeors Vt , vertical air velocity w, horizontal wind contributions as a result of aircraft attitude deviations, and aircraft motion. Aircraft motion was removed through standard processing. To retrieve vertical radial velocity W, Vr was corrected using rawinsonde data and aircraft attitude measurements; w was then calculated by subtracting the mean W ( W ¯ ) at a given height along a flight leg long enough for W ¯ to equal the mean reflectivity-weighted terminal velocity V t ¯ at that height. The accuracy of the w and V t ¯ retrievals were dependent on satisfying assumptions along a given flight leg that the winds at a given altitude above/below the aircraft did not vary, the vertical air motions at a given altitude sum to 0 m s−1, and V t ¯ at a given altitude did not vary. The uncertainty in the w retrieval associated with each assumption is evaluated. Case studies and a projectwide summary show that this methodology can provide estimates of w that closely match gust probe measurements of w at the aircraft level. Flight legs with little variation in equivalent reflectivity factor at a given height and large horizontal echo extent were associated with the least retrieval uncertainty. The greatest uncertainty occurred in regions with isolated convective turrets or at altitudes where split cloud layers were present.

Free access