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Campbell D. Watson and Todd P. Lane

Abstract

This study examines how variations in relatively simple terrain geometries influence orographic precipitation and its spatial patterns of sensitivity to small changes in upstream conditions. An idealized three-dimensional model is used to simulate a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. A variety of simulations are conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height of approximately unity. It is found that for the straight and convex ridges, the flow response is mostly linear for the conditions examined here and passes over the obstacles with little lateral deflection. The concave ridge, however, exhibits strengthened flow deceleration, wave breaking in the lee, and flow confluence between the ridge arms. The concave ridge also generates substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funneling mechanism near the ridge vertex. However, for some concave ridge configurations the results feature dual-precipitation maxima, which is an important difference from previous findings. Finally, results from a simple ensemble of simulations elucidate the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries.

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Campbell D. Watson and Todd P. Lane

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This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.

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Campbell D. Watson and Todd P. Lane

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This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ = 0.66–2.0 and β = 1.0–8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry; variations to the low-level relative humidity are also explored.

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Campbell D. Watson, Ronald B. Smith, and Alison D. Nugent

Abstract

A sharp reduction in precipitation was observed on the island of Dominica in the Caribbean during a 2011 field campaign when the trade winds weakened and convection transitioned from mechanically to thermally driven. The authors propose four hypotheses for this reduction, which relate to (i) the triggering mechanism, (ii) dry-air entrainment, (iii) giant sea-salt aerosol, and (iv) small-island-derived aerosol. The plausibility of the first three hypotheses is the focus of this study.

Aircraft observations show the dynamics of the orographic cumulus clouds at flight level are surprisingly similar, irrespective of how they are triggered. However, the orographic cumulus clouds are consistently shallower when the trade winds are weak, which the authors attribute to a drier and shallower cloud layer compared to days with stronger trade winds. The strong negative influence of dry-air entrainment in a drier environment on cumulus depth and liquid water content is qualitatively demonstrated using an entraining plume model and the WRF Model. Although the models appear more sensitive than observations to entrainment and cloud size, the sensitivity tests have some resemblance to observations. The authors also find evidence of sea-salt aerosol entering the base of marine cumulus on strong wind days using an aircraft-mounted lidar and other instruments. Although each hypothesis is plausible, the complex interplay of these processes makes determining the controlling mechanisms difficult. Ultimately, the authors’ analysis rejects the hypothesis (i) triggering, while supporting (ii) entrainment and (iii) sea-salt aerosol.

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Todd P. Lane, James D. Doyle, Robert D. Sharman, Melvyn A. Shapiro, and Campbell D. Watson

Abstract

Historical records of aviation turbulence encounters above Greenland are examined for the period from 2000 to 2006. These data identify an important flow regime that contributes to the occurrence of aircraft turbulence encounters, associated with the passage of surface cyclones that direct easterly or southeasterly flow over Greenland’s imposing terrain. The result of this incident flow is the generation of mountain waves that may become unstable through interactions with the background directional wind shear. It is shown that this regime accounted for approximately 40% of the significant turbulent events identified in the 7-yr database. In addition, two specific cases from the database are examined in more detail using a high-resolution mesoscale model. The model simulations highlight the important role of three-dimensional gravity wave–critical level interactions and demonstrate the utility of high-resolution forecasts in the prediction of such events.

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Alison D. Nugent, Campbell D. Watson, Gregory Thompson, and Ronald B. Smith

Abstract

Observations from the Dominica Experiment (DOMEX) field campaign clearly show aerosols having an impact on cloud microphysical properties in thermally driven orographic clouds. It is hypothesized that when convection is forced by island surface heating, aerosols from the mostly forested island surface are lofted into the clouds, resulting in the observed high concentration of aerosols and the high concentration of small cloud droplets. When trying to understand the impact of these surface-based aerosols on precipitation, however, observed differences in cloud-layer moisture add to the complexity. The WRF Model with the aerosol-aware Thompson microphysics scheme is used to study six idealized scenarios of thermally driven island convection: with and without a surface aerosol source, with a relatively dry cloud layer and with a moist cloud layer, and with no wind and with a weak background wind. It is found that at least a weak background wind is needed to ensure Dominica-relevant results and that the effect of cloud-layer moisture on cloud and precipitation formation dominates over the effect of aerosol. The aerosol impact is limited by the dominance of precipitation formation through accretion. Nevertheless, in order to match observed cloud microphysical properties and precipitation, both a relatively dry cloud layer and a surface aerosol source are needed. The impact of a surface aerosol source on precipitation is strongest when the environment is not conducive to cloud growth.

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David C. Fritts, Ronald B. Smith, Michael J. Taylor, James D. Doyle, Stephen D. Eckermann, Andreas Dörnbrack, Markus Rapp, Bifford P. Williams, P.-Dominique Pautet, Katrina Bossert, Neal R. Criddle, Carolyn A. Reynolds, P. Alex Reinecke, Michael Uddstrom, Michael J. Revell, Richard Turner, Bernd Kaifler, Johannes S. Wagner, Tyler Mixa, Christopher G. Kruse, Alison D. Nugent, Campbell D. Watson, Sonja Gisinger, Steven M. Smith, Ruth S. Lieberman, Brian Laughman, James J. Moore, William O. Brown, Julie A. Haggerty, Alison Rockwell, Gregory J. Stossmeister, Steven F. Williams, Gonzalo Hernandez, Damian J. Murphy, Andrew R. Klekociuk, Iain M. Reid, and Jun Ma

Abstract

The Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropsondes, and a microwave temperature profiler on the GV and by in situ probes and a Doppler lidar aboard the German DLR Falcon. Extensive ground-based instrumentation and radiosondes were deployed on South Island, Tasmania, and Southern Ocean islands. Deep orographic GWs were a primary target but multiple flights also observed deep GWs arising from deep convection, jet streams, and frontal systems. Highlights include the following: 1) strong orographic GW forcing accompanying strong cross-mountain flows, 2) strong high-altitude responses even when orographic forcing was weak, 3) large-scale GWs at high altitudes arising from jet stream sources, and 4) significant flight-level energy fluxes and often very large momentum fluxes at high altitudes.

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