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  • Author or Editor: MICHAEL L. KAPLAN x
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Michael L. Kaplan
,
Yuh-Lang Lin
,
David W. Hamilton
, and
Robert A. Rozumalski

Abstract

Meso-beta-scale numerical model simulations and observational data are synthesized in an effort to develop a multistage paradigm for use in forecasting tornadic convection in the southeastern United States. The case study to be utilized as an example of the multistage sequence of events is the Palm Sunday 1994 outbreak, which culminates with the development of an unbalanced mesoscale jet streak or jetlet that focuses a given region for significant values of low-level vertical wind shear, low-level confluence and vertical vorticity, midtropospheric cooling, and storm-relative helicity. The five-stage paradigm includes 1) the existence of a jet exit region accompanying a deep balanced thermally indirect circulation south of the outbreak and a return branch ageostrophic low-level southerly jet, both typically accompanying the subtropical jet stream and the leading edge of hot continental air; 2) the existence of a jet entrance region accompanying a deep balanced thermally direct circulation north of the outbreak and a return branch ageostrophic low-level northerly jet, both typically accompanying the polar jet stream and the leading edge of rain-cooled air; 3) the geostrophic adjustment of the wind in the southern jet to the emerging/intensifying mass field perturbation, that is, intensification of the cross-stream mesoscale pressure gradient force, caused by the juxtaposition of the rain-cooled air southeast of the polar front and hot air accompanying the continental front where evaporational cooling as well as surface heating merge resulting in unbalanced jetlet formation; 4) the low-level mass adjustment underneath the new mesoscale midtropospheric unbalanced jetlet induces a return branch low-level unbalanced jetlet as well as vertical motion patterns oriented along the stream; and 5) adiabatic cooling ahead of the unbalanced jetlet and sinking behind it accompanying the along-stream vertical circulation increases the intensity of downstream destabilization and upstream downward momentum fluxes, producing a favorable environment for severe convection.

This new synoptic/dynamical overview assigns added importance to the subtropical jet, defines the continental air front, and also defines the unbalanced jetlet for use in predicting the presevere storm environment.

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Tracy M. Backes
,
Michael L. Kaplan
,
Rina Schumer
, and
John F. Mejia

Abstract

This study presents the climatology of the vertical structure of water vapor flux above the Sierra Nevada during significant cool season (November–April) precipitation events. Atmospheric river (AR) and non-AR events are analyzed to better understand the effect of this structure on precipitation patterns. Daily measurements of cool season precipitation at seven weather stations around the Tahoe basin from 1974 to 2012 and NCEP/CPC gridded daily precipitation analysis along the Sierra crest for the period 1948–2012 are examined. NCEP–NCAR reanalysis and soundings from Oakland are used to look at upper atmospheric conditions, including the presence of vapor transport by low- and midlevel jets on storm days as well as upstream static stability in relation to significant precipitation events. Key findings are as follows: 1) ARs play a disproportionately large role in generating Tahoe basin precipitation during the cool season; 2) strong midlevel vapor transport needs to occur in tandem with low-level transport to achieve the most extreme 2-day precipitation in the Tahoe basin; 3) when low- to midlevel vapor transport is present on days with a defined AR, the local maximum in 2-day precipitation intensity decreases with distance from the Sierra crest, and on non-AR days, the relative increase in 2-day precipitation intensity due to low- and midlevel vapor transport does not vary based on distance from the Sierra crest; 4) AR and non-AR moisture fluxes are significantly modified by upstream static stability; and 5) understanding the impacts of ARs and their lower- and midlevel moisture flux structure are crucial components of the hydrometeorology in this region.

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Michael L. Kaplan
,
Ramesh K. Vellore
,
Phillip J. Marzette
, and
John M. Lewis

Abstract

This study focuses on the meso-α- and meso-β-scale manifestations of the latent-heat-induced reduction of windward-side blocking to two flood-producing precipitation events on the leeside of the Sierra Nevada. Two simulations were performed—one employing full microphysics [control (CTRL)] and a second in which the latent heating terms are turned off in the microphysics [no latent heating (NLH)]. The differences between the CTRL and NLH are consistent with upstream latent heating—the moist, divergent, and ascending flow dominates the leeside of the mountain range in the CTRL producing copious spillover precipitation while dry high-momentum/downslope-descending flow dominates the NLH simulation on the leeside. A comprehensive sequence of events for spillover precipitation is formulated as follows: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet’s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow.

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Yuh-Lang Lin
,
Sen Chiao
,
Ting-An Wang
,
Michael L. Kaplan
, and
Ronald P. Weglarz

Abstract

The purpose of this paper is to synthesize some common synoptic and mesoscale environments conducive to heavy orographic rainfall. Previous studies of U.S. and Alpine cases and new analyses of some Alpine and east Asian cases have shown the following common synoptic and mesoscale environments are conducive to heavy orographic rainfall: 1) a conditionally or potentially unstable airstream impinging on the mountains, 2) a very moist low-level jet (LLJ), 3) a steep mountain, and 4) a quasi-stationary synoptic system to slow the convective system over the threat area. A deep short-wave trough is found to approach the threat area in the U.S. and European cases, but is not found in the east Asian cases. On the other hand, a high convective available potential energy (CAPE) value is observed in east Asian cases, but is not consistently observed in the U.S. and European cases. The enhancement of low-level upward motion and the increase of instability below the trough by the approaching deep short-wave trough in the U.S. and Alpine events may partially compensate the roles played by high CAPE in the East Asian events. In addition, the concave mountain geometry plays an important role in helping trigger the convection in Alpine and Taiwanese cases.

Based on an ingredient argument, it is found that a heavy orographic rainfall requires significant contributions from any combinations of the above four common synoptic and mesoscale environments or ingredients, and high precipitation efficiency of the incoming airstream, strong upward motion, and large convective system. These ingredients are also used to help explain the synoptic and mesoscale environments observed in some orographic flooding and heavy rainfall events in other regions, such as in New Zealand, China, and India. An index, U(∂h/∂x)q, where U is the flow velocity perpendicular to the mountain range, ∂h/∂x the mountain slope, and q the water vapor mixing ratio, is also proposed to help predict the occurrence of heavy orographic rainfall. Estimates of this proposed index indicate that it may serve as a good indicator for predicting east Asian heavy orographic rainfall events.

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Steven E. Koch
,
Fuqing Zhang
,
Michael L. Kaplan
,
Yuh-Lang Lin
,
Ronald Weglarz
, and
C. Michael Trexler

Abstract

Mesoscale model simulations have been performed of the second episode of gravity waves observed in great detail in previous studies on 11–12 July 1981 during the Cooperative Convective Precipitation Experiment. The dominant wave simulated by the model was mechanically forced by the strong updraft associated with a mountain–plains solenoid (MPS). As this updraft impinged upon a stratified shear layer above the deep, well-mixed boundary layer that developed due to strong sensible heating over the Absaroka Mountains, the gravity wave was created. This wave rapidly weakened as it propagated eastward. However, explosive convection developed directly over the remnant gravity wave as an eastward-propagating density current produced by a rainband generated within the MPS leeside convergence zone merged with a westward-propagating density current in eastern Montana. The greatly strengthened cool pool resulting from this new convection then generated a bore wave that appeared to be continuous with the movement of the incipient gravity wave as it propagated across Montana and the Dakotas.

The nonlinear balance equation and Rossby number were computed to explore the role of geostrophic adjustment in the forecast gravity wave generation, as suggested in previous studies of this wave event. These fields did indicate flow imbalance, but this was merely the manifestation of the MPS-forced gravity wave. Thus, the imbalance indicator fields provided no lead time for predicting wave occurrence.

Several sensitivity tests were performed to study the role of diabatic processes and topography in the initiation of the flow imbalance and the propagating gravity waves. When diabatic effects owing to precipitation were prevented, a strong gravity wave still was generated in the upper troposphere within the region of imbalance over the mountains. However, it did not have a significant impact because moist convection was necessary to maintain wave energy in the absence of an efficient wave duct. No gravity waves were present in either a simulation that disallowed surface sensible heating, or the “flat terrain” simulation, because the requisite MPS forcing could not occur.

This study highlights difficulties encountered in attempting to model the generation of observed gravity waves over complex terrain in the presence of strong diabatic effects. The complex interactions that occurred between the sensible heating over complex terrain, the incipient gravity wave, and convection highlight the need for much more detailed observations between wave generation regions over mountains and the plains downstream of such regions.

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Michael L. Kaplan
,
Christopher S. Adaniya
,
Phillip J. Marzette
,
K. C. King
,
S. Jeffrey Underwood
, and
John M. Lewis

Abstract

The synoptic structure of two case studies of heavy “spillover” or leeside precipitation—1–2 January 1997 and 30–31 December 2005—that resulted in Truckee River flooding are analyzed over the North Pacific beginning approximately 7 days prior to the events. Several sequential cyclone-scale systems are tracked across the North Pacific, culminating in the strengthening and elongation of a polar jet stream’s deep exit region over northern California and Nevada. These extratropical cyclones separate extremely cold air from Siberia from an active intertropical convergence zone with broad mesoscale convective systems and tropical cyclones. The development of moisture surges resulting in leeside flooding precipitation over the Sierra Nevada is coupled to adjustments within the last wave in the sequence of cyclone waves. Stage I of the process occurs as the final wave moves across the Pacific and its polar jet streak becomes very long, thus traversing much of the eastern Pacific. Stage II involves the development of a low-level return branch circulation [low-level jet (LLJ)] within the exit region of the final cyclone scale wave. Stage III is associated with the low-level jet’s convergence under the upper-level divergence within the left exit region, which results in upward vertical motions, dynamic destabilization, and the development of mesoscale convective systems (MCSs). Stage IV is forced by the latent heating and subsynoptic-scale ridging caused by each MCS, which results in a region of diabatic isallobaric accelerations downstream from the MCS-induced mesoridge. During stage IV the convectively induced accelerating flow, well to the southeast of the upper-level jet core, organizes a midlevel jet and plume of moisture or midlevel atmospheric river, which is above and frequently out of phase with (e.g., southeast of) the low-level atmospheric river described in Ralph et al. ahead of the surface cold front. Stage V occurs as the final sequential midlevel river arrives over the Sierra Nevada. It phases with the low-level river, allowing upslope and midlevel moisture advection, thus creating a highly concentrated moist plume extending from near 700 to nearly 500 hPa, which subsequently advects moisture over the terrain.

When simulations are performed without upstream convective heating, the horizontal moisture fluxes over the Sierra Nevada are reduced by ∼30%, indicating the importance of convection in organizing the midlevel atmospheric rivers. The convective heating acts to accelerate the midlevel jet flow and create the secondary atmospheric river between ∼500 and 700 hPa near the 305-K isentropic surface. This midlevel moisture surge slopes forward with height and transports warm moist air over the Sierra Nevada to typically rain shadowed regions on the lee side of the range. Both observationally generated and model-generated back trajectories confirm the importance of this convectively forced rapid lifting process over the North Pacific west of the California coast ∼12 h and ∼1200 km upstream prior to heavy leeside spillover precipitation over the Sierra Nevada.

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Benjamin J. Hatchett
,
Susan Burak
,
Jonathan J. Rutz
,
Nina S. Oakley
,
Edward H. Bair
, and
Michael L. Kaplan

Abstract

The occurrence of atmospheric rivers (ARs) in association with avalanche fatalities is evaluated in the conterminous western United States between 1998 and 2014 using archived avalanche reports, atmospheric reanalysis products, an existing AR catalog, and weather station observations. AR conditions were present during or preceding 105 unique avalanche incidents resulting in 123 fatalities, thus comprising 31% of western U.S. avalanche fatalities. Coastal snow avalanche climates had the highest percentage of avalanche fatalities coinciding with AR conditions (31%–65%), followed by intermountain (25%–46%) and continental snow avalanche climates (<25%). Ratios of avalanche deaths during AR conditions to total AR days increased with distance from the coast. Frequent heavy to extreme precipitation (85th–99th percentile) during ARs favored critical snowpack loading rates with mean snow water equivalent increases of 46 mm. Results demonstrate that there exists regional consistency between snow avalanche climates, derived AR contributions to cool season precipitation, and percentages of avalanche fatalities during ARs. The intensity of water vapor transport and topographic corridors favoring inland water vapor transport may be used to help identify periods of increased avalanche hazard in intermountain and continental snow avalanche climates prior to AR landfall. Several recently developed AR forecast tools applicable to avalanche forecasting are highlighted.

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Michael L. Kaplan
,
Yuh-Lang Lin
,
Joseph J. Charney
,
Karl D. Pfeiffer
,
Darrell B. Ensley
,
David S. DeCroix
, and
Ronald P. Weglarz

A state-of-the-science meso-β-scale numerical weather prediction model is being employed in a prototype forecast system for potential operational use at the Dallas–Fort Worth International Airport (DFW). The numerical model is part of a unique operational forecasting system being developed to support the National Aeronautics and Space Administration's (NASA) Terminal Area Productivity Program. This operational forecasting system will focus on meso-β-scale aviation weather problems involving planetary boundary layer (PBL) turbulence, and is named the Terminal Area PBL Prediction System (TAPPS). TAPPS (version 1) is being tested and developed for NASA in an effort to improve 1–6-h terminal area forecasts of wind, vertical wind shear, temperature, and turbulence within both stable and convective PBLs at major airport terminal areas. This is being done to enhance terminal area productivity, that is, aircraft arrival and departure throughput, by using the weather forecasts as part of the Aircraft Vortex Spacing System (AVOSS). AVOSS is dependent upon nowcasts or short-period forecasts of wind, temperature, and eddy dissipation rate so that the drift and dissipation of wake vortices can be anticipated for safe airport operation. This AVOSS system will be demonstrated during calendar year 2000 at DFW.

This paper describes the numerical modeling system, which has three basic components: the numerical model, the initial data stream, and the postprocessing system. Also included are the results of several case study simulations with the numerical model from a field program that occurred in September 1997 at DFW. During this field program, detailed local measurements throughout the troposphere, with special emphasis on the PBL, were taken at and surrounding DFW in an effort to verify the numerical model simulations. Comparisons indicate that the numerical model is capable of an accurate simulation of the vertical wind shear structure during the diurnal evolution of the PBL when compared directly to specific local observations. The case studies represent unambiguous examples of the dynamics of the Great Plains diurnal low-level jet stream. This diurnal jet stream represents the dominant low-level wind shear–production mechanism during quiescent synoptic-scale flow regimes. Five consecutive daily case studies, during which this phenomenon was observed over and in proximity to DFW, are compared to the products derived from TAPPS.

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