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F. Martin Ralph
,
Paul J. Neiman
, and
Richard Rotunno

Abstract

Dropsonde observations are used to document the mean vertical profiles of kinematic and thermodynamic conditions in the pre-cold-frontal low-level-jet (LLJ) region of extratropical cyclones over the eastern Pacific Ocean. This is the region within storms that is responsible not only for the majority of heavy rainfall induced by orography when such storms strike the coast, but also for almost all meridional water vapor transport at midlatitudes. The data were collected from NOAA’s P-3 aircraft in 10 storms during the California Land-falling Jets Experiment (CALJET) of 1998 and in 7 storms during the Pacific Land-falling Jets Experiment (PACJET) of 2001. The mean position of the dropsondes was 500 km offshore, well upstream of orographic influences. The availability of data from two winters that were characterized by very different synoptic regimes and by differing phases of ENSO—that is, El Niño in 1998 and La Niña in 2001—allowed examination of interannual variability.

The composite pre-cold-frontal profiles reveal a well-defined LLJ at 1.0-km altitude with a wind speed of 23.4 m s−1 and a wind direction of 216.7°, as well as vertical shear characteristic of warm advection. The composite thermodynamic conditions were also documented, with special attention given to moist static stability due to the nearly saturated conditions that were prevalent. Although the dry static stability indicated very stable conditions (4.5 K km−1), the moist static stability was approximately zero up to 2.8-km altitude. Although the composite winds, temperatures, and water vapor mixing ratios in 2001 differed markedly from 1998, the moist static stability remained near zero from the surface up to 2.8–3.0-km altitude for both seasons. Hence, orographic precipitation enhancement is favored in this sector of the storm, regardless of the phase of ENSO.

The dropsonde data were also used to characterize the depth and strength of atmospheric rivers, which are responsible for most of the meridional water vapor transport at midlatitudes. The vertically integrated along-river water vapor fluxes averaged 525 × 105 kg s−1 (assuming a 100-km-wide swath), while the meridional and zonal components were 387 × 105 kg s−1 and 302 × 105 kg s−1, respectively. Although the composite meridional transport in 2001 was less than half that in 1998 (230 × 105 kg s−1 versus 497 × 105 kg s−1), the characteristic scale height of the meridional water vapor transport remained constant; that is, 75% of the transport occurred below 2.25-km altitude.

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William C. Skamarock
,
Richard Rotunno
, and
Joseph B. Klemp

Abstract

Observations show that coastally trapped disturbances (CTDs) often accompany Catalina eddies that appear in the southern California bight. In a previous modeling study of CTDs using simple environments and forcings, simulations of CTDs also evolved a mesoscale eddy that played a critical role in CTD formation and evolution. In this study the simple environments and forcings are extended to model the southern California bight, and simulations produce both Catalina eddies and CTDs. The simulated Catalina eddies and the mesoscale eddies in the previous CTD studies are dynamically equivalent. The primary mechanism for eddy formation in the simulations is lee troughing, and eddies and CTDs are produced provided that 1) there is sufficient stratification in the environment; 2) the offshore flow is of sufficient strength, breadth, and duration; and 3) there is sufficient terrain to produce significant lee troughing. The simulations show that CTDs can propagate out of the bight region when synoptic winds leading to eddy formation are northeasterly onto the bight, but are blocked by the prevailing marine layer northwesterlies for the case where the synoptic winds are northwesterly. Potential vorticity (PV) generated during the course of eddy formation does not significantly contribute to the eddy structure or CTD formation.

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Teddie L. Keller
,
Richard Rotunno
,
Matthias Steiner
, and
Robert D. Sharman

Abstract

Previous studies have observed upstream-propagating modes in two-dimensional numerical simulations of idealized flow over topography with moist, nearly neutral conditions in the troposphere, topped by a stable stratosphere. The generation and propagation mechanisms for these modes were attributed to localized and dramatic changes in stability induced by the desaturation of the flow impinging on the mountain. In the present paper it is shown that these modes are transient upstream-propagating gravity waves, which are a fundamental feature of both moist and dry flow over topography of a two-layer troposphere–stratosphere atmospheric profile impulsively started from rest. The mode selection and propagation speeds of these transient waves are highly dependent on the tropospheric stability, as well as the wind speed and tropopause depth. In the moist case these modes appear to propagate according to an effective static stability that is intermediate to the normal dry stability and the lower moist stability. Comparisons with the linear, time-dependent, hydrostatic analytic solution show that these modes are similar to the transients observed in flow of a constant wind and stability layer over topography with a rigid upper boundary.

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Fuqing Zhang
,
Naifang Bei
,
Richard Rotunno
,
Chris Snyder
, and
Craig C. Epifanio

Abstract

A recent study examined the predictability of an idealized baroclinic wave amplifying in a conditionally unstable atmosphere through numerical simulations with parameterized moist convection. It was demonstrated that with the effect of moisture included, the error starting from small random noise is characterized by upscale growth in the short-term (0–36 h) forecast of a growing synoptic-scale disturbance. The current study seeks to explore further the mesoscale error-growth dynamics in idealized moist baroclinic waves through convection-permitting experiments with model grid increments down to 3.3 km. These experiments suggest the following three-stage error-growth model: in the initial stage, the errors grow from small-scale convective instability and then quickly [O(1 h)] saturate at the convective scales. In the second stage, the character of the errors changes from that of convective-scale unbalanced motions to one more closely related to large-scale balanced motions. That is, some of the error from convective scales is retained in the balanced motions, while the rest is radiated away in the form of gravity waves. In the final stage, the large-scale (balanced) components of the errors grow with the background baroclinic instability. Through examination of the error-energy budget, it is found that buoyancy production due mostly to moist convection is comparable to shear production (nonlinear velocity advection). It is found that turning off latent heating not only dramatically decreases buoyancy production, but also reduces shear production to less than 20% of its original amplitude.

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Daniel J. Kirshbaum
,
Richard Rotunno
, and
George H. Bryan

Abstract

A combination of idealized numerical simulations and analytical theory is used to investigate the spacing between convective orographic rainbands over the Coastal Range of western Oregon. The simulations, which are idealized from an observed banded precipitation event over the Coastal Range, indicate that the atmospheric response to conditionally unstable flow over the mountain ridge depends strongly on the subridge-scale topographic forcing on the windward side of the ridge. When this small-scale terrain contains only a single scale (λ) of terrain variability, the band spacing is identical to λ, but when a spectrum of terrain scales are simultaneously present, the band spacing ranges between 5 and 10 km, a value that is consistent with observations. Based on the simulations, an inviscid linear model is developed to provide a physical basis for understanding the scale selection of the rainbands. This analytical model, which captures the transition from lee waves upstream of the orographic cloud to moist convection within it, reveals that the spacing of orographic rainbands depends on both the projection of lee-wave energy onto the unstable cap cloud and the growth rate of unstable perturbations within the cloud. The linear model is used in tandem with numerical simulations to determine the sensitivity of the band spacing to a number of environmental and terrain-related parameters.

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George H. Bryan
,
Richard Rotunno
, and
J. Michael Fritsch

Abstract

In high-resolution numerical simulations (using horizontal grid spacing less than ∼1 km), the convective region of squall lines will sometimes overturn as quasi-horizontal convective rolls. The authors study one case in detail using output from a simulation with 125-m grid spacing. The rolls have an average spacing of 3 km and are aligned parallel to the vertical wind shear. Individual convective cells often have long-lived, undiluted cores that entrain primarily on the sides of the rolls (i.e., between the roll updraft and downdraft). The following set of conditions is proposed for obtaining roll overturning: the formation of a moist absolutely unstable layer (MAUL); vertical shear of the horizontal wind within the MAUL; an environment without large-amplitude perturbations; and quasi-horizontal flow over the squall line’s surface-based cold pool.

Further insight is gained through a series of more idealized simulations wherein a specified MAUL is perturbed by analytic potential temperature perturbations. These simulations confirm classical studies based on linear analysis because the smallest perturbations grow fastest (with the exception of the very smallest scales that are affected by diffusion). The results also confirm that, with shear, updrafts oriented across the shear vector are inhibited by the shear. An explanation for the ∼3-km roll spacing also emerges from these simulations. The argument focuses on the perturbations that exist in the cold pool underneath the MAUL; they induce pressure fields that extend upward into the overlying MAUL. The perturbations with large horizontal scale have pressure fields that extend farther vertically and with a greater amplitude, and thus are more effective at initiating motions in the overlying MAUL. The convective scale that ultimately emerges within the MAUL is somewhere between two scales, whereby comparatively large scales are perturbed more strongly by perturbations in the cold pool, but the comparatively small scales grow faster.

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Zhe-Min Tan
,
Fuqing Zhang
,
Richard Rotunno
, and
Chris Snyder
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Wendell T. Welch
,
Piotr Smolarkiewicz
,
Richard Rotunno
, and
Byron A. Boville

Abstract

Airflow over two-dimensional sinusoidal mesoscale topography is studied using simulations from a numerical model, with an eye toward quantification of the net effect on the large-scale flow. Analytic formulas are derived for the amount of form drag, that is, the total slowdown of the flow, as a function of mountain height, and predictions from such formulas are shown to agree well with model results. The vertical distribution of drag, due to gravity wave breaking at various altitudes, is briefly discussed.

The flow is divided into two regimes: a “linear” regime for small mountain heights, and a “blocked” regime for taller mountains. The latter is always accompanied by a layer of stagnant fluid in the valleys. Separate analytic arguments are used in each regime, and together they provide a prediction of form drag over a wide range of parameter space. The cutoff mountain height between the two regimes is also argued analytically.

A key difference from flow over isolated mountains is explained. This suggests that studies of flow over both isolated and periodic topography are needed in the development of orographic parameterizations for large-scale models.

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Zhe-Min Tan
,
Fuqing Zhang
,
Richard Rotunno
, and
Chris Snyder

Abstract

Recent papers by the authors demonstrated the possible influence of initial errors of small amplitude and scale on the numerical prediction of the “surprise” snowstorm of 24–25 January 2000. They found that initial errors grew rapidly at scales below 200 km, and that the rapid error growth was dependent on moist processes. In an attempt to generalize these results from a single case study, the present paper studies the error growth in an idealized baroclinic wave amplifying in a conditionally unstable atmosphere. The present results show that without the effects of moisture, there is little error growth in the short-term (0–36 h) forecast error (starting from random noise), even though the basic jet used here produces a rapidly growing synoptic-scale disturbance. With the effect of moisture included, the error is characterized by upscale growth, basically as found by the authors in their study of the numerical prediction of the surprise snowstorm.

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Richard Rotunno
,
Paul M. Markowski
, and
George H. Bryan

Abstract

Numerical models of supercell thunderstorms produce near-ground rotation about a vertical axis (i.e., vertical vorticity) after the development of rain-cooled outflows and downdrafts. The physical processes involved in the production of near-ground vertical vorticity in simulated supercells have been a subject of discussion in the literature for over 30 years. One cause for this lengthy discussion is the difficulty in applying the principles of inviscid vorticity dynamics in a continuous fluid to the viscous evolution of discrete Eulerian simulations. The present paper reports on a Lagrangian analysis of near-ground vorticity from an idealized-supercell simulation with enhanced vertical resolution near the lower surface. The parcel that enters the low-level maximum of vertical vorticity has a history of descent during which its horizontal vorticity is considerably enhanced. In its final approach to this region, the parcel’s enhanced horizontal vorticity is tilted to produce vertical vorticity, which is then amplified through vertical stretching as the parcel rises. A simplified theoretical model is developed that exhibits these same features. The principal conclusion is that vertical vorticity at the parcel’s nadir (its lowest point), although helpful, does not need to be positive for rapid near-surface amplification of vertical vorticity.

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