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Jennifer M. Cram and Michael L. Kaplan

Abstract

A variational method has been developed to assimilate VAS temperature and moisture gradient information into a mesoscale model. A series of experiments were conducted to test the sensitivity of both adiabatic and diabatic versions of the model to VAS data assimilations for the 20–21 July 1981 case.

The VAS data for this case are compared to the rawinsonde data and VAS moisture imagery. The retrieved VAS temperature fields captured the asynoptic development of strong mesoscale temperature gradients although the VAS relative humidity fields were generally too smooth.

The synoptic-scale effects of the assimilation of VAS data were negligible. The greatest impact was on the mesoscale forecasts of the patterns of convective instability. The assimilation of the strong VAS temperature gradients resulted in the short-term forecast of greater convective instabilities across Oklahoma, where observed convection subsequently developed. The additional assimilation of relative humidity gradients did not significantly change the patterns of the forecast instabilities. Increasing the number of successive assimilations improved the subsequent forecasts of convective instability.

For this case, the greatest improvements from assimilation resulted from the resolution of the strong mesoscale temperature gradients by the asynoptic VAS data. The assimilation of this structure into the model resulted in forecasts of convective instability and precipitation more closely resembling the patterns of the observed convection.

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Michael L. Kaplan and Douglas A. Paine

Abstract

A numerical model has been designed to link quasi-geostrophic and mesoscale forcing. Initializations performed at 127 km with either a barotropic forecast or a diagnostic omega equation are followed by a prediction with a moist nine-level primitive equation model operating on a 32-km grid mesh. The results of two short-period integrations indicate good correspondence between forecast omega fields and radar observations, observed snowfall, and diagnostic integrations. Vertical motions reach maxima of 25 and 50 cm sec−1, while the forecast dependent variables indicate an integral role played by long gravity waves in organizing mesoscale development.

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Michael L. Kaplan and Douglas A. Paine

Abstract

A numerical model has been developed in an effort to simulate the interaction between quasi-geostrophic and mesoscale forcing. Initializations performed at 127 km with either a diagnostic omega equation or barotropic forecast are followed by a prediction with a moist nine-level primitive equation model at 32 km. Several integrations are performed utilizing both real and artificial data for the problem of the water-induced heat island in the cold season. The results of these integrations indicate important variations in both the patterns and intensity of development as a function of initialization.

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John W. Zack and Michael L. Kaplan

Abstract

A series of mesoscale numerical simulations of the AVE-SESAME I case (10 April 1979) were performed in order to analyze the dynamical processes that result in the production of an environment favorable for the development of severe local convective storms. The investigation focused on the relative contributions of quasi-adiabatic inertial and isallobaric adjustments attributable to the geometry of the tropospheric flow and the fluxes of heat, moisture and momentum from the surface of the earth.

The model simulations support many of the conclusions deduced by Kocin et al. in their analyses of the observations taken during the field experiment. The quasi-adiabatic simulations support the existence of a coupled upper-tropospheric and lower-tropospheric jet streak system. However, the dynamical coupling is more complex than the straight line jet streak model utilized by Uccellini and Johnson. The departures are attributable to two sources. First, there is a time-varying curvature in the exit region due to the propagation of a meso-αscale trough through the area while a longer wave trough remains relatively stationary. Second, the exit region experiences significant changes in the mass field due to the presence of differential horizontal thermal advection. These two effects produce significant alterations to the classical exit region patterns of vertical motion and man divergence. In addition, them processes phase with a pattern of significant horizontal variations in the fluxes of heat, moisture and momentum in the planetary boundary layer. The combination of these processes result in the amplification of the low-level pressure tendencies and an increase in the strength of the low-level jet streak.

The combination of mass-momentum adjustments associated with the jet streak system and low-level flux gradients results in the creation of significant amounts of buoyant energy and the vertical motion necessary for its release. The simulation experiments suggest that the 6 h increase in buoyant energy over the areas that subsequently experience convection is approximately half the result of the quasi-adiabatic processes and half the result of the surface fluxes of heat and moisture.

This study has three major contributions. First, it indicates the possible importance of the phasing of deep tropospheric mass-momentum adjustments with differential surface fluxes of heat and momentum. Second, it extends the understanding of jet-streak exit region dynamics to the case of cyclonically curved flow in the presence of differential horizontal thermal advection. Third, it reveals the rapidity with which circulation patterns associated with a jet streak exit region can change.

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JAMES E. JIUSTO and MICHAEL L. KAPLAN

Abstract

Three yr of winter lake-storm data were analyzed to determine snowfall distribution patterns downwind of Lake Erie and Lake Ontario. The total amount of snowfall and the area of ground cover in each of 23 lake-effect storms were determined for both lakes. Total snowfall mass was highly dependent on time of year; November and early December storms were two to five times more productive than January storms. A considerable variation in snow density (snowfall depth to melt water ratio) could be attributed mainly to differences in snow crystal type.

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William H. Bauman III, Michael L. Kaplan, and Steven Businger

Abstract

Space shuttle landings at the shuttle landing facility at Kennedy Space Center are subject to strict weather-related launch commit criteria and flight rules. Complex launch commit criteria and end-of-mission flight rules demand very accurate nowcasts (forecasts of less than 2 h) of cloud, wind, visibility, precipitation, turbulence, and thunderstorms prior to shuttle launches and landings.

During easterly flow regimes the onset of convective activity has proven to be particularly difficult to predict. Contrasting weather ranging from clear skies to thunderstorms occurs on days with seemingly similar synoptic environments. Four days of easterly flow during the Convection and Precipitation/Electrification (CaPE) Experiment were investigated in an effort to identify and simulate key features that distinguish convectively active and suppressed conditions. Data from CaPE and operational data, including satellite imagery and National Centers for Environmental Prediction model analysis output over the Florida peninsula and surrounding data-sparse Atlantic Ocean, are combined in the research. It is found that elevated moisture in the midtroposphere above the marine boundary layer helps distinguish convectively active and passive days. Moreover, analysis reveals that the moisture distribution is related to jet dynamics in the upper troposphere.

A series of simulations using the Mesoscale Atmospheric Simulation System (MASS) model was undertaken. The MASS model run with a coarse grid (45 km) correctly simulates the development of the upper-level jet streak and its general impact on convective activity over the Florida peninsula. The MASS model run with a nested (11 km) grid and moisture enhancement of the initial model state from radar, satellite, and surface data results in the best short-term (6 h) forecast of relative humidity and precipitation patterns over the Florida peninsula and proximate coastal environment. Implications of the research results for nowcasting convective activity over Cape Canaveral are discussed.

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Brian F. O’Hara, Michael L. Kaplan, and S. Jeffrey Underwood

Abstract

The Sierra Nevada of eastern California receives heavy snowfall each year. However, it is the snowstorms that deposit heavy snowfall in a relatively short period of time that can cause major inconveniences and even life-threatening situations for the residents and visitors to the region. Some of these snowstorms are so extreme as to become legendary, and with increased population in this region a synoptic climatology of these extreme snowstorms can be a useful tool for assessing snowfall potential by operational forecasters. Additionally, the hydrological and climatological implications of extreme Sierra Nevada snowfalls are important for state and local resource- and emergency-planning purposes.

A climatology of these snowstorms will be presented. The period of study will include the snowfall seasons (October–May) 1949/50 through 2004/05. A total of 542 snowstorms occurred during these 56 snowfall seasons. These snowstorms were analyzed to determine any common synoptic features. The most intense snowstorms in the highest decile of snowfall totals were analyzed in more detail in order to determine the parameters associated with these strongest snowstorms.

Upper-level synoptic and thermodynamic characteristics associated with each snowstorm were then diagnosed to determine what common synoptic hydrodynamic and thermodynamic parameters the snowstorms share. Synoptic patterns were studied using the National Centers for Environmental Prediction (NCEP) model reanalysis data. Wind speeds at 200 hPa, and height anomalies at 500 hPa, were analyzed for each snowstorm from 3 days prior to the start of snowfall and continuing through the end of the storm. Anomalies and the transport of precipitable water were studied in order to determine the relative amount of moisture that was available to each snowstorm.

A conceptual model for forecasting the strongest snowstorms was developed. Key findings include the following: 1) the importance of a fetch of moisture from the subtropics with relatively large positive moisture anomalies, 2) the importance of the atmospheric moisture stream being normal to the Sierra, 3) the low static stability accompanying these snowstorms, and 4) the importance of relatively strong upper-level dynamics, which helped to intensify the systems as they approached the Sierra.

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Sen Chiao, Yuh-Lang Lin, and Michael L. Kaplan

Abstract

This paper investigates the local circulation associated with a heavy orographic rainfall event during 19–21 September 1999 [Mesoscale Alpine Programme Intensive Observing Period 2B (MAP IOP-2B)]. This event was simulated with a 5-km horizontal grid spacing using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The MM5 simulation reproduced the basic features such as the timing and location of the deep trough and the associated precipitation evolution, though the total amount of precipitation is slightly higher than that measured by rain gauges (∼30% in 24 h). The near-surface flow was dominated by an easterly jet originally from the Adriatic Sea and a southerly jet from the Gulf of Genoa. A significant westward turning occurred when the southerly flow approached the south side of the Alps. This deflection was caused by boundary layer friction and rotation, as well as mountain blocking effects. Flow was generally from the south above the surface. Precipitation was mainly concentrated on the windward slopes, especially near the Lago Maggiore region. Sensitivity experiments have been conducted to investigate the effects of upstream orography, the western flank of the Alps, boundary layer friction, and horizontal resolution. The results indicate that precipitation distribution and amount over the southern upslope region of the Alps were not directly related to either the coastal Apennine Mountains or the west flank of the Alps. The boundary layer friction reduces the total amount and alters the distribution of rainfall by weakening the wind near the surface. The 1.67-km horizontal grid spacing simulation indicates that heavy rainfall tended to be concentrated in the vicinity of individual mountain peaks. The total amount of rainfall was overpredicted along the windward slopes because of the strong upward motion that occurred on the upslope of the barrier. The results indicate the importance of dynamical forcing associated with upslope-induced and near-surface horizontal velocity convergence-induced vertical motion, which increases rapidly as horizontal resolution increases.

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S. Jeffrey Underwood, Michael L. Kaplan, and K. C. King

Abstract

Pacific-originating storms that produce heavy leeside liquid precipitation in the Sierra Nevada are rare compared to those that generate windward slope rainfall. However, these leeside precipitation events have a profound effect on the flood hydrology of leeside basins in the Sierra Nevada. This study identified 12 storms that affected the Truckee River basin in northeastern Nevada. The storms produced both moderate and extreme flooding in this leeside basin. A synoptic-scale analysis of conditions leading to leeside storms was produced using a compositing procedure. Composites for multiple pressure levels and multiple parameters were produced for class 1 storms—those storms producing moderate flood flow in the Truckee River basin—and class 2 storms—those producing extreme flooding [>10 000 cubic feet per second (cfs), or 283 m3 s−1] in this basin. The analysis confirms that the two flood populations are in fact generated by Pacific-originating storms with observably different synoptic-scale circulations. The class 2 storms are moister through a great depth in the troposphere (saturated to 750 hPa), and they occur coincident with warmer conditions in the lower and midtroposphere. Class 2 events exhibited more favorable upper-level jet streak structures in the eastern Pacific and over western North America. Both classes of leeside storms were shown to differ substantially from Pacific-originating storms that exclusively affect the windward slope of the Sierra and the coastal mountain ranges of California (California storms). The leeside storms were much warmer than California storms through much of the lower and midtroposphere, and the onshore flow was predominantly from the west-southwest in leeside storms compared to southerly flow in California storms. The findings suggest the existence of a midlevel atmospheric river delivering moisture to leeside basins of the Sierra Nevada.

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Craig M. Smith, Benjamin J. Hatchett, and Michael L. Kaplan

Abstract

Sundowners are downslope windstorms that occur over the southern slopes of the east–west-trending Santa Ynez range in Santa Barbara County, California. In the past, many extreme fires in the area, including the Painted Cave, Montecito Tea, Jesusita, and Sherpa fires, have occurred during sundowner events. A high-resolution 11-yr dynamically downscaled climatology was produced using a numerical weather prediction model in order to elucidate the general dynamical characteristics of sundowners. The downscaled climatology is validated with observations during the 2016 Sherpa fire. A sundowner index (SI) is computed from the climatology that quantifies the magnitude of adiabatic warming and northerly (downslope) wind component during sundowner events. The SI allows for the classification of historical events into categories of various strengths. The primary characteristics of strong sundowners from this classification include 1) internal gravity wave breaking over the Santa Ynez range, 2) initiation in the western Santa Ynez range with eastward progression over the course of a day, 3) a maximum likelihood of occurrence in April and May near 2000 Pacific standard time, and 4) a limited downstream extent for most events, such that the long-term historical weather station, Santa Barbara airport, often does not experience moderate events. Analysis of an operational forecast rubric composed of the surface pressure difference from Bakersfield to Santa Barbara indicates that this rubric is not skillful. However, offshore pressure gradients are skillful and are related to the strong northwesterly alongshore jet. The findings presented herein can be used to provide guidance for fire weather forecasts and support resource allocation during fire suppression efforts.

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