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John D. Marwitz

Abstract

Two deep, orographic storms were documented over the Sierra Nevada with an instrumented aircraft and with a single Doppler radar. In both storms the geographic winds were normal to the barrier with speeds of 15 to 30 m s−1. Because of blocking the measured wind component normal to the barrier was substantially less than the geostrophic wind component, especially below barrier crest height. In the very stable case, a barrier jet was present below 1 km AGL and had peak winds of 28 m s−1. In the slightly stable case a barrier jet was present at 1 km AGL and had peak winds of 21 m s−1. Other than the speed and altitude of the barrier jet, the other measured characteristics of the airflow and thermal fields were rather similar between these two storms. The forced ascent of stable air plus the diabatic process of melting caused the isotherms to descend near the barrier. The barrier jet can be expressed by the thermal wind relation and is, therefore, a quasi-geostrophic phenomenon. The derived vertical velocities were estimated to be 0.2 to 0.4 m s−1.

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John D. Marwitz

Abstract

The thermodynamic and kinematic structure of two stable orographic storms were described in Part I based on instrumented aircraft data and single Doppler radar data. The precipitation processes in these storms are described in this paper. The storms were deep with cloud top temperatures of about −25°C. Below the melting level the cloud droplet population was continental with a mean droplet diameter <10 μm. Above the melting level the cloud droplet population was maritime with mean droplet diameters of 20 to 30 μm. Near the −5°C level a peak in ice crystal concentration of 30 to 200 L−1 was observed. Since most of the ice crystals were needles, are rime-splintering secondary ice crystal production processes as generally described by Hallett and Mossop was probably occurring.

Calculations of the condensation supply rates were compared with the depletion rates by deposition and accretion. The depletion rates by deposition were less than half the condensation supply rates, and the liquid water contents remained low. Accretion is deduced to be the dominant process, which acts to deplete the condensate to near zero. Deep, stable orographic storms over the Sierra barrier, therefore, develop an efficient glaciation process.

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John D. Marwitz

Abstract

Two case studies of the kinematics of the airflow over the Sierra barrier are presented. The observations consisted of rawinsondes and single Doppler RHI and velocity azimuth display (VAD) analysis of PPI scans. The RHI scans were made orthogonal to the nearly two-dimensional Sierra barrier. The cloud in the first case study contained a strong stable layer at 0°C while the second storm was highly unstable.

The radar bright band and soundings near the radar indicated that an ∼250 m thick 0°C isothermal layer was present in response to the diabatic process of melting. When the bright band was impinging upon the barrier, the associated 0°C isothermal layer was inferred to expand until it finally reached the ground. At that point it was ∼1 km in depth. Direct thermodynamic soundings am presented for a similar situation which agrees with the modified soundings inferred for this study. The resulting effects on the airflow and precipitation are discussed.

The unstable case contained a line of deep convection oriented parallel to the crest The deep convection acted to transport a substantial amount of low-level unstable air upward and the convection also blocked the airflow such that a wake was present downwind of the line of convection.

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Terrence W. Krauss
and
John D. Marwitz

Abstract

An investigation was made into the precipitation processes operating within an Alberta supercell hailstorm which occurred on 22 July 1979. The main research tools employed in the study were an instrumented aircraft and an S-band meteorological radar.

Five cloud penetrations were conducted in and around the bounded weak-echo region associated with the main updraft of the storm during a 40 min period while the storm was producing 2–5 cm diam hail at the surface. The main updraft consisted of a relatively smooth, steady flow with adiabatic temperature. The main updraft acted as an obstacle to the mid-level environmental flow and caused the winds to split and accelerate around the southern side. Several smaller turbulent updrafts associated with time-dependent, fine-scale convective cells (feeder clouds) existed adjacent to the main updraft and appeared to be superimposed onto the quasi-steady, broader-scale dynamically forced circulation of the main storm. Graupel particles which originated within the feeder clouds were transported by the mid-level winds into and across the weak-echo region.

This study provides evidence that feeder clouds are also found within storms classed as supercells and that they provide a viable source of hailstone embryos and present a vital link among hail formation processes within a broader-scale continuum of hailstorm structures.

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Fikrettin Çelik
and
John D. Marwitz

Abstract

The “ripening process” occurs due to thermodynamic instability of droplet size spectra in clouds. This instability results from the existence of droplets with different salinity and size in the droplet spectra. The ripening process is independent of turbulent fluctuations of supersaturation for a closed cloud parcel. Because of the ripening process, droplet number concentration continuously decreases with time after the initial peak supersaturation, which occurs during the initial updraft. Droplet spectra broaden to large sizes by evaporation of small droplets. Both mean droplet size and the standard deviation of droplet size spectra increase with time. This mechanism is suggested to be a potential physical mechanism for the formation of droplet size spectra in stratiform clouds.

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Ronald E. Stewart
,
John D. Marwitz
,
John C. Pace
, and
Richard E. Carbone

Abstract

Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze ) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.

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