Search Results

You are looking at 1 - 10 of 17 items for :

  • Author or Editor: Roy M. Rasmussen x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: All Content x
Clear All Modify Search
David B. Johnson
and
Roy M. Rasmussen

Abstract

The transition between wet and dry growth for graupel and hail is examined, and new figures are presented illustrating the critical water contents necessary for transitions into or out of the wet-growth regime. These figures are extended to smaller sizes and lower bulk densities than considered in previous studies. In addition, the possibility of hysteresis in the transitions is examined.

Full access
Roy M. Rasmussen
and
Piotr K. Smolarkiewicz

Abstract

This paper extends our earlier discussion of the flow past the island of Hawaii and the accompanying cloud band to smaller-scale effects occurring on the scale of the Hilo Bay region. The evolution of cloud bands forming upwind of the island on 1 August 1985 is studied using a high-resolution numerical model and available field observations. The current work provides further evidence in support of the view that the phenomenon of Hawaiian cloud bands is closely linked to the dynamics of strongly stratified flows past three-dimensional obstacles. In particular, results are presented that document the cloud interaction with a secondary, vertically propagating gravity wave and the formation of horizontally oriented vortices in the lower upwind flow—two characteristic features encountered in studies of idealized low Froude number flows. Quantification of the effects due to nocturnal thermal forcing is attempted, and it is shown that cooling along the volcano slope doubles the depth of the dynamically induced downslope flow as well as its maximum wind speed, whereas it has a little effect upon the position of the mesoscale convergence line and coinciding leading edge of the downslope current. Downslope surges of cold air from the volcano slope are shown to temporarily enhance the depth and strength of the downslope flow, leading to invigorated cloud development at the leading edge of the current. Analysis of the Hawaiian Rainband Project (HaRP) sounding data relates cloud bands to the theory of squall lines and suggests that the trade wind environment upstream of the island is favorable to the formation of cloud bands consisting of isolated cells advected by the local cloud-layer winds.

Full access
Roy M. Rasmussen
and
Andrew J. Heymsfield

Abstract

A detailed model of the melting, shedding, and wet growth of spherical graupel and hail is presented. This model is based upon recent experimental studies by Rasmussen et al. and Lesins et al. The model is presented in the form of five easy-to-use tables. Important quantities considered were the heat transfer. terminal velocity behavior, and shedding of liquid water.

Full access
Roy M. Rasmussen
and
Andrew J. Heymsfield

Abstract

A sensitivity study on the melting and shedding behavior of individual graupel and hail is presented utilizing the detailed microphysical model presented in Part I. The influence of particle density and size, atmospheric temperature profile, relative humidity profile, liquid water content, shedding parameterization, and heat transfer rates are investigated. The results show that the melting of graupel and the melting and shedding behavior of hailstones are significantly affected by the initial particle density and size, temperature profile, and relative humidity.

When graupel and hail are grown in a Doppler-derived three-dimensional wind field, the results show that the melting and shedding behavior of the graupel and hail are relatively insensitive to changes as large as 25% in the heat transfer coefficient or to the differences between the Rasmussen et al. and the Chong and Chen shedding parameterizations.

Full access
Roy M. Rasmussen
and
Andrew J. Heymsfield

Abstract

The 1 August severe storm during the Cooperative Convective Precipitation Experiment (CCOPE) has been analyzed making use of T-28 aircraft data, CP-2 radar data, and a particle trajectory model in conjunction with a time-averaged Doppler-derived wind field. This analysis reveals that water drops are shed from wet hailstones in this storm, and that some of them are shed in favorable locations to grow back into hailstones. A particular region of the storm is identified as the most likely source region for shed drops, and particle trajectory calculations show that hailstones grown from this region fall out in locations consistent with the reflectivity structure of the storm. Some trajectories are shown to fall back through this same region while at the same time shedding, providing new hail embryos.

Full access
Robert Tardif
and
Roy M. Rasmussen

Abstract

To gain insights into the poorly understood phenomenon of precipitation fog, this study assesses the evaporation of freely falling drops departing from equilibrium as a possible contributing factor to fog formation in rainy conditions. The study is based on simulations performed with a microphysical column model describing the evolution of the temperature and mass of evaporating raindrops within a Lagrangian reference frame. Equilibrium defines a state where the latent heat loss of an evaporating drop is balanced by the sensible heat flux from the ambient air, hence defining a steady-state drop temperature. Model results show that the assumption of equilibrium leads to small but significant errors in calculated precipitation evaporation rates for drops falling in continuously varying ambient near-saturated or saturated conditions. Departure from equilibrium depends on the magnitude of the vertical gradients of the ambient temperature and moisture as well as the drop-size-dependent terminal velocity. Contrasting patterns of behavior occur depending on the stratification of the atmosphere. Raindrops falling in inversion layers remain warmer than the equilibrium temperature and lead to enhanced moistening, with supersaturation achieved when evaporation proceeds in saturated inversions. Dehydration occurs in layers with temperature and water vapor increasing with height due to the vapor flux from the environment to the colder drops. These contrasts are not represented when equilibrium is assumed. The role of nonequilibrium raindrop evaporation in fog occurrences is further emphasized with simulations of a case study characterized by fog forming under light rain falling in a developing frontal inversion. Good agreement is obtained between fog water content observations and simulations representing only the effects of rainfall evaporation. This study demonstrates the need to take into account the nonequilibrium state of falling raindrops for a proper representation of an important mechanism contributing to precipitation fog occurrences.

Full access
Roy M. Rasmussen
,
Piotr Smolarkiewicz
, and
John Warner

Abstract

This paper presents a detailed comparison study of three-dimensional model results with an aircraft wind field mapping for the island of Hawaii. Model runs were initialized using an aircraft sounding from 1 August 1985, and detailed predictions from the model are compared with observations from that day.

The strength and location of the upwind convergence zone were well simulated, as well as the strong deflection and deceleration of the flow around the island and the geometry and location of the upstream cloud bands. The good agreement between the model results and observations supports the results of our previous study in which we show that the flow pattern and associated cloud processes around the island of Hawaii can be understood by considering the flow of a stably stratified fluid around a large three-dimensional obstacle.

Model runs with different wind directions showed that increasing northerly tradewind flow resulted in the band clouds moving closer to the shore line, and the large scale flow pattern rotating counterclockwise. Model results were also compared with various aspects of the island climatology, and good agreement was found in both the temporal and spatial distribution of precipitation on the island.

Full access
Roy M. Rasmussen
,
István Geresdi
,
Greg Thompson
,
Kevin Manning
, and
Eli Karplus

Abstract

This study evaluates the role of 1) low cloud condensation nuclei (CCN) conditions and 2) preferred radiative cooling of large cloud drops as compared to small cloud drops, on cloud droplet spectral broadening and subsequent freezing drizzle formation in stably stratified layer clouds. In addition, the sensitivity of freezing drizzle formation to ice initiation is evaluated. The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical scheme implemented into the National Center for Atmospheric Research–Pennsylvania State University Mesoscale Model version 5. The height and width of the two-dimensional mountain were designed to produce an updraft pattern with extent and magnitude similar to documented freezing drizzle cases. The results of the model simulations were compared to observations and good agreement was found.

The key results of this study are 1) low CCN concentrations lead to rapid formation of freezing drizzle. This occurs due to the broad cloud droplet size distribution formed throughout the cloud in this situation, allowing for rapid broadening of the spectra to the point at which the collision–coalescence process is initiated. 2) Continental clouds can produce freezing drizzle given sufficient depth and time. 3) Radiative cooling of the cloud droplets near cloud top can be effective in broadening an initially continental droplet spectrum toward that of a maritime cloud droplet size distribution. 4) Any mechanism that only broadens the cloud droplet spectra near cloud top, such as radiative cooling, may not act over a sufficiently broad volume of the cloud to produce significant amounts of freezing drizzle. 5) Low ice-crystal concentrations (<0.08 L−1) in the region of freezing drizzle formation is a necessary condition for drizzle formation (from both model and observations). 6) Ice nuclei depletion is a necessary requirement for the formation of freezing drizzle. 7) The maximum cloud water mixing ratio and threshold amount for the onset of drizzle in stably stratified clouds was shown to depend strongly on the CCN concentration. 8) A key factor controlling the formation of freezing drizzle in stratified clouds is the lifetime of the mesoscale and synoptic conditions and the thickness and length of the cloud.

Full access
Kyoko Ikeda
,
Roy M. Rasmussen
,
William D. Hall
, and
Gregory Thompson

Abstract

Observations of supercooled drizzle aloft within two storms impacting the Oregon Cascades during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project are presented. The storms were characterized by a structure and evolution similar to the split-front model of synoptic storms. Both storms were also characterized by strong cross-barrier flow. An analysis of aircraft and radar data indicated the presence of supercooled drizzle during two distinct storm periods: 1) the intrafrontal period immediately following the passage of an upper cold front and 2) the postfrontal period. The conditions associated with these regions of supercooled drizzle included 1) temperatures between −3° and −19°C, 2) ice crystal concentrations between 1 and 2 L−1, and 3) bimodal cloud droplet distributions of low concentration [cloud condensation nuclei (CCN) concentration between 20 and 30 cm−3 and cloud drop concentration <35 cm−3].

Unique to this study was the relatively cold cloud top (<−15°C) and relatively high ice crystal concentrations in the drizzle region. These conditions typically hinder drizzle formation and survival; however, the strong flow over the mountain barrier amplified vertical motions (up to 2 m s−1) above local ridges, the mountain crest, and updrafts in embedded convection. These vertical motions produced high condensate supply rates that were able to overcome the depletion by the higher ice crystal concentrations. Additionally, the relatively high vertical motions resulted in a near balance of ice crystal fall speed (0.5–1.0 m s−1), leading to nearly terrain-parallel trajectories of the ice particles and a reduction of the flux of ice crystals from the higher levels into the low-level moisture-rich cloud, allowing the low-level cloud water and drizzle to be relatively undepleted.

One of the key observations in the current storms was the persistence of drizzle drops in the presence of significant amounts of ice crystals over the steepest portion of the mountain crest. Despite the high radar reflectivity produced by the ice crystals (>15 dBZ) in this region, the relatively high condensate supply rate led to hazardous icing conditions. The current study reveals that vertical motions generated by local topographic features are critical in precipitation processes such as drizzle formation and thus it is essential that microphysical models predict these motions.

Full access
Kyoko Ikeda
,
Edward A. Brandes
, and
Roy M. Rasmussen

Abstract

An unusual multiple freezing-level event observed with polarimetric radar during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE-2) field program is described. The event occurred on 28 November 2001 when a warm front moved over the Oregon Cascade Mountains. As the front approached, an elevated melting layer formed above a preexisting melting layer near ground. Continued warming of the lower atmosphere eventually dissipated the lower melting layer.

The polarimetric measurements are used to estimate the height of the freezing levels, document their evolution, and deduce hydrometeor habits. The measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar. As warming progressed, the character of particles entering the lower melting layer changed, possibly becoming ice pellets or frozen drops. Eventually, the refreezing of particles ended and only rain occurred below the elevated melting layer.

The Doppler radial winds showed a well-defined wind maximum apparently associated with a “warm conveyor belt.” The jet intensified and descended through the elevated melting layer with time. However, the increase in wind speed did not appear connected with melting or result in precipitation enhancement.

Full access