My WebLink
|
Help
|
About
|
Sign Out
Home
Browse
Search
WMOD00430
CWCB
>
Weather Modification
>
DayForward
>
WMOD00430
Metadata
Thumbnails
Annotations
Entry Properties
Last modified
7/28/2009 2:39:07 PM
Creation date
4/18/2008 10:00:36 AM
Metadata
Fields
Template:
Weather Modification
Title
A Diagnostic Technique for Targeting During Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada
Date
7/7/1988
Weather Modification - Doc Type
Report
There are no annotations on this page.
Document management portal powered by Laserfiche WebLink 9 © 1998-2015
Laserfiche.
All rights reserved.
/
18
PDF
Print
Pages to print
Enter page numbers and/or page ranges separated by commas. For example, 1,3,5-12.
After downloading, print the document using a PDF reader (e.g. Adobe Reader).
Show annotations
View images
View plain text
<br />JULY 1988 <br /> <br />RAUBER ET AI.. <br /> <br />825 <br /> <br />" <br /> <br />locities will approach or exceed I m s- I due to the <br />small cross-sectional area of the crystals. <br />In the targeting calculations, the diffusional growth <br />rate and habit parameterizations appear to correpond <br />well with observations in the temperature region from <br />-80 to -l3OC where slow diffusional growth rates were <br />observed both experimentally and in the laboratory. <br />The riming parameterization, based on the Heymsfield <br />(1982) and Heymsfield and Pflaum (1985) model, pro- <br />duced particles with fall velocities within ranges esti- <br />mated by Heymsfield (1987) for temperatures < -80C <br />and typical supercooled water contents encountered in <br />SCPP experiments. However, at temperatures> -80C <br />sub-water saturated conditions within seeded regions <br />were found to limit growth so that particles retained <br />habits more similar to that observed between -70 and <br />-I20C. As a result, the parameterization overestimated <br />particle growth rate at temperatures near -60e. How- <br />ever, the fall velocity of these particles was actually <br />underestimated because the assumed particle habit <br />(needles) was incorrect, except in one case. Particles <br />with needlelike habits have reduced fall velocities due <br />to their larger cross-sectional area (see Fig. 5). <br /> <br />c. Radar echo evolution within seeded cloud regions <br /> <br />Storm systems over the Sierra Nevada generally <br />contain sufficient numbers of natural large ice particles <br />so that seeding effects cannot be discerned within nat- <br />ural background reflectivity measurements made with <br />radar. However, in some systems, particularly those <br />characterized by weak convective regions (Huggins and <br />Rodi 1985), seeding effects have been observed with <br />radar. In this section the 24 February 1984 storm sys- <br />tem is discussed. This storm is one of the three best <br />storms when meteorological conditions permitted <br />comparison of radar measurements within seeded re- <br />gions with calculated particle trajectories. The other <br />two storms where the radar has been used to observe <br />seeding effects are discusse:d by Deshler and Reynolds <br />( 1987). <br />On 24 February, the clouds had the appearance of <br />stratocumulus during most of the experimental period. <br />Some convective cloud tops extended above 3 km. <br />Seeding was done with CO2 pellets near cloud top. <br />Seeding effects, documented previously by Martner <br />(1986), were noted in the aircraft data downwind of <br />two seeding lines. <br />Optical array probe images showed the largest par- <br />ticles, at 12 min after seeding, were on the upwind edge <br />of the curtain, and consisted of 400-500 ~m graupel, <br />or rimed columns and needles of similar size. If these <br />particles were nucleated at the time of seeding they <br />grew at a rate of -0.6 ~m s- I. However, most particles <br />were smaller than 0.4 mm. The diffusionaljaccretional <br />model of Cooper and Lawson (1984) predicted that a <br />particle growing at -80C in I g m-31iquid could reach <br />500 ~m diameter in about 15 min and I mm in 24 <br />min. The particles detectt~d by the aircraft at compa- <br /> <br />I ' <br />,po' <br /> <br />rable times were smaller ( - 300 ~m at 15 min), prob- <br />ably due to liquid water content being less than 1 g <br />m-3 within the seeded ice particle curtain. <br />Using the techniques of Huggins and Rodi (1985), <br />the radar data for 24 February were examined for seed- <br />ing effects. Several clear examples of radar echoes de- <br />veloping downwind of seeding were found. The de- <br />velopment of these echoes closely resembled the radar <br />seeding signatures found by Huggins and Rodi in weak <br />convective clouds seeded with CO2. In one case, a radar <br />echo formed about 10 min downwind of a region <br />seeded at 2040:30 UTe. The first echo was detected <br />between 2.15 and 2.90 km (-50 to -100C) and had a <br />maximum intensity of 4 dBZ. <br />The trajectory calculations for a crystal initiated on <br />the seedline at -70C was compared to the seeded echo <br />development. The predicted fallout location was 33 <br />min downwind at a point 16 km from the seed point. <br />Using a value of 0.1 g m-3 for liquid water content, <br />the pre:dicted crystal was a rimed column that attained <br />a major axis of 1.04 mm. <br />A time-height analysis of maximum dBZ for the <br />seeded cell is shown in Fig. 10. The echo increased <br />slowly in intensity to 10 dBZ by 2059 UTC (to + 19). <br />This was in contrast to typical Huggins and Rodi (1985) <br />cases where aggregation resulted in a very rapid increase <br />of reflectivity after echo formation. The 10 dBZ con- <br />tour intercepted the surface at about 2113 UTC (to <br />+ 33). A plot of the predicted crystal trajectory on the <br />same figure shows similarity to the 10 dBZ contour <br />slope. The crystal was computed to impact the surface <br />at a point 33 min downwind of the SLCP, identical to <br />the radar echo 10 dBZ contour. The radar showed <br />weaker echo impacted the surface at 2108 UTC from <br />a region below the SLCP, possibly as low as 2.0 km <br /> <br />4.0 <br /> <br />3.0 <br /> <br /> <br />.s <br /> <br />.... 2.0 <br />:I: <br />'" <br />"' <br />:I: <br /> <br />1.0 <br /> <br />- <br /> <br />~n_- <br /> <br />0.0 <br />2040 <br />, <br />CI <br /> <br />I <br />2050 <br /> <br />I I I <br />2100 2110 2120 TIME (UTe) <br />I I I I <br />10 15 20 25 DISTANCE (km) <br /> <br />FIG. 10. Time-height development of radar echo (dBZ) in a seeded <br />cloud on 24 February 1984 compared to calculated trajectory (solid <br />line with arrows) for a rimed column initiated at -70C. Vertical <br />dashed line represents seeding curtain at to. <br />
The URL can be used to link to this page
Your browser does not support the video tag.