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Richard E. Danielson, John R. Gyakum, and David N. Straub

Abstract

The impact of eddy energy growth and radiation from a western North Pacific cyclone on the intensity of an eastern North Pacific cyclone a few days later is examined. Associated with the western cyclone is an upstream ridge and trough couplet, initially over Siberia on 8 March 1977. The amplitude of this couplet is perturbed in 5-day numerical simulations of the two marine cyclones. Balanced initial conditions are created by potential vorticity inversion. The magnitude of the upper-level couplet governs much of the subsequent growth of eddy energy in the western cyclone as well as the propagation of eddy energy between the two cyclones. This culminates in measurable changes in the maximum intensity of the eastern surface cyclone. The broader question of the sensitivity of this cyclone to upstream perturbations is also briefly addressed.

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Richard E. Danielson, John R. Gyakum, and David N. Straub

Abstract

The sequential development of a western, and then an eastern, North Pacific cyclone is examined in terms of eddy energy and a phase-independent wave activity. Based on the propagation of both a contiguous wave activity center and eddy energy, the development of the western cyclone appears to influence its downstream neighbor. A quantitative comparison of these two diagnoses is made in terms of group velocity, and only minor differences are found during much of the initial evolution. It is only once the tropopause undulations lose their wavelike appearance (at which point, application of the group-velocity concept itself becomes quite tenuous) that the downstream propagation of eddy energy seems faster than that of wave activity. Conventional methods of tracking this wave packet are also briefly discussed.

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David W. Reynolds, Thomas B. McKee, and Kelvin S. Danielson

Abstract

The relationship between a cumulus clouds' brightness, horizontal dimension and internal microphysical structure are investigated. Cumulus clouds located over the South Park region of Colorado are observed by the SMS-2 satellite and their brightness and size are determined. Aircraft observations were made in-cloud to obtain the drop size distributions and liquid water content (LWC) of the cloud. A Monte Carlo cloud model is used to imitate the sun-satellite-cloud geometry in an effort to understand the role of cloud size and microphysical structure in affecting cloud brightness.

Results show that for clouds of optical thickness between 20 and 60 (i.e., LWC of 0.037 gm−3 and 0.11 gm−3 for a 2 km deep cloud), information about a cloud's LWC may be obtained through monitoring cloud brightness for clouds of uniform depth and variable width. Theoretical results using this Monte Carlo method approximate very closely the relative brightness changes of clouds of the size and depth monitored by the SMS-2 satellite for these few days. Theory and observation both conclude that a cloud having a width to depth ratio of approximately 10:1 (and constant optical thickness) is nearly reaching its maximum brightness. Theory predicts that geometric factors affect cloud brightness more than microphysical changes.

It is also discussed that the previously reported work on the cloud height-cloud brightness relationship may indeed be seeing increasing brightness with increasing horizontal size changes (size being related to height) with finite small perturbations on top of the growing cloud slowing its approach to maximum brightness.

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Steven Marcus, Jinwon Kim, Toshio Chin, David Danielson, and Jayme Laber

Abstract

The effects of precipitable water vapor (PWV) retrievals from the Southern California Integrated GPS Network (SCIGN) on quantitative precipitation forecast (QPF) skill are examined over two flood-prone regions of Southern California: Santa Barbara (SB) and Ventura County (VC). Two sets of QPFs are made, one using the initial water vapor field from the NCEP 40-km Eta initial analysis, and another in which the initial Eta water vapor field is modified by incorporating the PWV data from the SCIGN receivers. Lateral boundary data for the QPFs, as well as the hydrostatic component of the GPS zenith delay data, are estimated from the Eta analysis. Case studies of a winter storm on 2 February during the 1997/98 El Niño, and storms leading up to the La Conchita, California, landslide on 10 January 2005, show notably improved QPFs for the first 3–6 h with the addition of GPS PWV data. For a total of 47 winter storm forecasts between February 1998 and January 2005 the average absolute QPF improvement is small; however, QPF improvements exceed 5 mm in several underpredicted rainfall events, with GPS data also improving most cases with overpredicted rainfall. The GPS improvements are most significant (above or near the 2σ level) when the low-level winds off the coast of Southern California are from the southern (SW to SE) quadrant. To extend the useful forecast skill enhancement beyond six hours, however, additional sources of water vapor data over broader areas of the adjacent Pacific Ocean are needed.

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