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Ping Chang
and
S. George Philander

Abstract

Recent observational studies have suggested that interactions between the atmosphere and the ocean play an important role in the pronounced annual cycle of the eastern equatorial Pacific and Atlantic Oceans. The key to this atmosphere–ocean interaction is a positive feedback between the surface winds and the local SST gradients in the cold tongue/ITCZ complex regions, which leads to an instability in the coupled system. By means of linear instability analyses and numerical model experiments, such an instability mechanism is explored in a simple coupled ocean-atmosphere system. The instability analysis yields a family of antisymmetric and symmetric unstable SST modes. The antisymmetric mode has the most rapid growth rate. The most unstable antisymmetric mode occurs at zero wavenumber and has zero frequency. The symmetric SST mode, although its growth rate is smaller, has a structure at annual period that appears to resemble the observed westward propagating feature in the annual cycle of near-equatorial zonal wind and SST. Unlike the ENSO type of coupled unstable modes, the modes of relevance to the seasonal cycle do not involve changes in the thermocline depth. The growth rates of these modes are linearly proportional to the mean vertical temperature gradient and inversely proportional to the depth of mean thermocline in the ocean. Because of the shallow thermocline and strong subsurface thermal gradients in the eastern Pacific and Atlantic Oceans, these coupled unstable modes strongly influence the seasonal cycles of those regions. On the basis of theoretical analyses and the observational evidence, it is suggested that the antisymmetric SST mode may be instrumental in rapidly reestablishing the cold tongues in the eastern Pacific and Atlantic Oceans during the Northern Hemisphere summer, whereas the symmetric SST mode contributes to the westward propagating feature in the annual cycle of near-equatorial zonal winds and SST.

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S. J. Ying
and
C. C. Chang

Abstract

A laboratory model of the tornado-like vortex near the ground is developed and studied. The circulation is produced by a rotating cylindrical screen and the updraft is produced by an exhaust fan at the opening of the top hood. By means of kerosene smoke, the vortex core and a reverse flow zone were observed in the experiment. The profiles of velocity and pressure were measured at three different circulation strengths. The maximum inward radial velocity in the boundary layer is approximately proportional to the circulation strength. Outside the vortex core, the top hood and ground boundary layers, the flow is a potential vortex flow with a very small inward radial velocity. The vertical velocity distribution generally has a Gaussian profile except that it is slightly downward in the annular reverse flow region. The diameter of the reverse flow region is controlled by the opening size of the outlet on the top hood. The reverse flow region extends to the top of the ground boundary layer only when the circulation is strong enough. The maximum downward flow speed observed in the experiments was less than 30 cm sec−1. A minimum pressure occurs at 1.27 cm from the ground on the vortex axis and shows the complexity of the flow in the conjunction region of the vortex core and the ground boundary layer.

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Sam S. Chang
and
Roscoe R. Braham Jr.

Abstract

Using aircraft data collected during the University of Chicago Lake-Effect Snow Storm project, the results of a case study of the convective thermal internal boundary layer (TIBL) over Lake Michigan are presented. An intense cold air outbreak on 20 January 1984 featured a rapid growth of the convective TIBL thickness and the concurrent development of cloud and snow. The average slope of the TIBL top over a fetch of 123.7 km was 1.0%. Microphysical characteristics of cloud and snow along with the TIBL development are also presented. Results of the TIBL integrated budgets of heat and total water (including cloud and snow water) are given in detail. Over the surface of Lake Michigan the average downward snow flux (snow precipitation rate) was 0.79 mm (water) per day. The average sensible and latent heat fluxes at the water surface were 323 and 248 W m−2, respectively. About 13 percent of the total warming of this cloud-topped TIBL was due to radiation.

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William W. Hager
,
John S. Nisbet
,
John R. Kasha
, and
Wei-Chang Shann

Abstract

Numerical simulations based on a three-dimensional model for the electric fields in a thunderstorm are presented. In some of the simulations, we solve problems with known analytical solutions in order to determine the relevant physical properties that must be incorporated in a thunderstorm model. We then examine the inverse problem: Given measurements for the electric fields in a thunderstorm what are the associated current generators? Fits based on an analytic formula that neglects conduction currents give approximations to the current generators while simulations based on the thunderstorm model yield refinements to the generators. As a specific illustration, we obtain estimates for current generators associated with a storm observed at the Kennedy Space Center on 11 July 1978. Finally, we explore qualitative properties of our method used to simulate lightning. It is observed that as the charged particles associated with the thunderstorm are spread over a larger and larger volume, the flesh rate decreases while the charge transfer associated with each flash increases. Also, it is seen that a series of intracloud flashes can produce a charge imbalance in the cloud that will eventually lead to a cloud-to-ground discharge.

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J. A. Coakley Jr.
,
P. A. Durkee
,
K. Nielsen
,
J. P. Taylor
,
S. Platnick
,
B. A. Albrecht
,
D. Babb
,
F.-L. Chang
,
W. R. Tahnk
,
C. S. Bretherton
, and
P. V. Hobbs

Abstract

The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon, NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes, visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why the majority of ship tracks observed from satellites off the coast of California are found south of 35°N. A small rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when low-level clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.

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Steven P. Oncley
,
Carl A. Friehe
,
John C. Larue
,
Joost A. Businger
,
Eric C. Itsweire
, and
Sam S. Chang

Abstract

An atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Kármán constant. New instruments were constructed to minimize flow distortion effects on the turbulence measurements and to provide high-resolution gradient measurements. In addition, a hot-wire anemometer directly measured the turbulent kinetic energy dissipation rate.

An average value of the von Kármán constant of 0.365 ± 0.015 was obtained from 91 runs (31 h) in near-neutral stability conditions. However, four near-neutral runs when snow covered the ground gave an average value of 0.42. This result suggests that the von Kármán constant depends on the roughness Reynolds number, which may resolve some of the differences in previous determinations over different surfaces. The one-dimensional Kolmogorov inertial subrange constant was found to have a value of 0.54 ± 0.03, slightly larger than previous results.

The flux-profile relations for momentum and temperature variance were evaluated, and humidity variance data behaved similarly to temperature. Dissipation of turbulent kinetic energy was found to be less than production under near-neutral conditions, which suggests that turbulent or pressure transport may be significant.

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E. A. Smith
,
J. E. Lamm
,
R. Adler
,
J. Alishouse
,
K. Aonashi
,
E. Barrett
,
P. Bauer
,
W. Berg
,
A. Chang
,
R. Ferraro
,
J. Ferriday
,
S. Goodman
,
N. Grody
,
C. Kidd
,
D. Kniveton
,
C. Kummerow
,
G. Liu
,
F. Marzano
,
A. Mugnai
,
W. Olson
,
G. Petty
,
A. Shibata
,
R. Spencer
,
F. Wentz
,
T. Wilheit
, and
E. Zipser

Abstract

The second WetNet Precipitation Intercomparison Project (PIP-2) evaluates the performance of 20 satellite precipitation retrieval algorithms, implemented for application with Special Sensor Microwave/Imager (SSM/I) passive microwave (PMW) measurements and run for a set of rainfall case studies at full resolution–instantaneous space–timescales. The cases are drawn from over the globe during all seasons, for a period of 7 yr, over a 60°N–17°S latitude range. Ground-based data were used for the intercomparisons, principally based on radar measurements but also including rain gauge measurements. The goals of PIP-2 are 1) to improve performance and accuracy of different SSM/I algorithms at full resolution–instantaneous scales by seeking a better understanding of the relationship between microphysical signatures in the PMW measurements and physical laws employed in the algorithms; 2) to evaluate the pros and cons of individual algorithms and their subsystems in order to seek optimal “front-end” combined algorithms; and 3) to demonstrate that PMW algorithms generate acceptable instantaneous rain estimates.

It is found that the bias uncertainty of many current PMW algorithms is on the order of ±30%. This level is below that of the radar and rain gauge data specially collected for the study, so that it is not possible to objectively select a best algorithm based on the ground data validation approach. By decomposing the intercomparisons into effects due to rain detection (screening) and effects due to brightness temperature–rain rate conversion, differences among the algorithms are partitioned by rain area and rain intensity. For ocean, the screening differences mainly affect the light rain rates, which do not contribute significantly to area-averaged rain rates. The major sources of differences in mean rain rates between individual algorithms stem from differences in how intense rain rates are calculated and the maximum rain rate allowed by a given algorithm. The general method of solution is not necessarily the determining factor in creating systematic rain-rate differences among groups of algorithms, as we find that the severity of the screen is the dominant factor in producing systematic group differences among land algorithms, while the input channel selection is the dominant factor in producing systematic group differences among ocean algorithms. The significance of these issues are examined through what is called “fan map” analysis.

The paper concludes with a discussion on the role of intercomparison projects in seeking improvements to algorithms, and a suggestion on why moving beyond the “ground truth” validation approach by use of a calibration-quality forward model would be a step forward in seeking objective evaluation of individual algorithm performance and optimal algorithm design.

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Weiqing Qu
,
A. Henderson-Sellers
,
A. J. Pitman
,
T. H. Chen
,
F. Abramopoulos
,
A. Boone
,
S. Chang
,
F. Chen
,
Y. Dai
,
R. E. Dickinson
,
L. Dümenil
,
M. Ek
,
N. Gedney
,
Y. M. Gusev
,
J. Kim
,
R. Koster
,
E. A. Kowalczyk
,
J. Lean
,
D. Lettenmaier
,
X. Liang
,
J.-F. Mahfouf
,
H.-T. Mengelkamp
,
K. Mitchell
,
O. N. Nasonova
,
J. Noilhan
,
A. Robock
,
C. Rosenzweig
,
J. Schaake
,
C. A. Schlosser
,
J.-P. Schulz
,
A. B. Shmakin
,
D. L. Verseghy
,
P. Wetzel
,
E. F. Wood
,
Z.-L. Yang
, and
Q. Zeng

Abstract

In the PILPS Phase 2a experiment, 23 land-surface schemes were compared in an off-line control experiment using observed meteorological data from Cabauw, the Netherlands. Two simple sensitivity experiments were also undertaken in which the observed surface air temperature was artificially increased or decreased by 2 K while all other factors remained as observed. On the annual timescale, all schemes show similar responses to these perturbations in latent, sensible heat flux, and other key variables. For the 2-K increase in temperature, surface temperatures and latent heat fluxes all increase while net radiation, sensible heat fluxes, and soil moistures all decrease. The results are reversed for a 2-K temperature decrease. The changes in sensible heat fluxes and, especially, the changes in the latent heat fluxes are not linearly related to the change of temperature. Theoretically, the nonlinear relationship between air temperature and the latent heat flux is evident and due to the convex relationship between air temperature and saturation vapor pressure. A simple test shows that, the effect of the change of air temperature on the atmospheric stratification aside, this nonlinear relationship is shown in the form that the increase of the latent heat flux for a 2-K temperature increase is larger than its decrease for a 2-K temperature decrease. However, the results from the Cabauw sensitivity experiments show that the increase of the latent heat flux in the +2-K experiment is smaller than the decrease of the latent heat flux in the −2-K experiment (we refer to this as the asymmetry). The analysis in this paper shows that this inconsistency between the theoretical relationship and the Cabauw sensitivity experiments results (or the asymmetry) is due to (i) the involvement of the β g formulation, which is a function of a series stress factors that limited the evaporation and whose values change in the ±2-K experiments, leading to strong modifications of the latent heat flux; (ii) the change of the drag coefficient induced by the changes in stratification due to the imposed air temperature changes (±2 K) in parameterizations of latent heat flux common in current land-surface schemes. Among all stress factors involved in the β g formulation, the soil moisture stress in the +2-K experiment induced by the increased evaporation is the main factor that contributes to the asymmetry.

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