3 resultados para Hydrodynamic weather forecasting.
em CaltechTHESIS
Resumo:
Arid and semiarid landscapes comprise nearly a third of the Earth's total land surface. These areas are coming under increasing land use pressures. Despite their low productivity these lands are not barren. Rather, they consist of fragile ecosystems vulnerable to anthropogenic disturbance.
The purpose of this thesis is threefold: (I) to develop and test a process model of wind-driven desertification, (II) to evaluate next-generation process-relevant remote monitoring strategies for use in arid and semiarid regions, and (III) to identify elements for effective management of the world's drylands.
In developing the process model of wind-driven desertification in arid and semiarid lands, field, remote sensing, and modeling observations from a degraded Mojave Desert shrubland are used. This model focuses on aeolian removal and transport of dust, sand, and litter as the primary mechanisms of degradation: killing plants by burial and abrasion, interrupting natural processes of nutrient accumulation, and allowing the loss of soil resources by abiotic transport. This model is tested in field sampling experiments at two sites and is extended by Fourier Transform and geostatistical analysis of high-resolution imagery from one site.
Next, the use of hyperspectral remote sensing data is evaluated as a substantive input to dryland remote monitoring strategies. In particular, the efficacy of spectral mixture analysis (SMA) in discriminating vegetation and soil types and detennining vegetation cover is investigated. The results indicate that hyperspectral data may be less useful than often thought in determining vegetation parameters. Its usefulness in determining soil parameters, however, may be leveraged by developing simple multispectral classification tools that can be used to monitor desertification.
Finally, the elements required for effective monitoring and management of arid and semiarid lands are discussed. Several large-scale multi-site field experiments are proposed to clarify the role of wind as a landscape and degradation process in dry lands. The role of remote sensing in monitoring the world's drylands is discussed in terms of optimal remote sensing platform characteristics and surface phenomena which may be monitored in order to identify areas at risk of desertification. A desertification indicator is proposed that unifies consideration of environmental and human variables.
Resumo:
The Madden-Julian Oscillation (MJO) is a pattern of intense rainfall and associated planetary-scale circulations in the tropical atmosphere, with a recurrence interval of 30-90 days. Although the MJO was first discovered 40 years ago, it is still a challenge to simulate the MJO in general circulation models (GCMs), and even with simple models it is difficult to agree on the basic mechanisms. This deficiency is mainly due to our poor understanding of moist convection—deep cumulus clouds and thunderstorms, which occur at scales that are smaller than the resolution elements of the GCMs. Moist convection is the most important mechanism for transporting energy from the ocean to the atmosphere. Success in simulating the MJO will improve our understanding of moist convection and thereby improve weather and climate forecasting.
We address this fundamental subject by analyzing observational datasets, constructing a hierarchy of numerical models, and developing theories. Parameters of the models are taken from observation, and the simulated MJO fits the data without further adjustments. The major findings include: 1) the MJO may be an ensemble of convection events linked together by small-scale high-frequency inertia-gravity waves; 2) the eastward propagation of the MJO is determined by the difference between the eastward and westward phase speeds of the waves; 3) the planetary scale of the MJO is the length over which temperature anomalies can be effectively smoothed by gravity waves; 4) the strength of the MJO increases with the typical strength of convection, which increases in a warming climate; 5) the horizontal scale of the MJO increases with the spatial frequency of convection; and 6) triggered convection, where potential energy accumulates until a threshold is reached, is important in simulating the MJO. Our findings challenge previous paradigms, which consider the MJO as a large-scale mode, and point to ways for improving the climate models.
Resumo:
I. Foehn winds of southern California.
An investigation of the hot, dry and dust laden winds
occurring in the late fall and early winter in the Los Angeles
Basin and attributed in the past to the influences of the desert
regions to the north revealed that these currents were of a
foehn nature. Their properties were found to be entirely due
to dynamical heating produced in the descent from the high level
areas in the interior to the lower Los Angeles Basin. Any dust
associated with the phenomenon was found to be acquired from the
Los Angeles area rather than transported from the desert. It was
found that the frequency of occurrence of a mild type foehn of this
nature during this season was sufficient to warrant its classification
as a winter monsoon. This results from the topography of
the Los Angeles region which allows an easy entrance to the air
from the interior by virtue of the low level mountain passes north
of the area. This monsoon provides the mild winter climate of
southern California since temperatures associated with the foehn
currents are far higher than those experienced when maritime air
from the adjacent Pacific Ocean occupies the region.
II. Foehn wind cyclo-genesis.
Intense anticyclones frequently build up over the high level
regions of the Great Basin and Columbia Plateau which lie between
the Sierra Nevada and Cascade Mountains to the west and the Rocky
Mountains to the east. The outflow from these anticyclones produce
extensive foehns east of the Rockies in the comparatively low
level areas of the middle west and the Canadian provinces of
Alberta and Saskatchewan. Normally at this season of the year very
cold polar continental air masses are present over this territory
and with the occurrence of these foehns marked discontinuity surfaces
arise between the warm foehn current, which is obliged to slide over
a colder mass, and the Pc air to the east. Cyclones are
easily produced from this phenomenon and take the form of unstable
waves which propagate along the discontinuity surface between the
two dissimilar masses. A continual series of such cyclones was
found to occur as long as the Great Basin anticyclone is maintained
with undiminished intensity.
III. Weather conditions associated with the Akron disaster.
This situation illustrates the speedy development and
propagation of young disturbances in the eastern United States
during the spring of the year under the influence of the conditionally
unstable tropical maritime air masses which characterise the
region. It also furnishes an excellent example of the superiority
of air mass and frontal methods of weather prediction for aircraft
operation over the older methods based upon pressure distribution.
IV. The Los Angeles storm of December 30, 1933 to January 1, 1934.
This discussion points out some of the fundamental interactions
occurring between air masses of the North Pacific Ocean in connection
with Pacific Coast storms and the value of topographic and
aerological considerations in predicting them. Estimates of rainfall
intensity and duration from analyses of this type may be made and
would prove very valuable in the Los Angeles area in connection with
flood control problems.