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  • Author or Editor: G. D. Emmitt x
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W. M. Frank
,
G. D. Emmitt
, and
C. Warner

Abstract

Data from a wide variety of measurement platforms are integrated to analyze a GATE cloud cluster and its environment. Lower tropospheric mass and moisture fluxes are computed on several scales using rawinsonde, aircraft, radar and satellite data from 18 September 1974. Lower tropospheric mass and moisture budget analyses are combined with GATE tethered-balloon measurements of cloud properties at cloud base to diagnose active cloud and turbulent fluxes in three areas of different scale ranging from 2.3-34.8 × 1010 m 2 (1.8-28.2° latitude)2.

The significant convection was concentrated in the region of strongest and deepest low-level convergence and highest conditional instability. Observed layer clouds were well correlated with levels of synoptic-scale divergence. Despite large differences in low-level convergence and echo-area coverage, all three regions exhibited similar vertical eddy moisture fluxes at cloud base. Increased area coverage by precipitating clouds was found to be associated with increased subsidence and reduced turbulent moisture flux in the air outside of these clouds.

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L. P. Riishøjgaard
,
R. Atlas
, and
G. D. Emmitt

Abstract

Through the use of observation operators, modern data assimilation systems have the capability to ingest observations of quantities that are not themselves model variables but are mathematically related to those variables. An example of this is the so-called line-of-sight (LOS) winds that a spaceborne Doppler wind lidar (DWL) instrument would provide. The model or data assimilation system ideally would need information about both components of the horizontal wind vectors, whereas the observations in this case would provide only the projection of the wind vector onto a given direction. The estimated or analyzed value is then calculated essentially as a weighted average of the observation itself and the model-simulated value of the observed quantity. To assess the expected impact of a DWL, it is important to examine the extent to which a meteorological analysis can be constrained by the LOS winds. The answer to this question depends on the fundamental character of the atmospheric flow fields that are analyzed, but, just as important, it also depends on the real and assumed error covariance characteristics of these fields. A single-level wind analysis system designed to explore these issues has been built at the NASA Data Assimilation Office. In this system, simulated wind observations can be evaluated in terms of their impact on the analysis quality under various assumptions about their spatial distribution and error characteristics and about the error covariance of the background fields. The basic design of the system and experimental results obtained with it are presented. The experiments were designed to illustrate how such a system may be used in the instrument concept definition phase.

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S. Pal
,
S. F. J. De Wekker
, and
G. D. Emmitt

Abstract

Spatiotemporal variability in the convective boundary layer height z i over complex terrain is governed by numerous factors such as land surface processes, topography, and synoptic conditions. Observational datasets to evaluate weather forecast models that simulate this variability are sparse. This study aims to investigate the z i spatial variability (along a total leg length of 1800 km) around and over a steep isolated mountain (Granite Mountain) of horizontal and vertical dimensions of 8 and 0.9 km, respectively. An airborne Doppler lidar was deployed on seven flights during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) campaign conducted at Dugway Proving Ground (Utah) from 25 September to 24 October 2012. During the afternoon, an east–west z i gradient over the region with z i that was approximately 200 m higher on the eastern side than on the western side of Granite Mountain was observed. This gradient illustrates the impact of two different land surface properties on z i spatial variability, with a sparsely vegetated desert steppe region on the east and a dry, bare lake-bed desert with high subsurface soil moisture to the west of Granite Mountain. Additionally, the z i spatial variability was partly attributed to the impact of Granite Mountain on the downwind z i . Differences in z i were also observed by the radiosonde measurements in the afternoon but not in the morning as the z i variability in morning were modulated by the topography. The high-resolution lidar-derived z i measurements were used to estimate the entrainment zone thickness in the afternoon, with estimates ranging from 100 to 250 m.

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S. F. J. De Wekker
,
K. S. Godwin
,
G. D. Emmitt
, and
S. Greco

Abstract

Three-dimensional winds obtained with an airborne Doppler lidar are used to investigate the spatial structure of topographically driven flows in complex coastal terrain in Southern California. The airborne Doppler lidar collected four hours of data between the surface and 3000 m MSL along a 40-km segment of the Salinas Valley during the afternoon of 12 November 2007. The airborne lidar measurements, obtained at horizontal and vertical resolutions of approximately 1500 and 50 m, respectively, reveal a detailed spatial structure of the atmospheric flows within the valley and their associated aerosol features. Clear skies prevailed on the flight day with northwesterly synoptic flows around 10 m s−1. The data document a shallow sea breeze making a transition into an upvalley flow in the Salinas Valley that accelerates in the upvalley direction. Along with the acceleration of the upvalley wind, the lidar data indicate the presence of enhanced sinking motions. No return flows associated with the sea-breeze or upvalley flows are observed. While synoptic flows are aligned along the valley axis in the upvalley direction, lidar data indicate the presence of a northerly cross-valley flow around the height of the surrounding ridges. This flow intrudes into the valley atmosphere and induces, along with thermally driven slope flows on the sunlit valley sidewall, a cross-valley circulation that causes an asymmetric distribution of the aerosols. This study demonstrates the large potential of airborne Doppler lidar data in describing flows in complex terrain.

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K. S. Godwin
,
S. F. J. De Wekker
, and
G. D. Emmitt

Abstract

Airborne Doppler wind lidars are increasingly being used to measure winds in the lower atmosphere at higher spatial resolution than ever before. However, wind retrieval in the range gates closest to the earth’s surface remains problematic. When a laser beam from a nadir-pointing airborne Doppler wind lidar intercepts the ground, the return signal from the ground mixes with the windblown aerosol signal. As a result, winds in a layer adjacent to the surface are often unreliable and removed from wind profiles. This paper describes the problem in detail and discusses a two-step approach to improve near-surface wind retrievals. The two-step approach involves removing high-intensity ground returns and identifying and tracking aerosol radial velocities in the layer affected by ground interference. Using this approach, it is shown that additional range gates closer to the surface can be obtained, thereby further enhancing the potential of airborne Doppler lidar in atmospheric applications. The benefits of the two-step approach are demonstrated using measurements acquired over the Salinas Valley in central California. The additional range gates reveal details of the wind field that were previously not quantified with the original approach, such as a pronounced near-surface wind speed maximum.

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H. J. S. Fernando
,
E. R. Pardyjak
,
S. Di Sabatino
,
F. K. Chow
,
S. F. J. De Wekker
,
S. W. Hoch
,
J. Hacker
,
J. C. Pace
,
T. Pratt
,
Z. Pu
,
W. J. Steenburgh
,
C. D. Whiteman
,
Y. Wang
,
D. Zajic
,
B. Balsley
,
R. Dimitrova
,
G. D. Emmitt
,
C. W. Higgins
,
J. C. R. Hunt
,
J. C. Knievel
,
D. Lawrence
,
Y. Liu
,
D. F. Nadeau
,
E. Kit
,
B. W. Blomquist
,
P. Conry
,
R. S. Coppersmith
,
E. Creegan
,
M. Felton
,
A. Grachev
,
N. Gunawardena
,
C. Hang
,
C. M. Hocut
,
G. Huynh
,
M. E. Jeglum
,
D. Jensen
,
V. Kulandaivelu
,
M. Lehner
,
L. S. Leo
,
D. Liberzon
,
J. D. Massey
,
K. McEnerney
,
S. Pal
,
T. Price
,
M. Sghiatti
,
Z. Silver
,
M. Thompson
,
H. Zhang
, and
T. Zsedrovits

Abstract

Emerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort—the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program—that draws the expertise of a multidisciplinary, multi-institutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms, has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent submesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting.

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Wayman E. Baker
,
Robert Atlas
,
Carla Cardinali
,
Amy Clement
,
George D. Emmitt
,
Bruce M. Gentry
,
R. Michael Hardesty
,
Erland Källén
,
Michael J. Kavaya
,
Rolf Langland
,
Zaizhong Ma
,
Michiko Masutani
,
Will McCarty
,
R. Bradley Pierce
,
Zhaoxia Pu
,
Lars Peter Riishojgaard
,
James Ryan
,
Sara Tucker
,
Martin Weissmann
, and
James G. Yoe

The three-dimensional global wind field is the most important remaining measurement needed to accurately assess the dynamics of the atmosphere. Wind information in the tropics, high latitudes, and stratosphere is particularly deficient. Furthermore, only a small fraction of the atmosphere is sampled in terms of wind profiles. This limits our ability to optimally specify initial conditions for numerical weather prediction (NWP) models and our understanding of several key climate change issues.

Because of its extensive wind measurement heritage (since 1968) and especially the rapid recent technology advances, Doppler lidar has reached a level of maturity required for a space-based mission. The European Space Agency (ESA)'s Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) Doppler wind lidar (DWL), now scheduled for launch in 2015, will be a major milestone.

This paper reviews the expected impact of DWL measurements on NWP and climate research, measurement concepts, and the recent advances in technology that will set the stage for space-based deployment. Forecast impact experiments with actual airborne DWL measurements collected over the North Atlantic in 2003 and assimilated into the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model are a clear indication of the value of lidar-measured wind profiles. Airborne DWL measurements collected over the western Pacific in 2008 and assimilated into both the ECMWF and U.S. Navy operational models support the earlier findings.

These forecast impact experiments confirm observing system simulation experiments (OSSEs) conducted over the past 25–30 years. The addition of simulated DWL wind observations in recent OSSEs performed at the Joint Center for Satellite Data Assimilation (JCSDA) leads to a statistically significant increase in forecast skill.

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