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Albert D. Anderson
and
Henry J. Mastenbrook

A new concept of upper-air data collection utilizes instrumented balloons controlled to float along given constant-pressure surfaces in the atmosphere. A system of instrumentation, named the transosonde (trans-oceanic-sonde) has been developed for implementing this concept. Field tests have established the technical and meteorological feasibility of the system. In the course of the tests, transosonde balloons were tracked over distances of thousands of miles using a network of shore-based high-frequency radio-direction-finder stations. Emphasis has been placed upon the trajectory of the balloon as the primary source of meteorological data. Wind velocities and accelerations can be derived directly from constant-pressure surface trajectories, providing valuable synoptic and research data. Balloon trajectories in passing through major troughs and ridges define these features, giving information of importance for synoptic analysis and long-range forecasting. In addition, a sequence of trajectories provides a measure of the acceleration and deceleration of these entities. The transosonde system has additional data-gathering potentials for temperature, lapse rate, wind shear and other parameters. It is concluded that the system can be employed over those regions of the globe where upper-air data are lacking at a cost competitive with present-day systems.

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N. Žagar
,
J. Tribbia
,
J. L. Anderson
, and
K. Raeder

Abstract

This paper quantifies the linear mass–wind field balance and its temporal variability in the global data assimilation system Data Assimilation Research Testbed/Community Atmosphere Model (DART/CAM), which is based on the ensemble adjustment Kalman filter. The part of the model state that projects onto quasigeostrophic modes represents the balanced state. The unbalanced part corresponds to inertio-gravity (IG) motions. The 80-member ensemble is diagnosed by using the normal-mode function expansion. It was found that the balanced variance in the prior ensemble is on average about 90% of the total variance and about 80% of the wave variance. Balance depends on the scale and the largest zonal scales are best balanced. For zonal wavenumbers greater than k = 30 the balanced variance stays at about the 45% level. There is more variance in the westward- than in the eastward-propagating IG modes; the difference is about 2% of the total wave variance and it is associated with the covariance inflation. The applied inflation field has a major impact on the structure of the prior variance field and its reduction by the assimilation step. The shape of the inflation field mimics the global radiosonde observation network (k = 2), which is associated with the minimum variance reduction in k = 2. Temporal variability of the ensemble variance is significant and appears to be associated with changes in the energy of the flow. A perfect-model assimilation experiment supports the findings from the real-observation experiment.

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N. Žagar
,
J. Tribbia
,
J. L. Anderson
, and
K. Raeder

Abstract

This paper presents the application of the normal-mode functions to diagnose the atmospheric energy spectra in terms of balanced and inertia–gravity (IG) contributions. A set of three-dimensional orthogonal normal modes is applied to four analysis datasets from July 2007. The datasets are the operational analysis systems of NCEP and ECMWF, the NCEP–NCAR reanalyses, and the Data Assimilation Research Testbed–Community Atmospheric Model (DART–CAM), an ensemble analysis system developed at NCAR. The differences between the datasets can be considered as a measure of uncertainty of the IG contribution to the global energetics.

The results show that the percentage of IG motion in the present NCEP, ECMWF, and DART–CAM analysis systems is between 1% and 2% of the total energy field. In the wave part of the flow (zonal wavenumber k ≠ 0), the IG energy contribution is between 9% and 15%. On the contrary, the NCEP–NCAR reanalyses contain more IG motion, especially in the Southern Hemisphere extratropics. Each analysis contains more energy in the eastward IG motion than in its westward counterpart. The difference is about 2%–3% of the total wave energy and it is associated with the motions projected onto the Kelvin wave in the tropics.

The selected truncation parameters of the expansion (zonal, meridional, and vertical truncation) ensure that the projection provides the optimal fit to the input data on model levels. This approach is different from previous applications of the normal modes and under the linearity assumption it allows the application of the inverse projection to obtain details of circulation associated with a selected type of motion. The bulk of the IG motion is confined to the tropics. For the successful reproduction of three-dimensional circulations by the normal modes it is important that the expansion includes a number of vertical modes.

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N. Žagar
,
J. Tribbia
,
J. L. Anderson
, and
K. Raeder

Abstract

This paper analyzes the spectra and spatiotemporal features of the large-scale inertia-gravity (IG) circulations in four analysis systems in the tropics. Of special interest is the Kelvin wave (KW), which represents between 7% and 25% of the total IG wave (zonal wavenumber k ≠ 0) energy. The mixed Rossby–gravity (MRG) mode comprises between 4% and 15% of the IG wave energy. At the longest scales, the KW spectra are fitted by a law while the MRG energy spectrum appears flat. At shorter scales both modes follow a −3 law. Energy spectra of the total IG wave motion at long zonal scales (zonal wavenumber smaller than 7) have slopes close to −1.

The average circulation associated with KW is characterized by reverse flows in the upper and lower troposphere consistent with the ideas behind simple tropical models. The inverse projection is used to quantify the role of Kelvin and MRG waves in current analysis systems in the upper troposphere over the Indian Ocean. At these levels, easterlies between 10°S and 30°N are represented by the KW to a significant degree while the cross-equatorial flow toward the descending branch of the Hadley cell at 10°S is associated with the MRG waves.

The transient structure of equatorial waves is presented in the space of normal modes defined by the zonal wavenumbers, meridional Hough functions, and the vertical eigenfunctions. The difference in the depth of the model domain in DART–CAM and NCEP–NCAR on one hand and ECMWF and NCEP on the other appears to be one reason for different wave propagation properties. In the latter case the vertical energy propagation is diagnosed by filtering the propagating KW modes back to physical space. The results agree with the linear theory of vertically propagating equatorial waves.

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N. Žagar
,
J. Tribbia
,
J. L. Anderson
, and
K. Raeder
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Allen J. Riordan
,
J. Thomas Anderson
, and
S. Chiswell

Abstract

The analysis of the rainband structure and wind fields associated with a coastal front along the North Carolina shoreline is described. Dual-Doppler radar and the augmented GALE (Genesis of Atlantic Lows Experiment) ensemble of in situ stations depict shallow, convective rainbands that overtake the front from the warm-air sector and intensify at the surface front location. Clockwise band rotation is shown to be caused by the difference in alignment between the echo motion and the rainband axes and by new development ahead of the front.

Radar measurements depict the circulation systems associated with a portion of one rainband in the cold air ahead of the front. Here shallow precipitation cores are vertically tilted due to the frontal wind shear. Circulation cells and most precipitation cores are centered just above the frontal inversion, as inferred by the wind shift line aloft. This feature is nearly horizontal in the cross-frontal direction but slopes downward in a direction roughly parallel to the front.

Ahead of the front, main updrafts in and above the cold air are found near the upwind portion of precipitation cores and along two well-defined lines aligned roughly perpendicular to the front. These lines propagate northward and affect several nearby surface sites prior to frontal passage. The speed of northward propagation is consistent with gravity wave theory, while on the larger scale the front appears to behave as the leading edge of a density current. The major features found in this case are compared and contrasted with those of a synoptic-scale warm front.

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J. E. Kay
,
K. Raeder
,
A. Gettelman
, and
J. Anderson

Abstract

This study documents and evaluates the boundary layer and energy budget response to record low 2007 sea ice extents in the Community Atmosphere Model version 4 (CAM4) using 1-day observationally constrained forecasts and 10-yr runs with a freely evolving atmosphere. While near-surface temperature and humidity are minimally affected by sea ice loss in July 2007 forecasts, near-surface stability decreases and atmospheric humidity increases aloft over newly open water in September 2007 forecasts. Ubiquitous low cloud increases over the newly ice-free Arctic Ocean are found in both the July 2007 and the September 2007 forecasts. In response to the 2007 sea ice loss, net surface [top of the atmosphere (TOA)] energy budgets change by +19.4 W m−2 (+21.0 W m−2) and −17.9 W m−2 (+1.4 W m−2) in the July 2007 and September 2007 forecasts, respectively. While many aspects of the forecasted response to sea ice loss are consistent with physical expectations and available observations, CAM4’s ubiquitous July 2007 cloud increases over newly open water are not. The unrealistic cloud response results from the global application of parameterization designed to diagnose stratus clouds based on lower-tropospheric stability (CLDST). In the Arctic, the well-mixed boundary layer assumption implicit in CLDST is violated. Requiring a well-mixed boundary layer to diagnose stratus clouds improves the CAM4 cloud response to sea ice loss and increases July 2007 surface (TOA) energy budgets over newly open water by +11 W m−2 (+14.9 W m−2). Of importance to high-latitude climate feedbacks, unrealistic stratus cloud compensation for sea ice loss occurs only when stable and dry atmospheric conditions exist. Therefore, coupled climate projections that use CAM4 will underpredict Arctic sea ice loss only when dry and stable summer conditions occur.

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Chad J. Daniel
,
Raymond W. Arritt
, and
Christopher J. Anderson

Abstract

The authors have evaluated the performance of operational hourly data from a NOAA Wind Profiler Network 404-MHz radar profiler for detecting low-level jet (LLJ) events in the central United States. Independent, collocated rawinsonde and radar profiler data were time matched, producing 2614 paired observations over a 2-yr period. These observations were used to determine the impacts of the height of the first profiler range gate (500 m) and contamination of the hourly data by migrating birds on the ability of the profiler to accurately diagnose LLJ events. The profilers tend to underrepresent both the strength and frequency of occurrence of the LLJ. It was found that about 50% of LLJ events with wind speed maxima below 500 m were detected, increasing to 70%–80% for events having their wind speed maxima above 500 m. To reduce contamination by migrating birds when using profilers to detect the LLJ, a second-moment filtering technique with a threshold of approximately 2–2.5 m2 s−2 is suggested as an effective compromise between maximizing threat score and probability of detection while maintaining a low false alarm rate.

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J. M. Rees
,
J. C. W. Denholm-Price
,
J. C. King
, and
P. S. Anderson

Abstract

Internal gravity waves are frequently observed in stably stratified regions of the atmospheric boundary layer. In order to determine the statistical influence of such waves on the dynamics of the boundary layer it is necessary to compile information concerning properties of the waves such as frequency of occurrence, propagation, and spectral characteristics. Gravity wave climatologies have been compiled from relatively few locations. In this paper a climatological study of gravity waves, in the period range 1–20 min, propagating in the stably stratified atmospheric boundary layer overlying an Antarctic ice shelf is presented. An extensive set of boundary layer measurements were compiled throughout 1991. Surface pressure fluctuations were recorded from a spatial array of six sensitive microbarographs. Wind and temperature records from an instrumented mast were also available. A beam-steering technique has been used to determine wave parameters from the surface pressure data. The microbarographs detected the presence of internal gravity waves throughout the observational campaign. Root-mean-square pressure values were typically in the region 16–40 μb, but a significant number of isolated events with amplitudes of up to 180 μb were also found. Wave properties have been studied in conjunction with the mean wind and temperature profiles in the boundary layer. It was found that most of the wave activity did not originate locally, but from shear layers aloft, or, more commonly, from the katabatic flow regime where the ice shelf joins the Antarctic continent.

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James Correia Jr.
,
Raymond W. Arritt
, and
Christopher J. Anderson

Abstract

The development and propagation of mesoscale convective systems (MCSs) was examined within the Weather Research and Forecasting (WRF) model using the Kain–Fritsch (KF) cumulus parameterization scheme and a modified version of this scheme. Mechanisms that led to propagation in the parameterized MCS are evaluated and compared between the versions of the KF scheme. Sensitivity to the convective time step is identified and explored for its role in scheme behavior. The sensitivity of parameterized convection propagation to microphysical feedback and to the shape and magnitude of the convective heating profile is also explored.

Each version of the KF scheme has a favored calling frequency that alters the scheme’s initiation frequency despite using the same convective trigger function. The authors propose that this behavior results in part from interaction with computational damping in WRF. A propagating convective system develops in simulations with both versions, but the typical flow structures are distorted (elevated ascending rear inflow as opposed to a descending rear inflow jet as is typically observed). The shape and magnitude of the heating profile is found to alter the propagation speed appreciably, even more so than the microphysical feedback. Microphysical feedback has a secondary role in producing realistic flow features via the resolvable-scale model microphysics. Deficiencies associated with the schemes are discussed and improvements are proposed.

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