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S. A. Ackerman, A. S. Bachmeier, K. Strabala, and M. Gunshor

Abstract

A cold, dry arctic air mass occupied southeastern Canada and the northeastern United States on 13–14 January 2004. This air mass was quite dry—total column precipitable water values at Pickle Lake, Ontario, Canada, and The Pas, Manitoba, Canada, were as low as 0.02 in. (0.5 mm)—allowing significant amounts of radiation originating from the surface to be detected using Geostationary Operational Environmental Satellite (GOES) 6.5-μm “water vapor channel” imagery. On this day the strong thermal gradient between the very cold snow-covered land surface in southern Canada and the warmer, unfrozen, cloud-free water along the northern portion of the Great Lakes was quite evident in GOES-12 imager water vapor channel data. Several hours later, as the cold dry air mass moved eastward, the coast of Maine, Cape Cod, and the Saint Lawrence River were also apparent in the water vapor channel imagery.

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B. C. Maddux, S. A. Ackerman, and S. Platnick

Abstract

Characterizing the earth’s global cloud field is important for the proper assessment of the global radiation budget and hydrologic cycle. This characterization can only be achieved with satellite measurements. For complete daily coverage across the globe, polar-orbiting satellites must take observations over a wide range of sensor zenith angles. This paper uses Moderate Resolution Imaging Spectroradiometer (MODIS) Level-3 data to determine the effect that sensor zenith angle has on global cloud properties including the cloud fraction, cloud-top pressure, effective radii, and optical thickness. For example, the MODIS cloud amount increases from 57% to 71% between nadir and edge-of-scan (∼67°) observations, for clouds observed between 35°N and 35°S latitude. These increases are due to a combination of factors, including larger pixel size and longer observation pathlength at more oblique sensor zenith angles. The differences caused by sensor zenith angle bias in cloud properties are not readily apparent in monthly mean regional or global maps because the averaging of multiple satellite overpasses together “washes out” the zenith angle artifact. Furthermore, these differences are not constant globally and are dependent on the cloud type being observed.

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Richard A. Frey, S. A. Ackerman, and Brian J. Soden

Abstract

An automated method of monitoring various climate parameters using collocated Advanced Very High Resolution Radiometer (AVHRR) and High-Resolution Infrared Sounder-2 (HIRS/2) observations has been developed. The method, referred to as CHAPS (collocated HIRS/2 and AVHRR products) was implemented during the months of July 1993 and January and July 1994. This paper presents the oceanic cloud screening method and analysis of the spectral greenhouse parameter (g λ) for July 1993 and January 1994. In addition, the CHAPS derived clear-sky parameters are compared to the NESDIS historical dataset. There is agreement between NESDIS and CHAPS for the g 6.7 and g 7.3. The NESDIS 8.2-µm data appears to be cloud contaminated. Through comparison with CHAPS, it is suggested that the mode, rather than the mean, provides the better estimate of the central tendency of the NESDIS clear-sky 8.2-µm radiance distribution, particularly for regions with extensive low-level cloud cover.

It is shown that the spectral greenhouse parameter at wavelengths sensitive to middle and upper atmospheric water vapor content is dependent on SST via its connection to large-scale atmospheric circulation patterns. It is also shown that the variability of the spectral greenhouse parameter is strongly a function of latitude at these wavelengths, as well as in spectral regions sensitive to lower-level water vapor. Standard deviations are largest in the Tropics and generally decrease poleward. In contrast, variability in the spectral regions sensitive to upper-tropospheric temperature peaks in the middle latitudes and has its minimum in tropical latitudes.

Variability in the relationship between g λ and SST shows only a weak dependence on season for channels sensitive to water vapor content. A strong seasonal dependence is found in the g 14.2 for the middle-latitude regions associated with changes in the temperature structure of the upper troposphere.

Thee relationship between the spectral greenhouse parameter and the broadband greenhouse parameter is presented and discussed. It is found that the range in broadband g for warm tropical SSTs is driven by spectral changes at wavelengths sensitive to upper-troposheric water vapor. For cooler SSTs associated with the middle latitudes, the range in g is a function of the spectral greenhouse parameter sensitive to the temperature structure of the upper troposphere.

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Agnieszka A. Mrowiec, O. M. Pauluis, A. M. Fridlind, and A. S. Ackerman

Abstract

Application of an isentropic analysis of convective motions to a simulated mesoscale convective system is presented. The approach discriminates the vertical mass transport in terms of equivalent potential temperature. The scheme separates rising air at high entropy from subsiding air at low entropy. This also filters out oscillatory motions associated with gravity waves and isolates the overturning motions associated with convection and mesoscale circulation. The mesoscale convective system is additionally partitioned into stratiform and convective regions based on the radar reflectivity field. For each of the subregions, the mass transport derived in terms of height and an isentropic invariant of the flow is analyzed. The difference between the Eulerian mass flux and the isentropic counterpart is a significant and symmetric contribution of the buoyant oscillations to the upward and downward mass fluxes. Filtering out these oscillations results in substantial reduction of the diagnosed downward-to-upward convective mass flux ratio. The analysis is also applied to graupel and snow mixing ratios and number concentrations, illustrating the relationship of the particle formation process to the updrafts.

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A. Lenz, K. M. Bedka, W. F. Feltz, and S. A. Ackerman

Abstract

Transverse cirrus bands have commonly been observed in the outflow of thunderstorms, though little literature exists on the subject. The primary objective of this paper is to characterize the transverse band signature in satellite imagery with references to storm location, movement, and life cycle. The transverse band signature was observed in nearly half of all convective systems analyzed between May and August 2006, commonly in the mature and decay stages of the system. Storm size and propagation did not appear to influence transverse bands, though the bands did appear to be associated with negative 300-hPa relative vorticity and positive divergence. Transverse bands lasted an average duration of 9 h and generally occurred during the nighttime hours. The satellite analysis was combined with eddy dissipation rate (EDR) atmospheric turbulence observations collected by commercial aircraft. At least one observation of light (moderate) turbulence was found within transverse bands for 93% (44%) of events, indicating that the presence of transverse bands in satellite imagery is a strong indicator for aviation turbulence.

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Steven A. Ackerman, Jean M. Phillips, Thomas A. Achtor, and Daniel S. Bull

This paper examines the mission success of a federally funded research center by using bibliometric methods that include quantitative, descriptive, and citation analyses. We developed a methodology to facilitate examination of patterns in research, publishing, and collaborations, quantification and categorization of research partners, classification of topics, historical and emerging areas of research, and publishing venues. These patterns over a 12-yr period are used to assess whether the institute is achieving its mission goals of 1) fostering collaborative research, 2) becoming a center of excellence, and 3) educating scientists and students. Our findings indicate that a self-study of publishing activities yields useful results about programmatic strengths and weaknesses. This could be a first step of a larger study of federal government research and programmatic evaluation

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D. D. Turner, S. A. Ackerman, B. A. Baum, H. E. Revercomb, and P. Yang

Abstract

A new technique for ascertaining the thermodynamic cloud phase from high-spectral-resolution ground-based infrared measurements made by the Atmospheric Emitted Radiance Interferometer (AERI) is presented. This technique takes advantage of the differences in the index of refraction of ice and water between 11 and 19 μm. The differences in the refractive indices translate into differences in cloud emissivity at the various wavelengths, which are used to determine whether clouds contain only ice particles or only water particles, or are mixed phase. Simulations demonstrate that the algorithm is able to ascertain correctly the cloud phase under most conditions, with the exceptions occurring when the optical depth of the cloud is dominated by liquid water (>70%). Several examples from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment are presented, to demonstrate the capability of the algorithm, in which a collocated polarization-sensitive lidar is used to provide insight to the true thermodynamic phase of the clouds. Statistical comparisons with this lidar during the SHEBA campaign demonstrate that the algorithm identifies the cloud as either an ice or mixed-phase cloud approximately 80% of time when a single-layer cloud with an average depolarization above 10% exists that is not opaque to the AERI. For single-layer clouds having depolarization of less than 10%, the algorithm identifies the cloud as a liquid water cloud over 50% of the time. This algorithm was applied to 7 months of data collected during SHEBA, and monthly statistics on the frequency of ice, water, and mixed-phase clouds are presented.

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W. F. Feltz, K. M. Bedka, J. A. Otkin, T. Greenwald, and S. A. Ackerman

Abstract

Prior work has shown that pilot reports of severe turbulence over Colorado often occur when complex interference or crossing wave patterns are present in satellite water vapor imagery downstream of the Rocky Mountains. To improve the understanding of these patterns, a high-resolution (1-km) Weather Research and Forecasting (WRF) model simulation was performed for an intense mountain-wave event that occurred on 6 March 2004. Synthetic satellite imagery was subsequently generated by passing the model-simulated data through a forward radiative transfer model. Comparison with concurrent Moderate Resolution Imaging Spectroradiometer (MODIS) water vapor imagery demonstrates that the synthetic satellite data realistically captured many of the observed mesoscale features, including a mountain-wave train extending far downstream of the Colorado Front Range, the deformation of this wave train by an approaching cold front, and the substantially warmer brightness temperatures in the lee of the major mountain ranges composing the Colorado Rockies. Inspection of the model data revealed that the mountain waves redistributed the water vapor within the lower and middle troposphere, with the maximum column-integrated water vapor content occurring one-quarter wavelength downstream of the maximum ascent within each mountain wave. Due to this phase shift, the strongest vertical motions occur halfway between the locally warm and cool brightness temperature couplets in the water vapor imagery. Interference patterns seen in the water vapor imagery appear to be associated with mesoscale variability in the ambient wind field at or near mountaintop due to flow interaction with the complex topography. It is also demonstrated that the synergistic use of multiple water vapor channels provides a more thorough depiction of the vertical extent of the mountain waves since the weighting function for each channel peaks at a different height in the atmosphere.

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Y. Cheng, V. M. Canuto, A. M. Howard, A. S. Ackerman, M. Kelley, A. M. Fridlind, G. A. Schmidt, M. S. Yao, A. Del Genio, and G. S. Elsaesser

Abstract

We formulate a new second-order closure turbulence model by employing a recent closure for the pressure–temperature correlation at the equation level. As a result, we obtain new heat flux equations that avoid the long-standing issue of a finite critical Richardson number. The new, structurally simpler model improves on the Mellor–Yamada and Galperin et al. models; a key feature includes enhanced mixing under stable conditions facilitating agreement with observational, experimental, and high-resolution numerical datasets. The model predicts a planetary boundary layer height deeper than predicted by models with low critical Richardson numbers, as demonstrated in single-column model runs of the GISS ModelE general circulation model.

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A. D. Collard, S. A. Ackerman, W. L. Smith, X. Ma, H. E. Revercomb, R. O. Knuteson, and S-C. Lee

Abstract

During FIRE II, cirrus clouds were observed in the wavelength range 3–19, µm with two High Resolution Interferometer Sounders as described in the Part I companion paper. One, known as AC-HIS, was mounted on the NASA ER-2 aircraft in order to look down on the clouds; these results are described in the Part II companion paper. The other, GB-HIS, also known as the Atmospheric Emitted Radiance Interferometer (AERI), was ground based. The AERI observations have been simulated, assuming scattering from spherical ice particles, using a single-layer doubling model for the cloud, for two atmospheric windows at 700–1250 and 2650–3000 cm−1. The second of these windows is affected by scattered sunlight, which has been included in the calculations. The sensitivity of the cloud signal to quantities such as the ice water path (IWP) and effective radius (r eff) have been determined. Using the cloud model, best fits have been derived for IWP and r eff, for both windows individually and together. Possible errors in these derivations have been investigated.

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