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Matthew D. Eastin
,
Peter G. Black
, and
William M. Gray

Abstract

Flight-level thermodynamic errors caused by the wetting of temperature and moisture sensors immersed within the airstream are studied using data from 666 radial legs collected in 31 hurricanes at pressure levels ranging from 850 to 500 mb. Concurrent measurements from a modified Barnes radiometer and a Rosemount 102 immersion thermometer are compared to identify regions, called instrument wetting events (IWE), in which Rosemount temperatures are significantly cooler than radiometer-derived temperatures by a specified amount. A total of 420 IWE are identified in the dataset. Roughly 50% of the radial legs contain at least one instrument wetting event. More than 90% of IWE are associated with updrafts containing cloud water and are confined to scales less than 10 km. IWE are also found to be more frequent in eyewalls and intense hurricanes.

Thermodynamic errors within IWE and convective updrafts and downdrafts are summarized as distributions of average temperature, specific humidity, virtual potential temperature, and equivalent potential temperature error. Distributions are skewed toward larger error values at all levels. Median average errors within IWE indicate that the thermodynamic quantities are typically too low by ∼1°C, ∼1 g kg−1, ∼1.5 K, and ∼5 K, respectively. The largest errors (>90% of the distribution) are nearly twice the median values. Error magnitudes tend to increase with height, but rarely achieve theoretical predictions. In addition, more than 65% of updrafts and 35% of downdrafts are found to contain significant thermodynamic errors. A correction method used in earlier studies was found to be inadequate at removing the majority of errors, but reduced the errors by ∼30%–50% on average.

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Matthew D. Eastin
,
Peter G. Black
, and
William M. Gray

Abstract

The implications of flight-level instrument wetting error removal upon the mean thermodynamic structure across the eyewall, buoyancy of rainband vertical motions, and vertical energy fluxes near the top of the inflow layer are studied. Thermodynamic quantities across the mean eyewall are found to increase at all levels. As a result, maximum radial gradients of each quantity are shifted from the center of the eyewall cloud toward the outer edge. The increase in equivalent potential temperature lifts eyewall values to comparable magnitudes observed in the eye. The mean virtual potential temperature deviation of rainband updrafts increases from slightly negative to slightly positive. This increase and shift in sign are more pronounced in stronger updrafts. The mean deviation in rainband downdrafts decreases slightly toward neutral conditions. Vertical sensible heat fluxes near the top of the inflow layer are found to shift from downward to upward. Upward latent heat fluxes increase. Implications of these results upon hurricane structure and evolution are discussed.

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Matthew D. Eastin
,
William M. Gray
, and
Peter G. Black

Abstract

The buoyancy of hurricane convective vertical motions is studied using aircraft data from 175 radial legs collected in 14 intense hurricanes at four altitudes ranging from 1.5 to 5.5 km. The data of each leg are initially filtered to separate convective-scale features from background mesoscale structure. Convective vertical motion events, called cores, are identified using the criteria that the convective-scale vertical velocity must exceed 1.0 m s−1 for at least 0.5 km. A total of 620 updraft cores and 570 downdraft cores are included in the dataset. Total buoyancy is calculated from convective-scale virtual potential temperature, pressure, and liquid water content using the mesoscale structure as the reference state.

Core properties are summarized for the eyewall and rainband regions at each altitude. Characteristics of core average convective vertical velocity, maximum convective vertical velocity, and diameter are consistent with previous studies of hurricane convection. Most cores are superimposed upon relatively weak mesoscale ascent. The mean eyewall (rainband) updraft core exhibits small, but statistically significant, positive total buoyancy below 4 km (between 2 and 5 km) and a modest increase in vertical velocity with altitude. The mean downdraft core not superimposed upon stronger mesoscale ascent also exhibits positive total buoyancy and a slight decrease in downward vertical velocity with decreasing altitude. Buoyant updraft cores cover less than 5% of the total area in each region but accomplish ∼40% of the total upward transport.

A one-dimensional updraft model is used to elucidate the relative roles played by buoyancy, vertical perturbation pressure gradient forces, water loading, and entrainment in the vertical acceleration of ordinary updraft cores. Small positive total buoyancy values are found to be more than adequate to explain the vertical accelerations observed in updraft core strength, which implies that ordinary vertical perturbation pressure gradient forces are directed downward, opposing the positive buoyancy forces. Entrainment and water loading are also found to limit updraft magnitudes.

The observations support some aspects of both the hot tower hypothesis and symmetric moist neutral ascent, but neither concept appears dominant. Buoyant convective updrafts, however, are integral components of the hurricane’s transverse circulation.

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Matthew D. Eastin
,
William M. Gray
, and
Peter G. Black

Abstract

This is the second of two papers on the buoyancy of convective vertical motions in the inner core of intense hurricanes. This paper uses extensive airborne radar, dropwindsonde, and flight-level observations in Hurricanes Guillermo (1997) and Georges (1998) to illustrate typical azimuthal distribution of buoyant convection and demonstrate that the low-level eye can be an important source region for buoyant eyewall convection.

In both hurricanes, eyewall vertical velocity and radar reflectivity are asymmetric and exhibit persistent relationships with the direction of the environmental vertical wind shear. Mesoscale vertical motions exhibit a wavenumber-1 structure with maximum ascent downshear and weak descent upshear. The mesoscale reflectivity maxima are located left-of-shear. Buoyant eyewall updraft cores and transient convective-scale reflectivity cells are predominantly downshear and left-of-shear. Most eyewall downdraft cores that transport significant mass downward are located upshear. Negative buoyancy was most common in left-of-shear downdrafts, with positive buoyancy dominant in upshear downdrafts. Inward-spiraling rainbands located outside the eyewall exhibit upband/downband asymmetries. Upband segments contain more convective reflectivity cells and buoyant updraft cores than the more stratiform downband segments. Equal numbers of downdraft cores are found upband and downband, but the majority exhibit negative buoyancy.

Several buoyant updraft cores encountered in the midlevel eyewall exhibit equivalent potential temperatures (θe ) much higher than the θe observed in the low-level eyewall, but equivalent to the θe observed in the low-level eye. Asymmetric low-wavenumber circulations appear responsible for exporting the high-θe eye air into the relatively low-θe eyewall and generating the locally buoyant updraft cores.

Implications of these results upon conceptual models of hurricane structure are discussed. Three mechanisms, whereby an ensemble of asymmetric buoyant convection could contribute to hurricane evolution, are also discussed.

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J. G. Hosking
,
C. D. Stow
,
S. G. Bradley
, and
W. R. Gray

Abstract

Evaluation of an earlier raingage design based on counting drops formed on the tip of a small-diameter stainless-steel tube shows a defect due to resonant oscillation of the water column in the dropper unit. The defect causes nonlinearity in the drop rate-flow rate relationship, precluding useful integration of the gage output. An improved design is presented which eliminates this defect. Laboratory tests on the new design show linear performance over two orders of magnitude of rainfall intensity with time resolution of better than 5 s and accuracy limited by sampling errors. The new gage is also shown to perform well in field testing.

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L. M. McMillin
,
D. G. Gray
,
H. F. Drahos
,
M. W. Chalfant
, and
C. S. Novak

Abstract

Root-mean-square differences between satellite and radiosondes for the past three years that TIROS-N has been operational are examined. They show a pronounced annual cycle because the statistics are dominated by the Northern Hemisphere. Differences are smaller in the summer and are larger in the winter, but they reflect a change in the effect of location differences as well as retrieval error. In addition to the annual cycle, there is an increase in retrieval accuracy with time. For the partly cloudy retrievals, the increase approaches 1.3 K for some levels.

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G. R. Gray
,
R. J. Serafin
,
D. Atlas
,
R. E. Rinehart
, and
J. J. Boyajian

Color enhancement of images has become a powerful tool in rapid evaluation of grey-scale information. Recent advances in semiconductor technology have made possible the construction of an inexpensive digital real-time color-enhanced (or false-color) display for meteorological radar information such as reflectivity and Doppler velocities. Variable magnification allows detailed analysis of selected areas of the radar coverage.

The display was interfaced to a Doppler/reflectivity processor on the NHRE S-band radar at Grover, Colorado, during the 1974 hail season. A preliminary meteorological analysis of the Doppler color displays of the storm of 7 August 1974 demonstrates a large variety of significant features which may be observed either in real-time or subsequently. These include the regions of convergence and vorticity, major inflow and outflow regions, and turbulence. Most importantly, it is shown that the updraft cores can be identified with the easterly-momentum air which has been transported upward with the drafts from the lower levels. In view of the slow eastward motion of the storm system, the very large Doppler components found at the leading edge of the higher-level echo pattern also indicate rapid evaporation of the particles as they move out into the clear, dry environmental air. It is the resulting evaporative cooling which is responsible for the downdrafts in this vicinity. Among the many real-time applications of the color Doppler display, perhaps the most important in the artificial modification of convective storms is the location of the major inflow and updraft regions. These determine where seeding should be focused. The use of the color display also permits the ready discrimination of storm echoes from ground clutter in which they are frequently obscured. Its applicability to the detection of tornado cyclones and hurricane velocity mapping is also self-evident.

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H. W. Baynton
,
R. J. Serafin
,
C. L. Frush
,
G. R. Gray
,
P. V. Hobbs
,
R. A. Houze Jr
, and
J. D. Locatelli

Abstract

Color displays of the velocities of precipitation particles detected with a C-band Doppler radar in wide-spread cyclonic storms provide a variety of real-time information on the atmospheric wind field.Vertical profiles of wind speed and direction indicated by the real-time color displays agree well withrawinsonde measurements. Veering winds (or warm advection) produce a striking S-shaped pattern onthe color display and backing winds (or cold advection) produce a backward S. A maximum in the verticalprofile of wind speed is indicated by a pair of concentric colored rings, one upwind and one downwind ofthe radar. Vertically sloping velocity maxima are indicated by asymmetries in the color displays, as areconfluent and difluent winds. Divergence and convergence computed from the real-time color displays areof reasonable magnitude.

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Michael D. King
,
Steven Platnick
,
Ping Yang
,
G. Thomas Arnold
,
Mark A. Gray
,
Jérôme C. Riedi
,
Steven A. Ackerman
, and
Kuo-Nan Liou

Abstract

A multispectral scanning spectrometer was used to obtain measurements of the bidirectional reflectance and brightness temperature of clouds, sea ice, snow, and tundra surfaces at 50 discrete wavelengths between 0.47 and 14.0 μm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) Arctic Clouds Experiment, conducted over a 1600 km × 500 km region of the north slope of Alaska and surrounding Beaufort and Chukchi Seas between 18 May and 6 June 1998. Multispectral images in eight distinct bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) were used to derive a confidence in clear sky (or alternatively the probability of cloud) over five different ecosystems. Based on the results of individual tests run as part of this cloud mask, an algorithm was developed to estimate the phase of the clouds (liquid water, ice, or undetermined phase). Finally, the cloud optical thickness and effective radius were derived for both water and ice clouds that were detected during one flight line on 4 June.

This analysis shows that the cloud mask developed for operational use on MODIS, and tested using MAS data in Alaska, is quite capable of distinguishing clouds from bright sea ice surfaces during daytime conditions in the high Arctic. Results of individual tests, however, make it difficult to distinguish ice clouds over snow and sea ice surfaces, so additional tests were added to enhance the confidence in the thermodynamic phase of clouds over the Chukchi Sea. The cloud optical thickness and effective radius retrievals used three distinct bands of the MAS, with a recently developed 1.62- and 2.13-μm-band algorithm being used quite successfully over snow and sea ice surfaces. These results are contrasted with a MODIS-based algorithm that relies on spectral reflectance at 0.87 and 2.13 μm.

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R. T. Sutton
,
G. D. McCarthy
,
J. Robson
,
B. Sinha
,
A. T. Archibald
, and
L. J. Gray

Abstract

Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and rainfall patterns, hurricane activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.

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