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David Mayers and Christopher Ruf

Abstract

A new method is described for determining the center location of a tropical cyclone (TC) using wind speed measurements by the NASA Cyclone Global Navigation Satellite System (CYGNSS). CYGNSS measurements made during TC overpasses are used to constrain a parametric wind speed model in which storm center location is varied. The “MTrack” storm center location is selected to minimize the residual difference between model and measurement. Results of the MTrack center fix are compared to the National Hurricane Center (NHC) Best Track, the Automated Rotational Center Hurricane Eye Retrieval (ARCHER), and aircraft reconnaissance fixes for category 1–category 3 TCs during the 2017 and 2018 hurricane seasons. MTrack produces storm center locations at intermediate times between NHC fixes with a factor of 5.6 overall reduction in sensitivity to uncertainties in the NHC fixes between which it interpolates. The MTrack uncertainty is found to be larger in the cross-track direction than the along-track direction, although this behavior and the absolute accuracy of position estimates require further investigation.

Open access
David Mayers and Christopher Ruf

Abstract

The maximum sustained wind speed Vm of a tropical cyclone (TC) observed by a sensor varies with its spatial resolution. If unaccounted for, the difference between the “true” and observed Vm results in an error in estimation of Vm. The magnitude of the error is found to depend on the radius of maximum wind speed Rm and Vm itself. Quantitative relationships are established between Vm estimation errors and the TC characteristics. A correction algorithm is constructed as a scale factor to estimate the true Vm from coarsely resolved wind speed measurements observed by satellites. Without the correction, estimates of Vm made directly from the observations have root-mean-square differences of 1.77, 3.41, and 6.11 m s−1 given observations with a spatial resolution of 25, 40, and 70 km, respectively. When the proposed scale factors are applied to the observations, the errors are reduced to 0.69, 1.23, and 2.12 m s−1. A demonstration of the application of the correction algorithm throughout the life cycle of Hurricane Sergio in 2018 is also presented. It illustrates the value of having the scale factor depend on Rm and Vm, as opposed to using a fixed value, independent of TC characteristics.

Open access
David Mayers and Christopher Ruf

Abstract

MTrack is an automated algorithm which determines the center location (latitude and longitude) of a tropical cyclone from a scalar wind field derived from satellite observations. Accurate storm centers are useful for operational forecasting of tropical cyclones and for their reanalysis (e.g. research on storm surge modeling). Currently, storm center fixes have significantly larger errors for weak, disorganized storms. The MTrack algorithm presented here improves storm centers in some of those cases. It is also automated and, therefore, less subjective than manual fixes made by forecasters. The MTrack algorithm, which was originally designed to work with CYGNSS wind speed measurements, is applied to SMAP winds for the first time. The average difference between MTrack and Best Track storm center locations is 21, 36 and 46 km for major hurricanes, category 1-2 hurricanes, and tropical storms, respectively. MTrack is shown to operate successfully when a storm is only partially sampled by the observing satellite and when the eye of the storm is not resolved.

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Linda Forster, Anthony B. Davis, David J. Diner, and Bernhard Mayer

Abstract

For passive satellite imagers, current retrievals of cloud optical thickness and effective particle size fail for convective clouds with 3D morphology. Indeed, being based on 1D radiative transfer (RT) theory, they work well only for horizontally homogeneous clouds. A promising approach for treating clouds as fully 3D objects is cloud tomography, which has been demonstrated for airborne observations. However, more efficient forward 3D RT solvers are required for cloud tomography from space. Here, we present a path forward by acknowledging that optically thick clouds have “veiled cores” (VCs). Sunlight scattered into and out of this deep region does not contribute significant information about the inner structure of the cloud to the spatially detailed imagery. We investigate the VC location for the MISR and MODIS imagers. While MISR provides multiangle imagery in the visible and near-infrared (IR), MODIS includes channels in the shortwave IR, albeit at a single view angle. This combination will enable future 3D retrievals to disentangle the cloud’s effective particle size and extinction fields. We find that, in practice, the VC is located at an optical distance of ~5, starting from the cloud boundary along the line of sight. For MODIS’s absorbing wavelengths the VC covers a larger volume, starting at smaller optical distances. This concept will not only lead to a reduction in the number of unknowns for the tomographic reconstruction but also significantly increase the speed and efficiency of the 3D RT solver at the heart of the algorithm by applying, say, the photon diffusion approximation inside the VC.

Open access
Amber L. Pearson, Jonathan D. Mayer, and David J. Bradley

Abstract

Even as millions live without reliable access to water, very little is known about how households cope with scarcity. The aims of this research were to 1) understand aspects of water scarcity in three rural villages in southwestern Uganda, 2) examine differences by demographics and type of source, 3) assess relationships between different factors related to water access, and 4) explore coping strategies used. Health implications and lessons learned that relate to future climate change are discussed.

Demographic data, water accessibility, and coping strategies used were recorded using a survey. Descriptive statistics were calculated, and Spearman’s rank correlations were calculated between self-reported level of access, walking minutes to source, ranked ownership of source, and source accessibility during the last two weeks of April (16–30 April). Changes in water source type across seasons and demographic and access measures by coping strategies were examined.

Over half of the households relied on seasonal water sources. Of those accessing “permanent” sources, ~30% experienced inaccessibility within the last two weeks of April. Self-reported better access to water was correlated with minutes spent walking to source and to some degree with the source being more public or shared. Those without access to public sources tended to migrate as the primary coping strategy. Water sharing and reciprocity appears crucial between wealthy and poor households; however, those from outside ethnic groups appear to be partially excluded. Middle income households followed by the poorest had the largest reliance on purchasing water to cope. These findings underscore how access to water resources, particularly in times of insecurity, involves social networks.

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Sebastian W. Hoch, C. David Whiteman, and Bernhard Mayer

Abstract

The Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres (MYSTIC) three-dimensional radiative transfer model was used in a parametric study to determine the strength of longwave radiative heating and cooling in atmospheres enclosed in idealized valleys and basins. The parameters investigated included valley or basin shape, width, and near-surface temperature contrasts. These parameters were varied for three different representative atmospheric temperature profiles for different times of day. As a result of counterradiation from surrounding terrain, nighttime longwave radiative cooling in topographic depressions was generally weaker than over flat terrain. In the center of basins or valleys with widths exceeding 2 km, cooling rates quickly approached those over flat terrain, whereas the cooling averaged over the entire depression volume was still greatly reduced. Valley or basin shape had less influence on cooling rates than did valley width. Strong temperature gradients near the surface associated with nighttime inversion and daytime superadiabatic layers over the slopes significantly increased longwave radiative cooling and heating rates. Local rates of longwave radiative heating ranged between −30 (i.e., cooling) and 90 K day−1. The effects of the near-surface temperature gradients extended tens of meters into the overlying atmospheres. In small basins, the strong influence of nocturnal near-surface temperature inversions could lead to cooling rates exceeding those over flat plains. To investigate the relative role of longwave radiative cooling on total nighttime cooling in a basin, simulations were conducted for Arizona’s Meteor Crater using observed atmospheric profiles and realistic topography. Longwave radiative cooling accounted for nearly 30% of the total nighttime cooling observed in the Meteor Crater during a calm October night.

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Warren M. Washington, Albert J. Semtner Jr., Gerald A. Meehl, David J. Knight, and Thomas A. Mayer

Abstract

This paper describes the construction and results of a comprehensive, three-dimensional general circulation model (GCM) of the earth's climate. The model, developed at the National Center for Atmospheric Research (NCAR), links separate existing models of the atmosphere, ocean and sea ice. The atmospheric model is a version of the third-generation NCAR GCM which has a relatively complete treatment of physical processes. It uses a generalized vertical coordinate with eight layers (∼3 km thick) and 5° horizontal grid spacing over the entire globe. The ocean model, using the primitive equations and the hydrostatic and Boussinesq approximations, was changed to the world domain from an earlier model developed by Bryan (1969) and reprogrammed by Semtner (1974). The model has four unequally spaced vertical layers and 5° horizontal grid structure. The sea ice model is a simple thermodynamic model using a simplified calculation of heat flux through sea ice (Semtner, 1976).

The method of coupling the atmosphere and ocean models is an attempt to deal with the two different time scales of the atmosphere and ocean in a computationally efficient fashion. By means of four relatively short integrations, the atmospheric model provides samples (10–30 days in length) of four seasonal months—January, April, July and October. The data from the four atmospheric model months are fitted to annual and semiannual harmonics and are used to drive the ocean model for five years. The process is iterated for a number of cycles to achieve an approximate equilibrium.

The atmospheric circulation in the coupled model is similar to that obtained previously by Washington et al. (1979) with climatological ocean forcing. The simulated ocean surface temperature pattern is reasonably similar to the observed pattern, but the calculated ocean temperatures tend to be as much as 3°C too cold locally in the tropics and up to 4°C too warm in the midlatitudes. Possible reasons for these discrepancies are discussed. The major mean ocean current gyre systems are reproduced in the ocean model second layer where effects of non-geostrophic Ekman drift and short-term wind-stress averaging bias are not felt. These effects, however, tend to complicate somewhat the computed surface current pattern. The computed horizontal oceanic heat flux compares favorably with the observed of Oort and Vonder Haar (1976) in phase and amplitude. Vertical velocities at the bottom of the 50 m surface layer, which can be considered a simple mixed layer, have the same general pattern as those calculated using observed wind stress. The simulation of sea ice thickness and seasonal geographical extent is closer to the observed in the Arctic than in the Antarctic region.

The experiment described here must be regarded as preliminary; even though many first-order aspects of the climate system are simulated, improvements are still needed.

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Mark W. Maier, VA Frank W. Gallagher III, Karen St. Germain, Richard Anthes, Cinzia Zuffada, Robert Menzies, Jeffrey Piepmeier, David Di Pietro, Monica M. Coakley, and Elena Adams

Capsule

A recent NOAA study investigated numerous alternatives for its operational constellation of weather satellites in the 2030 era. The study identified key options for orbits and levels of performance.

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S. G. Gopalakrishnan, David P. Bacon, Nash'at N. Ahmad, Zafer Boybeyi, Thomas J. Dunn, Mary S. Hall, Yi Jin, Pius C. S. Lee, Douglas E. Mays, Rangarao V. Madala, Ananthakrishna Sarma, Mark D. Turner, and Timothy R. Wait

Abstract

The Operational Multiscale Environment model with Grid Adaptivity (OMEGA) is an atmospheric simulation system that links the latest methods in computational fluid dynamics and high-resolution gridding technologies with numerical weather prediction. In the fall of 1999, OMEGA was used for the first time to examine the structure and evolution of a hurricane (Floyd, 1999). The first simulation of Floyd was conducted in an operational forecast mode; additional simulations exploiting both the static as well as the dynamic grid adaptation options in OMEGA were performed later as part of a sensitivity–capability study. While a horizontal grid resolution ranging from about 120 km down to about 40 km was employed in the operational run, resolutions down to about 15 km were used in the sensitivity study to explicitly model the structure of the inner core. All the simulations produced very similar storm tracks and reproduced the salient features of the observed storm such as the recurvature off the Florida coast with an average 48-h position error of 65 km. In addition, OMEGA predicted the landfall near Cape Fear, North Carolina, with an accuracy of less than 100 km up to 96 h in advance. It was found that a higher resolution in the eyewall region of the hurricane, provided by dynamic adaptation, was capable of generating better-organized cloud and flow fields and a well-defined eye with a central pressure lower than the environment by roughly 50 mb. Since that time, forecasts were performed for a number of other storms including Georges (1998) and six 2000 storms (Tropical Storms Beryl and Chris, Hurricanes Debby and Florence, Tropical Storm Helene, and Typhoon Xangsane). The OMEGA mean track error for all of these forecasts of 101, 140, and 298 km at 24, 48, and 72 h, respectively, represents a significant improvement over the National Hurricane Center (NHC) 1998 average of 156, 268, and 374 km, respectively. In a direct comparison with the GFDL model, OMEGA started with a considerably larger position error yet came within 5% of the GFDL 72-h track error. This paper details the simulations produced and documents the results, including a comparison of the OMEGA forecasts against satellite data, observed tracks, reported pressure lows and maximum wind speed, and the rainfall distribution over land.

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Bjorn Stevens, Felix Ament, Sandrine Bony, Susanne Crewell, Florian Ewald, Silke Gross, Akio Hansen, Lutz Hirsch, Marek Jacob, Tobias Kölling, Heike Konow, Bernhard Mayer, Manfred Wendisch, Martin Wirth, Kevin Wolf, Stephan Bakan, Matthias Bauer-Pfundstein, Matthias Brueck, Julien Delanoë, André Ehrlich, David Farrell, Marvin Forde, Felix Gödde, Hans Grob, Martin Hagen, Evelyn Jäkel, Friedhelm Jansen, Christian Klepp, Marcus Klingebiel, Mario Mech, Gerhard Peters, Markus Rapp, Allison A. Wing, and Tobias Zinner

Abstract

A configuration of the High-Altitude Long-Range Research Aircraft (HALO) as a remote sensing cloud observatory is described, and its use is illustrated with results from the first and second Next-Generation Aircraft Remote Sensing for Validation (NARVAL) field studies. Measurements from the second NARVAL (NARVAL2) are used to highlight the ability of HALO, when configured in this fashion, to characterize not only the distribution of water condensate in the atmosphere, but also its impact on radiant energy transfer and the covarying large-scale meteorological conditions—including the large-scale velocity field and its vertical component. The NARVAL campaigns with HALO demonstrate the potential of airborne cloud observatories to address long-standing riddles in studies of the coupling between clouds and circulation and are helping to motivate a new generation of field studies.

Open access