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Lawrence S. Jackson and Piers M. Forster

Abstract

The diurnal temperature range (DTR) of surface air over land varies geographically and seasonally. The authors have investigated these variations using generalized additive models (GAMs), a nonlinear regression methodology. With DTR as the response variable, meteorological and land surface parameters were treated as explanatory variables. Regression curves related the deviation of DTR from its mean value to values of the meteorological and land surface variables. Cloud cover, soil moisture, distance inland, solar radiation, and elevation were combined as explanatory variables in an ensemble of 84 GAM models that used data grouped into seven vegetation types and 12 months. The ensemble explained 80% of the geographical and seasonal variation in DTR. Vegetation type and cloud cover exhibited the strongest relationships with DTR. Shortwave radiation, distance inland, and elevation were positively correlated with DTR, whereas cloud cover and soil moisture were negatively correlated. A separate analysis of the surface energy budget showed that changes in net longwave radiation represented the effects of solar and hydrological variation on DTR. It is found that vegetation and its associated climate is important for DTR variation in addition to the climatic influence of cloud cover, soil moisture, and solar radiation. It is also found that surface net longwave radiation is a powerful diagnostic of DTR variation, explaining over 95% of the seasonal variation of DTR in tropical regions.

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Ting-I. Wang, K. B. Earnshaw, and R. S. Lawrence

Abstract

Path-averaged terminal velocity distribution of raindrops is determined from the temporal covariance function of signals from two vertically spaced linear optical detectors that respond to raindrop-induced amplitude scintillations of a projected laser beam. The known monotonic relationship between drop size and terminal velocity permits the measured velocity distribution to be converted to path-averaged drop-size distribution and, in turn, to rain rate. The large capture area of the measurements over a 200 m path allows drop-size distribution to be measured in short time intervals. We present measurements of path-averaged rain rate and raindrop size distribution made at 42 s intervals. The terminal velocity distribution during a storm that contained a mixture of rain and hail clearly shows the two-component nature of the precipitation.

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John C. Andrews, Martin W. Lawrence, and Carl S. Nilsson

Abstract

Ship and air surveys were conducted in 1978–79 to examine the thermal structure in and near the Tasman Front between Australia and New Zealand at latitudes between Brisbane and Bass Strait. After separation from the continental slope, the East Australian Current feeds into the conjunction of the warm South Coral and cool Tasman Seas. This conjunction is seen as an abrupt change of temperature at all depths peaking at ∼6°C between 150 and 300 m depth. Extreme north-south excursions of the Tasman Front occur and waters of the Tasman or South Coral Sea origin follow the distortions to form equatorward cyclonic meanders or poleward anticyclonic meanders, respectively. These meanders occupy a latitude span of about 400–700 km and form a wave pattern stretching coherently across the Tasman Sea with a wavelength of ∼370 km; the average fall in surface dynamic height across the span is 30 cm. Eddy intensification near the Tasman Front products very large transports but this is mostly recirculating, so in the time-averaged sense only ∼15 × 106 m3 s−1 flows east into the southern limb of the South Pacific subtropical gyre. The abrupt change in the shape and temperature of the thermocline in crossing the Front reinforces Warren's (1970) argument that a zonal jet is maintained near latitude 34°S by baroclinic adjustment in order to connect the western boundary currents which flow along Australia and New Zealand; this composite boundary current is required to close the interior (Sverdrup) wind-driven circulation in the South Pacific. Ship and satellite infrared measurements show that in addition to there being an abrupt change in surface temperature at the Front, the East Australian Current advects very warm water from the north, down the coast and out along the Tasman Front.

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M. K. Davey, D. L. T. Anderson, and S. Lawrence

Abstract

In many prediction schemes, the skill of long-range forecasts of ENSO events depends on the time of year. Such variability could be directly due to seasonal changes in the basic ocean-atmosphere system or due to the state of ENSO itself.

A highly idealized delayed oscillator model with seasonally varying internal parameters is used here to simulate such behavior. The skill of the artificial forecasts shows dependence on both seasonal and ENSO phase. Experiments with ENSO phase-locked to the seasonal cycle. but with no seasonal variation of model parameters. show that the ENSO cycle alone can induce variability in skill. Inclusion of seasonal parameters enhances seasonal skill dependence. It is suggested that the seasonal skill variations found in practice am due to a combination of seasonal changes in the basic state and the phase-locking of the ENSO and annual cycles.

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Larry M. McMillin, Lawrence J. Crone, and David S. Crosby

Abstract

Many methods for converting satellite radiances to temperatures require a comparison of observed radiances with radiances calculated from a first guess. Usually, measured values must be tuned to agree with theoretical calculations. A regression in which the calculated radiances are predicted from the observed ones is a method of making the adjustment, but results in unrealistic coefficients. Several modifications to standard regression are tried, and it is shown that rotated regression, a technique developed in this paper, provides the required accuracy with coefficients that satisfy the physical constraints.

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Daniel S. Wilks, Charles J. Neumann, and Miles B. Lawrence

Abstract

U.S. National Hurricane Center (NHC) forecasts for tropical cyclone tracks and wind speeds are extended in time to produce spatially disaggregated probability forecasts for landfall location and intensity, using a weighted bootstrap procedure. Historical analogs, with respect to the forecast characteristics (location, heading, and wind speed) of a current storm, are selected. These are resampled by translating their locations to random positions consistent with the current forecast, and recent NHC forecast accuracy statistics. The result is a large number of plausible Monte Carlo realizations that jointly approximate a probability distribution for the future track and intensity of the storm. Performance of the resulting forecasts is assessed for U.S. tropical cyclone landfall probabilities during 1998–2006, and the forecasts are shown to be skillful and exhibit excellent reliability, even beyond the 120-h forecast horizon of the NHC advisory forecasts upon which they are based.

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A. B. White, R. J. Lataitis, and R. S. Lawrence

Abstract

It is well known that the width of a clear-air Doppler radar spectrum can be used to estimate the small-scale variability of the wind. To do this accurately requires that all contributions to the spectral width be accounted for. Recently, an approximate formula for correcting Doppler spectral widths for spatial and temporal filtering effects was proposed. The formula assumes independent additive contributions to the spectral width from the inherent volume averaging of the radar-sampling volume and the finite measurement interval. In the present paper the approximate formula is compared to an exact triple-integral formulation in which the spatial and temporal effects are shown to be coupled in a nonlinear fashion. The required integrations are evaluated numerically and are carried out over the full range of scales in wavenumber space, in contrast to earlier work, where truncated forms of isotropic, inertial-subrange spectral forms were used to obtain a simple, closed-form expression. Comparisons show that the approximate formula provides a good approximation to the integral solution over small to moderate length scales, but that it diverges from the integral solution at larger scales. Asymptotic limits to the exact integral formulation for large and small scales are presented. Finally, a double-integral solution that can be rapidly evaluated by any one of a number of commercial mathematics packages has been developed and shown to agree within 2% of the exact solution over all scales.

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S. M. Rosier, B. N. Lawrence, D. G. Andrews, and F. W. Taylor

Abstract

Dynamical fields based on temperature measurements from the Improved Stratospheric and Mesospheric Sounder on the Upper Atmosphere Research Satellite are presented for the Northern Hemisphere stratosphere for the period 28 October 1991 through 18 January 1992. Interpretation of these fields gives a picture of the dynamical evolution of this period in terms of the zonal-mean fields and the synoptic structures. Among the features of interest are the movements of the zonal-mean jets and several periods of stratospheric warming, culminating in a near-major warming in January.

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Ting-I. Wang, R. Lataitis, R. S. Lawrence, and G. R. Ochs

Abstract

Prototype Laser Weather Identifier (LWI) systems designed to detect fog, rain and snow were tested for several months at Stapleton International Airport in Denver, and at the AFGL Weather Test Facility at Otis Air Force Base, Massachusetts. We present a detailed analysis of the performance of these systems, compared with human weather observations and tipping-bucket raingages, and suggest modifications for future operational instruments.

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Janusz Eluszkiewicz, Richard S. Hemler, Jerry D. Mahlman, Lori Bruhwiler, and Lawrence L. Takacs

Abstract

The age of air has recently emerged as a diagnostic of atmospheric transport unaffected by chemical parameterizations, and the features in the age distributions computed in models have been interpreted in terms of the models’ large-scale circulation field. This study shows, however, that in addition to the simulated large-scale circulation, three-dimensional age calculations can also be affected by the choice of advection scheme employed in solving the tracer continuity equation. Specifically, using the 3.0° latitude × 3.6° longitude and 40 vertical level version of the Geophysical Fluid Dynamics Laboratory SKYHI GCM and six online transport schemes ranging from Eulerian through semi-Lagrangian to fully Lagrangian, it will be demonstrated that the oldest ages are obtained using the nondiffusive centered-difference schemes while the youngest ages are computed with a semi-Lagrangian transport (SLT) scheme. The centered-difference schemes are capable of producing ages older than 10 years in the mesosphere, thus eliminating the “young bias” found in previous age-of-air calculations.

At this stage, only limited intuitive explanations can be advanced for this sensitivity of age-of-air calculations to the choice of advection scheme. In particular, age distributions computed online with the National Center for Atmospheric Research Community Climate Model (MACCM3) using different varieties of the SLT scheme are substantially older than the SKYHI SLT distribution. The different varieties, including a noninterpolating-in-the-vertical version (which is essentially centered-difference in the vertical), also produce a narrower range of age distributions than the suite of advection schemes employed in the SKYHI model. While additional MACCM3 experiments with a wider range of schemes would be necessary to provide more definitive insights, the older and less variable MACCM3 age distributions can plausibly be interpreted as being due to the semi-implicit semi-Lagrangian dynamics employed in the MACCM3. This type of dynamical core (employed with a 60-min time step) is likely to reduce SLT’s interpolation errors that are compounded by the short-term variability characteristic of the explicit centered-difference dynamics employed in the SKYHI model (time step of 3 min). In the extreme case of a very slowly varying circulation, the choice of advection scheme has no effect on two-dimensional (latitude– height) age-of-air calculations, owing to the smooth nature of the transport circulation in 2D models.

These results suggest that nondiffusive schemes may be the preferred choice for multiyear simulations of tracers not overly sensitive to the requirement of monotonicity (this category includes many greenhouse gases). At the same time, age-of-air calculations offer a simple quantitative diagnostic of a scheme’s long-term diffusive properties and may help in the evaluation of dynamical cores in multiyear integrations. On the other hand, the sensitivity of the computed ages to the model numerics calls for caution in using age of air as a diagnostic of a GCM’s large-scale circulation field.

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