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Thomas W. Schlatter

Abstract

The physical processes occurring within a perennial snow cover and at its surface are discussed with particular emphasis on monthly variations in Antarctic snow. Results of a one-year simulation of physical processes In snow are compared with 1958 data at Mirny and Pionerskaya, Antarctica. The simulation demonstrates the following: first, solar radiation is effective to a depth of 50 cm or so, but longwave radiation cools the surface alone, causing the “greenhouse” effect first observed in snow at least four decades ago. Highest summertime temperature and initial snowmelt thus occur not at the surface but roughly 10 cm below It. Second, the percolation of meltwater with eventual refreezing during the subsequent winter is the most efficient mechanism for modifying the temperature profile at depths of several meters of more. Third, concerning the surface energy budget, at temperatures below −20C, atmospheric sensible heat flux approximately compensates net radiation throughout the year. When the temperature nears 0C (only in summer), the surplus of net radiation Is used primarily for changes of phase—sublimation and possibly melting; little energy is used to heat the snow. Then latent beat flux approximately compensates net radiation.

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Thomas W. Schlatter

Abstract

A statistical scheme for simultaneous analysis of the wind and geopotential height fields has been developed based upon optimum interpolation. The matrix weights applied to the observation vectors depend upon covariances of observed-minus-forecast differences. The simplest possible forecasts (or first guesses)—climatology, persistence and damped persistence—are used. The geostrophic relationship and the height-height covariances computed from historical data are used to derive the other required covariances. The scheme has been tested at a single point based upon 500-mb winter U.S. radiosonde data. Results are promising; when either climatology or damped persistence is used as a first guess, the root-men-square (rms) differences between analyzed and observed values were about 13 m for the height and 4.0 m s−1 for the u-and v–components of the wind. When persistence is used as a fist guess, results are slightly worse. The multivariate approach is clearly superior to a univariate approach for height analyses. On the other hand, geopotential height data do not significantly improve wind analyses. The applicability of the scheme to objective analysis over large areas is briefly mentioned.

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Thomas W. Schlatter

Abstract

For a 14-day period in late March and early April 1979, TIROS-N operational soundings have been compared with independent temperature analyses produced by the National Meteorological Center. Over the United States and southern Canada, analyzed layer-mean virtual temperatures were interpolated in space and time to the TIROS-N soundings. Differences between retrieved and analyzed layer temperatures were calculated at each sounding location. Statistical analysis of those differences reveals the following. For this data sample, the average TIROS-N sounding is too warm near the surface, too cool in the mid troposphere and too warm at the tropopause. Soundings made under cloudy conditions, which depend heavily on radiances in the microwave channels, have larger temperature biases than soundings made in clear air. Root-mean-square differences between retrieved and analyzed layer temperatures are generally less than 2°C except in the 1000–850 and 70–50 mb layers, where they are larger. The horizontal correlation of TIROS-N retrieval errors is substantial, largerthan O.5 out to 5OOkm. There is strong evidence that horizontal temperature gradients inferred from TIROS-N soundings are systematically too weak.

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Dezső Dévényi and Thomas W. Schlatter

Abstract

Statistical properties of observed residuals from the Mesoscale Analysis and Prediction System (MAPS), a real-time data assimilation system, were investigated. Observed residuals are defined as differences between rawinsonde observations interpolated vertically to the model levels and the predicted values from MAPS interpolated horizontally to the radiosonde locations. One-point statistical moments up to order 4 (including skewness and flatness) were computed to investigate the normality of the probability distribution of observed residuals. The finding of near-zero skewness indicates symmetry in the distribution of observed residuals, but values of flatness significantly different from 3 indicate deviations from a normal (Gaussian) distribution. These results are supported by an effective statistical test. The spatial distributions of these statistical moments show strong local variability, which is ascribed to occasional gross errors in the rawinsonde data.

The spatial correlation of observed residuals was computed for the Montgomery streamfunction and the components of the horizontal wind, following a model proposed by Roger Daley and used at the European Centre for Medium-Range Weather Forecasts. This model allows for divergence in the analyzed wind field. Complications arising from lateral boundary conditions were addressed. The spatial correlation was also computed from observed residuals of condensation pressure, which is the moisture variable in MAPS. All empirical correlations were approximated by truncated series of Bessel functions. The results are similar to those of other authors, with the exception that 3-h prediction errors in the MAPS model tend to be less geostrophic than 12-h prediction errors in global models, which have coarser resolution. The correlation range for condensation pressure was large, approaching 1000 km, reflecting the conservation of this quantity on isentropic surfaces in nonsaturated flow.

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Thomas W. Schlatter and Grant W. Branstator

Abstract

Using an 8-day series (18–26 August 1975) of multivariate statistical analyses of European radiosonde data together with a measure of analysis error, we have estimated error statistics from 959 Nimbus 6 temperature profiles for 10 isobaric layers in the troposphere and lower stratosphere. The mean error or bias is largest near the tropopause (+0.9°C) but changes sign several times in the vertical so that the integrated mean error for the atmospheric column 1000–70 mb is small (−0.1°C). The root-mean-square error peaks at the tropopause (2.9°C) with a minimum in the midtroposphere (1.0°C). In all layers, the horizontal correlation of retrieval error shows little systematic dependence on direction but strong dependence on distance. The correlation is greater than 0.50 at distances less than 400 km and less than 0.10 at 800 km and beyond, and it can be approximated by a Gaussian curve. The vertical correlations are greatest between adjacent layers (∼0.50); negative correlations exist between layers on opposite sides of the tropopause. This information is useful in any statistical objective analysis which accounts for observational error.

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Paul T. Schlatter, Thomas W. Schlatter, and Charles A. Knight

Abstract

An unusual, isolated hailstorm descended on Boulder, Colorado, on the evening of 24 June 2006. Starting with scattered large, flattened, disk-shaped hailstones and ending with a deluge of slushy hail that was over 4 cm deep on the ground, the storm lasted no more than 20 min and did surprisingly little damage except to vegetation. Part I of this two-part paper examines the meteorological conditions preceding the storm and the signatures it exhibited on Weather Surveillance Radar-1988 Doppler (WSR-88D) displays. There was no obvious upper-tropospheric forcing for this storm, vertical shear of the low-level wind was minimal, the boundary layer air feeding the storm was not very moist (maximum dewpoint 8.5°C), and convective available potential energy calculated from a modified air parcel was at most 1550 J kg−1. Despite these handicaps, the hail-producing storm had low-level reflectivity exceeding 70 dBZ, produced copious low-density hail, exhibited strong rotation, and generated three extensive bounded weak-echo regions (BWERs) in succession. The earliest of these filled with high reflectivities as the second one to the south poked up through precipitation-filled air. This has implications for low-density hail growth, as discussed in Part II.

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Charles A. Knight, Paul T. Schlatter, and Thomas W. Schlatter

Abstract

The 24 June 2006 Boulder hailstorm produced very heavy precipitation including disklike hailstones that grew with low density. These disklike hailstones, 4–5 cm in diameter, are unusual, and some of them appear to have accumulated graupel while aloft. A large amount of very fine-grained slush was left on the ground along with the hail. The hail and the great amount of slush suggest that most of the hydrometeor growth in the cloud was by low- or very-low-density riming. Consistent with that, the radar data suggest that the storm updraft had substantially depleted liquid water content. There is evidence that low-density hydrometeor growth within storms may be considerably more frequent than is commonly suspected.

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Thomas W. Schlatter, Grant W. Branstator, and Linda G. Thiel

Abstract

A multivariate statistical analysis procedure has been developed for estimating geopotential height h and wind (u, v) on a global latitude-longitude grid. Estimates are obtained by modifying the “first guess” from a prediction model by a linear combination of forecast errors deduced from observed data. Because the scheme is multivariate, the regression coefficients (weights) are matrices, which depend upon covariance among forecast errors in h, u and v. These covariances are modeled mathematically with geostrophic constraints. In the tropics, however, only the wind field is analyzed, covariances are modeled under the constraint of nondivergence, and heights are obtained from a balance equation. At high latitudes, analyses are performed in polar stereographic coordinates.

The objective analysis scheme fits observed data as well as the “Cressman scheme” that was used operationally at the National Meteorological Center until recently and also as well as a skilled analyst. In data-rich areas, the analyses are insensitive to the type of fist guess. Realistic ageostrophic and divergent components are present in the analyzed winds, and the kinetic energy spectrum at 40°N is reasonable at zonal wavenumbers less than 20. When both wind and height observations are plentiful, two univariate schemes (one for height, one for wind) fit the data as well as the multivariate scheme, but forecasts based upon the latter are consistently better. Experiments suggest that for a fixed amount of initial data, small gains in forecast accuracy can be made by improving the analysis procedure.

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Thomas W. Schlatter, Grant W. Branstator, and Linda G. Thiel

Abstract

No abstract available.

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Margaret A. LeMone, Thomas W. Schlatter, and Robert T. Henson

Scientific investigation is supposed to be objective and strictly logical, but this is not always the case: the process that leads to a good conclusion can be messy. This narrative describes interactions among a group of scientists trying to solve a simple problem that had scientific implications. It started with the observation of a cloud exhibiting behavior associated with supercooled water and temperatures around −20°C. However, other aspects of the cloud suggested an altitude where the temperature was around −40°C. For several months following the appearance of the cloud on 23 March 2011, the people involved searched for evidence, formed strong opinions, argued, examined evidence more carefully, changed their minds, and searched for more evidence until they could reach agreement. While they concluded that the cloud was at the higher and colder altitude, evidence for supercooled liquid water at that altitude is not conclusive.

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