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P. J. Mason and A. R. Brown

Abstract

Large eddy simulations use a subgrid model, which is characterized by a length scale that is often related to the scale of the computational mesh by a numerical constant, C s. Mason and Callen argued that this subgrid model and its length scale define and impose the filter operation of the simulation. They saw C s as a measure of numerical accuracy. Others have sought to link the filter operation to the computational mesh and have viewed C s as needing determination for correct implementation. Here tests with a high resolution of 224 × 224 × 200 grid points are found to confirm Mason and Callen’s view. These simulations are also used together with lower-resolution simulations to illustrate the degree of convergence achieved. Some erroneous features of the simulations are identified through this test.

For the case of buoyant convection, the buoyancy dependence of the subgrid model is further examined. Most available subgrid models allow for buoyancy fluxes changing the level of the subgrid energy but only allow stable buoyancy gradients to modify the subgrid length scale—a reduction in this case. In contrast to most applications, it has been suggested that for a fixed filter operation, the subgrid length scale should always have a buoyancy dependence and should increase, in a finite way, with unstable buoyant transfer. Here an examination of spectral behavior in high-resolution simulations supports such an approach and shows that the model with the buoyancy-dependent length scale is indeed consistent with a fixed filter operation. The more conventional models are shown to have less satisfactory behavior.

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D. P. Brown and R. A. Harvey

An integrator, which automatically and periodically records the weighted average solar radiation from the previous period, has been developed. The instrument is used to obtain a measure of the average solar radiant energy per day. This measurement was previously obtained by manual—and often tedious—graphical analysis.

Theoretical considerations involving averaging by passive networks are presented with experimental results which show the accuracy of the instrument.

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A. R. Brown, M. K. MacVean, and P. J. Mason

Abstract

Large eddy simulations sometimes use monotone advection schemes. Such schemes are dissipative, and the effective subgrid model then becomes the combined effect of the intended model and of the numerical dissipation. The impacts on simulation reliability are examined for the cases of dry convective and neutral planetary boundary layers. In general it is found that the results in the well-resolved flow interior are insensitive to the details of the advection scheme. However, unsatisfactory results may be obtained if numerical dissipation dominates where the flow becomes less well resolved as the surface is approached.

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P. R. Field, R. Wood, P. R. A. Brown, P. H. Kaye, E. Hirst, R. Greenaway, and J. A. Smith

Abstract

Ice particle interarrival times have been measured with a fast forward scattering spectrometer probe (FSSP). The distribution of interarrival times is bimodal instead of the exponential distribution expected for a Poisson process. The interarrival time modes are located at ∼10−2 and ∼10−4 s. This equates to horizontal spacings on both the centimeter and meter scales. The characteristics of the interarrival times are well modeled by a Markov chain process that couples together two independent Poisson processes operating at different scales. The possibility that ice crystals shattering on the probe tip causes the bimodal interarrival times is explored and cannot be ruled out. If the observations are indicating real spacings of particles in clouds, then the observations show very localized (centimeter scale) concentrations of ∼100 s cm−3 embedded within an average concentration of typically ∼1 cm−3. If the localized high concentrations are produced by the ice crystals shattering, then the concentration measured by the FSSP is overcounted by a factor of 5 in the worst case measured here, but more typically by a factor of 2. This uncertainty in concentration will adversely affect the predicted radiative influence of these clouds.

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G. M. Martin, M. R. Bush, A. R. Brown, A. P. Lock, and R. N. B. Smith

Abstract

A new turbulent mixing scheme, described in Part I of this paper, is tested in the climate and mesoscale configurations of the U.K. Met. Office’s Unified Model (UM). In climate configuration, the scheme is implemented along with increased vertical resolution below 700 hPa (the same as that in the mesoscale model), in order to allow the different boundary layer types and processes to be identified and treated properly. In both configurations, the new boundary layer (PBL-N) mixing scheme produces some improvement over the current boundary layer (PBL-C) scheme. The PBL-N scheme is able to diagnose different boundary layer types that appear to be consistent with the observed conditions, and the boundary layer structure is improved in comparison with observations. In the climate model, the boundary layer and cloud structure in the semipermanent stratocumulus regions of the eastern subtropical oceans are noticeably improved with the PBL-N scheme. The deepening and decoupling of the boundary layer toward the trade cumulus regime is also simulated more realistically. However, the cloud amounts in the stratocumulus regions, which were underestimated with the PBL-C scheme, are reduced further when the PBL-N scheme is included. Tests of the PBL-N scheme in the UM single-column model and in a development version of the UM, where the dynamics, time stepping, and vertical grid are different from the standard version, both show that realistic stratocumulus cloud amounts can be achieved. Thus, it is thought that the performance of the PBL-N scheme in the standard UM may be being limited by other aspects of that model. In the mesoscale model, improvements in the simulation of a convective case are achieved with the PBL-N scheme through reductions in layer cloud amount, while the simulation of a stratocumulus case is improved through better representation of the cloud and boundary layer structure. Other mesoscale model case studies show that there is a consistent improvement in fog probabilities and forecasts of cloud-base height. The root-mean-square errors in screen-level temperature are also reduced slightly. The weak daytime bias in wind strength is improved greatly through a systematic increase in the 10-m wind speed in unstable conditions. As a result of these trials, the scheme has been implemented operationally in the mesoscale model.

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A. P. Lock, A. R. Brown, M. R. Bush, G. M. Martin, and R. N. B. Smith
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A. P. Lock, A. R. Brown, M. R. Bush, G. M. Martin, and R. N. B. Smith

Abstract

A new boundary layer turbulent mixing scheme has been developed for use in the UKMO weather forecasting and climate prediction models. This includes a representation of nonlocal mixing (driven by both surface fluxes and cloud-top processes) in unstable layers, either coupled to or decoupled from the surface, and an explicit entrainment parameterization. The scheme is formulated in moist conserved variables so that it can treat both dry and cloudy layers. Details of the scheme and examples of its performance in single-column model tests are presented.

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P. R. A. Brown, A. J. Illingworth, A. J. Heymsfield, G. M. McFarquhar, K. A. Browning, and M. Gosset

Abstract

The purpose of this paper is to assess the potential of a spaceborne 94-GHz radar for providing useful measurements of the vertical distribution and water content of ice clouds on a global scale.

Calculations of longwave (LW) fluxes for a number of model ice clouds are performed. These are used to determine the minimum cloud optical depth that will cause changes in the outgoing longwave radiation or flux divergence within a cloud layer greatear than 10 W m−2, and in surface downward LW flux greater than 5 W m−2, compared to the clear-sky value. These optical depth values are used as the definition of a “radiatively significant” cloud. Different “thresholds of radiative significance” are calculated for each of the three radiation parameters and also for tropical and midlatitude cirrus clouds. Extensive observational datasets of ice crystal size spectra from midlatitude and tropical cirrus are then used to assess the capability of a radar to meet these measurement requirements. A radar with a threshold of −30 dBZ should detect 99% (92%) of “radiatively significant” clouds in the midlatitudes (Tropics). This detection efficiency may be reduced significantly for tropical clouds at very low temperatures (−80°C).

The LW flux calculations are also used to establish the required accuracy within which the optical depth should be known in order to estimate LW fluxes or flux divergence to within specified limits of accuracy. Accuracy requirements are also expressed in terms of ice water content (IWC) because of the need to validate cloud parameterization schemes in general circulation models (GCMs). Estimates of IWC derived using radar alone and also using additional information to define the mean crystal size are considered. With crystal size information available, the IWC for samples with a horizontal scale of 1–2 km may be obtained with a bias of less than 8%. For IWC larger than 0.01 g m−3, the random error is in the range +50% to −35%, whereas for a value of 0.001 g m−3 the random error increases to between +80% and −45%. This level of accuracy also represents the best that may be achieved for estimates of the cloud optical depth and meets the requirements derived from LW flux calculations. In the absence of independent particle size information, the random error is within the range +85% to −55% for IWC greater than 0.01 g m−3. For the same IWC range, the estimated bias is few than ±15%. This accuracy is sufficient to provide useful constraints on GCM cloud parameteriation schemes.

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K. A. Browning, C. G. Collier, P. R. Larke, P. Menmuir, G. A. Monk, and R. G. Owens

Abstract

This paper is concerned with the quantitative forecasting of hourly rainfall for the period 0–6 h ahead using linear extrapolation techniques. It deals with results obtained as part of the Meteorological Office Short Period Forecasting Pilot Project. The primary data used in this study are composite maps of rainfall echo distribution generated automatically and in real time using digital data received from a network of four weather radars covering parts of England and Wales. Forecasts have been derived during a total of 29 frontal rainfall events between November 1979 and June 1980. The forecasts wore derived both subjectively in real time and objectively using a computerized echo centroid tracking technique. The objective procedure, which was used to derive forecasts on a grid of 32 × 32, 20 km squares, is a practical way of quickly producing detailed forecasts for a large, number of target areas but its accuracy suffers from a number of factors. The subjective procedure, which was applied to a single target zone, was used to investigate some of the sources of error and their impact on forecasts. It is shown that radar rainfall measurement errors accounted for as much as half of the errors in the forecasts, and it is suggested that the biggest improvements in forecast accuracy are likely to accrue from improved analysis of the radar data prior to input into the forecast procedure. The radar measurement errors are due more to the variability of echo intensity with height than to straightforward radar calibration difficulties. Subtle procedures are required to identify these errors based on an analysis of the meteorological situation in which the radar data are viewed in the context of other kinds of meteorological information. Factors such as the development and decay of rainfall systems, which lead to the breakdown of the basic assumption underlying the linear extrapolation approach, accounted for about a quarter of the errors in the forecasts.

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J. Ström, R. Busen, M. Quante, B. Guillemet, P. R. A. Brown, and J. Heintzenberg

Abstract

During the pre-EUCREX (European Cloud and Radiation Experiment) intercomparison of airborne instrumentation in January 1992, nine hygrometers mounted on three different aircraft were compared. Although the different instruments are based on completely different principles and the three aircraft have very different flying characteristics, humidity data from both vertical profiles as well as horizontal flight legs showed good agreement. Despite the different aircraft limitations the intercomparison was done with the aircraft in close formation. In terms of relative difference in mixing ratio, most instruments agreed to within ±5% for values down to about 0.1 g kg−1. For mixing ratios between 0.03 and 0.1 g kg−1 most instruments agreed to within ±15%. Systematic differences between the instruments suggest that in joint experiments where data will be shared, the same algorithms for evaluating and converting humidity parameters should be used whenever possible.

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