Numerical simulations of optical properties of Saharan dust aerosols with emphasis on lidar applications

In the framework of the Saharan Mineral Dust Experiment (SAMUM) for the first time the spectral dependence of particle linear depolarization ratios was measured by combining four lidar systems. In this paper these measurements are compared with results from scattering theory based on the T-matrix method. For this purpose, in situ measurements—size distribution, shape distribution and refractive index—were used as input parameters; particle shape was approximated by spheroids. A sensitivity study showed that lidar-related parameters—lidar ratio   S _p   and linear depolarization ratio  δ _p   —are very sensitive to changes of all parameters. The simulated values of the  δ _p   are in the range of 20% and 31% and thus in the range of the measurements. The spectral dependence is weak, so that it could not be resolved by the measurements. Calculated lidar ratios based on the measured microphysics and considering equivalent radii up to 7.5 μm show a range of possible values between 29 and 50 sr at  λ= 532 nm  . Larger   S _p   might be possible if the real part of the refractive index is small and the imaginary part is large. A strict validation was however not possible as too many microphysical parameters influence   S _p   and  δ _p   that could not be measured with the required accuracy.     

Modification of a Parametrization of Shallow Convection in the Grey Zone Using a Mesoscale Model

In current operational numerical weather prediction models, the effect of shallow convection is parametrized. The grey zone of shallow convection is found between the horizontal resolutions of mesoscale numerical models (2–3 km) and large-eddy simulations (10–100 m or finer). At these horizontal scales the shallow convection is to some extent explicitly resolved by the model. The shallow-convection parametrization is still needed, but has to be regulated according to the model horizontal resolution. Here the behaviour of the non-hydrostatic mesoscale numerical weather prediction model Application of Research to Operations at Mesoscale is examined in the grey zone and a new scale-adaptive surface closure of its shallow-convection parametrization, dependent on horizontal resolution, is defined based on large-eddy simulation. The new closure is tested on a series of numerical experiments and validated on a 15-day-long real case period. Its impact on the development of deep convection is examined in detail. The idealized simulations show promising results, as the mean profiles of the subgrid and resolved turbulence change in the desired way. Based on the real case tests our modification has a low impact on model performance, but is part of a set of upgrades of the current parametrization that is aimed to treat the shallow convection grey zone.     

A method of automatic solar radiation integration with printed records including an assessment of its reliability

Summary Solar radiation intensity has been recorded on a single trace strip chart potentiometer and integrated automatically. The integral can be printed at frequencies from minutes to hours as required. The performance of the integration equipment has been assessed by (1) comparing chart area expected from a calibration of the printed digital output with the area as measured by planimeter and (2) estimating the scatter in the relation of chart area to printed output per hour on a number of days over periods up to a month. The errors (95% limits) are about ±1 cal cm^–2 for hourly values and ±4 cal cm^–2 for daily values.

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Zusammenfassung Die Intensität der Sonnenstrahlung wurde mit einem selbstabgleichenden Millivoltschreiber registriert und automatisch integriert. Das Integral kann nach Bedarf über Zeiten von der Größenordnung von Minuten bis Stunden gebildet werden. Die Leistungsfähigkeit des Integrators wurde abgeschätzt 1. durch Vergleich der auf Grund der Eichung des Integrators aus seinen Angaben errechneten Fläche unter der Registrierkurve mit der planimetrierten Fläche und 2. durch Abschätzung der Streuung der Werte des Zusammenhanges zwischen den pro Stunde angegebenen Integratorwerten und der Fläche auf dem Registrierstreifen an einer Anzahl von Tagen aus Perioden von einer Dauer bis zu einem Monat. Der Fehler (bei einer Genauigkeitsgrenze von 95%) liegt bet ±1 cal cm^–2 für Stundensummen und bei ±4 cal cm^–2 für Tagessummen.

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Résumé L'intensité du rayonnement solaire fut enregistrée par un millivoltmètre compensateur et intégrée automatiquement. L'intégration peut s'étendre à des temps allant de la minute à plusieurs heures. L'exactitude de l'intégrateur fut estimée 1) en comparant la surface résultant de ses indications à celle que l'on planimètre, et 2) en calculant la dispersion des écarts entre les valeurs données par heure par l'intégrateur et celles qui résultent de la planimétrie, cela pour un certain nombre de jours choisis dans des périodes pouvant atteindre le mois. L'erreur (pour un domaine de validité de 95%) est de ±1 cal cm^–2 pour des sommations horaires, de ±4 cal cm^–2 pour des sommations diurnes.

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With 4 Figures     

Some evaluations of drag and bulk transfer coefficients over water bodies of different sizes

Three recent experiments allow evaluation of the bulk transfer coefficients for momentum, water vapour and sensible heat over water bodies of different sizes. As part of a study of evaporation rates from a swamp, measurements of latent and sensible heat fluxes were made over Lake Wyangan in southern N.S.W., Australia. This lake is of several kilometers diameter. In a later experiment, Reynolds stress and sensible heat transfer were measured from a natural-gas platform standing in Bass Strait, south of mainland Australia. The most recent experiment involved the direct measurement of each of these turbulent fluxes from a fixed tower erected in Lake Michigan, U.S.A.Perhaps the most important of the results is the finding that drag coefficients measured over Bass Strait are not significantly different from those over Lake Michigan, despite the obvious differences in depth, fetch, and hence surface wave structure. At both locations, drag coefficients are found to increase slightly with increasing wind speed, while at low wind speeds they are not significantly different from those corresponding to aerodynamic smoothness.Near-neutral bulk transfer coefficients for sensible heat and for water vapour are found to be similar. An average value of about 1.4 × 10^–3 is obtained.It is emphasized that stability effects should be considered in any discussion of drag coefficients or bulk transfer coefficients. Large errors can result if near-neutrality is incorrectly assumed.     

Climatological aspects of the tropical convective boundary layer

The characteristics of a boundary layer depend both on conditions at the surface and in the interior of the medium. In the undisturbed tropics, the latter are largely determined by subsidence and by infrared radiational cooling. One-dimensional models are used to establish relationships between the inversion height, subsidence, upper-air humidity and sea-surface temperature. In particular, it is shown that a universally colder tropical ocean would probably be covered by more extensive clouds.Contribution No. 1700 Rosenstiel School of Marine and Atmospheric Science, University of Miami.     

Vertical cloud structures of the boreal summer intraseasonal variability based on CloudSat observations and ERA-interim reanalysis

The boreal summer intraseasonal variability (BSISV), which is characterized by pronounced meridional propagation from the equatorial zone to the Indian Continent, exerts significant modulation of the active/break phases of the south Asian monsoon. This form of variability provides a primary source of subseasonal predictive skill of the Asian summer monsoon. Unfortunately, current general circulation models display large deficiencies in representing this variability. The new cloud observations made available by the CloudSat mission provide an unprecedented opportunity to advance our characterization of the BSISV. In this study, the vertical structures of cloud water content and cloud types associated with the BSISV over the Indian Ocean and subcontinent are analyzed based on CloudSat observations from 2006 to 2008. These cloud structures are also compared to their counterparts as derived from ERA-interim reanalysis. A marked vertical tilting structure in cloud water is illustrated during the northward propagation of the BSISV based on both datasets. Increased cloud liquid water content (LWC) tends to appear to the north of the rainfall maximum, while ice water content (IWC) in the upper troposphere slightly lags the convection. This northward shift of increased LWC, which is in accord with local enhanced moisture as previously documented, may play an important role in the northward propagation of the BSISV. The transition in cloud structures associated with BSISV convection is further demonstrated based on CloudSat, with shallow cumuli at the leading edge, followed by the deep convective clouds, and then upper anvil clouds. Some differences in cloud water structures between CloudSat and ERA-interim are also noted, particularly in the amplitudes of IWC and LWC fields.     

A multifractal analysis of lidar measured water vapour

Accurate and fast-response measurements of space-time observations of specific humidity were made above a drying land surface at the University of California at Davis, using the Los Alamos water Raman-lidar. In an attempt to quantify the space-time intermittency features of turbulent flows in the lower atmosphere, a multifractal analysis of these water vapour measurements was performed. The structure of the specific humidity, (x, t), was analyzed quantifying a scalar gradient measure both in time and space, for all possible one-dimensional cuts, i.e. and . The results confirm the multifractal nature of this scalar gradient measure (a type of scalar dissipation rate) and show that humidity measurements at fixed times (x) are more intermittent (e.g. have less entropy dimension) than those at fixed locations in space (t). Similar multifractal behaviour of the spatial data, with and without a transformation from the observed wind velocities, supports the validity of Taylor's hypothesis for the studied fields.     

Characteristics of air flow above and within soybean canopies

Air flow was observed above and within canopies of a number of kinds of soybeans. The Clark cultivar and two isolines of the Harosoy cultivar were studied in 1979 and 1980, respectively. Wind speed above the canopy was measured with cup anemometers. Heated thermistor anemometers were used to measure air flow within the canopy. Above-canopy air flow was characterized in terms of the zero-plane displacement (d), roughness parameter (z _o) and drag coefficient (C _d). d and z _o were dependent on canopy height but were independent of friction velocity in the range 0.55 to 0.75 m s^?1 · C _d for the various canopies ranged from 0.027 to 0.035. Greater C _d values were measured over an erectophile canopy than over a planophile canopy. C _d was not measurably affected by differences in leaf pubescence. Within-canopy wind profiles were measured at two locations: within and between rows. The wind profile was characterized by a region of great wind shear in the upper canopy and by a region of relatively weak wind shear in the middle canopy. Considerable spatial variability in wind speed was evident, however. This result has significant implications for canopy flow modeling efforts aimed at evaluating transport in the canopy. In the lower canopy, wind speed within a row increased with depth whereas wind speed between two rows decreased with depth. The wind speeds at the two locations tended to converge to a common value at a height near 0.10 m. The attenuation of within-canopy air flow was stronger in canopies with greater foliage density. Canopy flow attenuation seemed to decrease with increasing wind speed, suggesting that high winds distorted the shape of the canopy in such a manner that the penetration of wind into the canopy increased.     

Observing and Simulating Wind-Turbine Wakes During the Evening Transition

Wind-turbine-wake evolution during the evening transition introduces variability to wind-farm power production at a time of day typically characterized by high electricity demand. During the evening transition, the atmosphere evolves from an unstable to a stable regime, and vertical stratification of the wind profile develops as the residual planetary boundary layer decouples from the surface layer. The evolution of wind-turbine wakes during the evening transition is examined from two perspectives: wake observations from single turbines, and simulations of multiple turbine wakes using the mesoscale Weather Research and Forecasting (WRF) model. Throughout the evening transition, the wake’s wind-speed deficit and turbulence enhancement are confined within the rotor layer when the atmospheric stability changes from unstable to stable. The height variations of maximum upwind-downwind differences of wind speed and turbulence intensity gradually decrease during the evening transition. After verifying the WRF-model-simulated upwind wind speed, wind direction and turbulent kinetic energy profiles with observations, the wind-farm-scale wake evolution during the evening transition is investigated using the WRF-model wind-farm parametrization scheme. As the evening progresses, due to the presence of the wind farm, the modelled hub-height wind-speed deficit monotonically increases, the relative turbulence enhancement at hub height grows by 50%, and the downwind surface sensible heat flux increases, reducing surface cooling. Overall, the intensifying wakes from upwind turbines respond to the evolving atmospheric boundary layer during the evening transition, and undermine the power production of downwind turbines in the evening.