S. D. Mobbs

The Convective and Orographically Induced Precipitation Study:A Research and Development Project of the World Weather Research Program for Improving Quantitative Precipitation Forecasting in Low-mountain Regions

Abstract The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional...

THE TERRAIN-INDUCED ROTOR EXPERIMENT : A Field Campaign Overview Including Observational Highlights

Abstract The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotem-poral characteristics ...

Profile measurements of blowing snow at Halley, Antarctica

Measurements of blowing snow particle concentration from the second Stable Antarctic Boundary Layer Experiment (STABLE 2) are presented. The measurements, made at Halley Station (75.6 degreesS, 26.7 degreesW) throughout the 1991 austral winter, are supplemented with profile measurements of wind speed, air temperature, and humidity. Threshold wind speeds for blowing snow are shown to be distinctly different for various episodes, often depending strongly on the the availability of loose snow. Blowing snow is measured to occur at Halley between 27 and 37% of the time during winter. Total winter (June, July, August) blowing snow sublimation at Halley is calculated to be around 4.7 mm water equivalent, 3.7% of total accumulation over the same period. Total winter blowing snow mass transport is calculated to be around 5.5 x 10(5) kg per metre width. Measured humidity profiles at Halley show that during winter blowing snow conditions, a layer of near-saturated air forms, causing total sublimation to be less than that for seasonal or limited fetch snow covers. The extent of sublimation is shown to be strongly dependent on wind speed, temperature and fetch.

The surface energy and mass balance at Halley, Antarctica, during winter

We present measurements of the components of the energy and mass balance of the snow surface at Halley Research Station, Antarctica. During the winter months, when insolation is small or zero, the surface energy balance is dominated by radiative cooling. This is mostly balanced by a downward transport of atmospheric sensible heat, with an upward conductive flux of heat through the snowpack making a secondary contribution. The average flux of atmospheric latent heat is downward but of negligible importance in the surface energy balance. During the winter, a significant imbalance is seen in the measured energy budget, with insufficient sensible and conductive heat fluxes to balance the radiative cooling. The wintertime surface mass balance is dominated by precipitation. Sublimation of blowing snow makes a small negative contribution to the budget and is observed to be highly dependent on wind speed. It is suggested that this may be an important mechanism for removing surface mass in some parts of Antarctica.

Experimental studies of strongly stratified flow past three-dimensional orography

Stably stratified flows past three-dimensional orography have been investigated using a stratified towing tank. Flows past idealized axisymmetric orography in which the Froude number, F h = U / Nh (where U is the towing speed, N is the buoyancy frequency and h is the height of the obstacle) is less than unity have been studied. The orography considered consists of two sizes of hemisphere and two cones of different slope. For all the obstacles measurements show that as F h decreases, the drag coefficient increases, reaching between 2.8 and 5.4 times the value in neutral flow (depending on obstacle shape) for F h [les ]0.25. Local maxima and minima in the drag also occur. These are due to the finite depth of the tank and can be explained by linear gravity-wave theory. Flow visualization reveals a lee wave train downstream in which the wave amplitude is O ( F h h ), the smallest wave amplitude occurring for the steepest cone. Measurements show that for all the obstacles, the dividing-streamline height, z s , is described reasonably well by the formula z s / h =1− F h . Flow visualization and acoustic Doppler velocimeter measurements in the wake of the obstacles show that vortex shedding occurs when F h [les ]0.4 and that the period of the vortex shedding is independent of height. Based on velocity measurements in the wake of both sizes of hemisphere (plus two additional smaller hemispheres), it is shown that a blockage-corrected Strouhal number, S 2 c = fL 2 / U c , collapses onto a single curve when plotted against the effective Froude number, F h c = U c / Nh . Here, U c is the blockage-corrected free-stream speed based on mass-flux considerations, f is the vortex shedding frequency and L 2 is the obstacle width at a height z s /2. Collapse of the data is also obtained for the two different shapes of cone and for additional measurements made in the wake of triangular and rectangular at plates. Indeed, the values of S 2 c for all these obstacles are similar and this suggests that despite the fact that the obstacle widths vary with height, a single length scale determines the vortex-street dynamics. Experiments conducted using a splitter plate indicate that the shedding mechanism provides a major contribution to the total drag (∼25%). The addition of an upstream pointing ‘verge region’ to a hemisphere is also shown to increase the drag significantly in strongly stratified flow. Possible mechanisms for this are discussed.

Wind-borne redistribution of snow across an Antarctic ice rise

Redistribution of snow by the wind can drive spatial and temporal variations in snow accumulation that may affect the reconstruction of paleoclimate records from ice cores. In this paper we investigate how spatial variations in snow accumulation along a 13 km transect across Lyddan Ice Rise, Antarctica, are related to wind-borne snow redistribution. Lyddan Ice Rise is an approximately two-dimensional ridge which rises about 130 m above the surrounding ice shelves. Local slopes on its flanks never exceed 0.04. Despite this very smooth profile, there is a pronounced gradient in snow accumulation across the feature. Accumulation is highest on the ice shelf to the east ( climatologically upwind) of the ice rise and decreases moving westward, with the lowest accumulation seen to the west ( climatologically downwind) of the ice rise crest. Superimposed on this broad-scale gradient are large ( 20-30%), localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of less than 0.01. The broad-scale accumulation gradient is consistent with estimates of wind-borne redistribution of snow made using wind speed observations from three automatic weather stations. The small-scale variability in accumulation is reproduced quite well using a snow transport model driven by surface winds obtained from an airflow model, providing that both the wind shear and static stability of the upwind flow are taken into account. We conclude that great care needs to be exercised in selecting ice core sites in order to avoid the possibility of blowing snow transport confounding climate reconstructions.

Observations of an internal gravity wave in the lower troposphere at Halley, Antarctica

Observations of internal gravity waves in the stably-stratified atmospheric boundary layer at Halley, Antarctica are presented. These were made on 1 February, 1986 and take the form of temperature measurements from a 30 m mast and a Sodar record. The temperature record shows a clearly defined, dominant wave period of around 11 min. A high-resolution radiosonde ascent made during the period of wave activity exhibits thin layers of low Richardson number and it is suggested that these are regions of dynamic instability where the waves are generated. A linear stability analysis of the radiosonde data supports this idea. It is argued from simple theoretical ideas and by means of a numerical model that only waves with a wavelength greater than a certain critical value are likely to be observed at the surface. The observations are shown to be consistent with this hypothesis.

The convective and orographically-induced precipitation study

Abstract The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional...

RESEARCH CAMPAIGN: The Convective and Orographically Induced Precipitation Study

Abstract The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional...

Some vorticity theorems and conservation laws for non-barotropic fluids

Some theorems concerning the vorticity in barotropic flows of perfect fluids are generalized for non-barotropic flows. The generalization involves replacing the velocity in certain parts of the equations by a time-dependent quantity which is a function of the velocity and thermodynamic properties of the fluid. Results which are generalized include Kelvins circulation theorem and conservation laws for potential vorticity and helicity. It is shown how the results can be further generalized to include dissipative effects. The possibility of using some of the results in deriving a complete set of Lagrangian conservation laws for perfect fluids is discussed.