The ultrafast aircraft thermometer, built for measuring temperature in clouds at flight speeds up to 100 m s(-1), employs a 2.5-mu m-thick platinum-coated tungsten wire as a sensing element. When temperature increases, the wire resistance increases. The changes are amplified by an electronic system. Temperature measurements made in a wind tunnel and during flights show noise that is related to the von Karman vortex street generated behind the shield that protects the sensing element against the impact of cloud droplets. To reduce both the level of turbulence and the amount of water collected on the shield, suction is applied through the slits in its sides. The effect of suction on the flow field is twofold. First, at the Reynolds numbers that the thermometer is operated suction eliminates aerodynamic disturbances. Second, suction diverts the inner part of the boundary layer into the slit. This inner part is a region of strong shear and, therefore, a region where intensive viscous heating takes place. When the suction is on much of the air that is heated in the boundary layer in the front part of the shield is removed through the slits and never reaches the sensor. To study the role of the shield with suction and confirm its chosen shape, two-dimensional (2D) direct numerical simulations (DNSs) are performed of the airflow and of the trajectories of droplets of various sizes and initial positions. The influence on the temperature distribution of the irreversible dissipation of energy due to air viscosity is also examined. This is found to have a small but measurable effect. The effects associated with sampling and processing of the analog signal obtained from the sensing wire are discussed. The results herein quantitatively explain the nature of the measured aerodynamic noise.

Small-scale inhomogeneities of the atmospheric temperature field are caused by air turbulence and result in refractive index fluctuations, which in turn influence the propagation of optical beams. Understanding small density fluctuations in the atmosphere is important for the free-space laser communication and for high-resolution imaging through the atmosphere. The ultra-fast aircraft resistance thermometer constructed in the Institute of Geophysics, Warsaw University, measures the temperature of cloudless air and of warm clouds with 10 kHz sampling frequency. During a flight at the speed of 100 m/s, at low altitudes up to 2 km, this corresponds to the spatial resolution of the order of one centimeter. This resolution is sufficient for studying small density fluctuations in the atmospheric boundary layer. A streamlined shield protects the sensing wire of the thermometer from cloud droplets and other small particles suspended in the air but introduce aerodynamic disturbances in the form of vortices. The thermometer records the resulting fluctuations of temperature as noise. The shield sucks air and water collected on its surface through the suction slits. This suction also suppresses the disturbances. In this paper we analyze how the temperature measurements are influenced by: (i) turbulence generated behind the shield placed in front of the sensing wire; (ii) suction of air through the shield slits; (iii) cloud droplets of various space distributions, masses and velocities. We have carried out the 2D numerical simulations of the time-dependent, incompressible, viscous flow (the Navier-Stokes equation) around the shield placed in a uniform stream. We solved the particle path equations for an ensamble of droplets in the Stokes approximation. All the simulations are oriented toward optimization of the shield shape in order to (i) reduce noise in measurements at low and high altitudes and (ii) protect the sensing wire against ice crystals in flights at high altitudes.

The recently improved ultra-fast aircraft resistance thermometer measures with a time constant of the order 0.1 ms. For an aircraft speed of 100 m/s this time constant corresponds to a spatial resolution of a few centimeters. Measurements made both in the atmosphere and in the low-turbulence wind tunnel at air speed 80 m/s are corrupted with noise of a few kHz frequency. Authors of the thermometer suggest that this noise results from turbulence introduced by vortex shedding from the protective shield. To achieve further improvement of the instrument we have to understand the nature of these aerodynamic disturbances. The present study is carried in two complementary directions. In the first, flow modeling is made with the FEATFLOW 1.2d - a finite element software for the incompressible Navier-Stokes equations. The results of flow simulation are in qualitative agreement with the experiment. In the second, we simulate visualization of the flow using two optical spatial filters: the Foucault filter that gives output intensity signal where bright bias is modulated with 1-D Hilbert transform of an object phase function and modified Zernike phase filter that shifts phase of the spectrum dc term by 0.2pi.

A number of techniques to track rainfall patterns by use of radar observations have been developed over the years. We present a method for radar-echo tracking based on Hu invariant moments. The method has been tried on several sequences of test images, and the derived displacement fields were in good agreement with the real motions of the tested objects. For the real data obtained from the conventional meteorological radar in Legionowo the method occasionally failed when changes in the radar echo between observations were too large.

A new version of an ultrafast aircraft resistance thermometer (UFT-F) with a time constant of the order 10(-4) s, for use in both cloudy and cloudless air, is described. It evolved from an earlier version (UFT-S). Its sensing element is similar to that in UFT-S and consists of a 5-mm-long and 2.5-mum-thick platinum-coated tungsten wire, located on a rotatable vane behind a thin vertical rod that protects the sensor against direct impact of cloud droplets and other objects. Such construction introduces much smaller thermal disturbances than do more massive housings of other types of immersion thermometers and permits taking full advantage of low thermal inertia of the sensing wire. However, aerodynamic disturbances created by vortex shedding from the protective rod induce adiabatic fluctuations of temperature, which appear on the temperature records as "noise." In the case of the UFT-S the level of this noise has become intolerable at airspeeds of about 40 m s(-1), limiting applicability of this instrument to slow aircraft or gliders. For UFT-F the shape of the protective rod has been redesigned and endowed with a special system of reducing aerodynamic disturbances behind it, which made it usable at airspeeds up to 100 m s(-1) in cloudless air or warm clouds. For use in supercooled clouds, a special variety of UFT-F (denoted here UFT-D) has been designed. As in its predecessor, its sensing element is a 5-mm-long, 2.5-mum-thick, platinum-coated tungsten resistive wire protected against impact of cloud droplets by an airfoil-shaped rod, but all its icing-sensitive parts are electrically heated to prevent buildup of ice. This modification required a total change of mechanical structure of the instrument. Tests during the Third Canadian Freezing Drizzle Experiment showed that UFT-D can perform fairly well in water clouds supercooled down to at least -8 degreesC and that its heating system introduces no intolerable disturbances into the record. Use of UFT-D in ice or mixed clouds is limited by the fact that the protective rod is not effective enough against ice crystals bigger than about 200 mum, which can quickly destroy the delicate sensing element.The paper gives details of construction as well as results of wind tunnel and in-flight tests of these instruments.

A new aircraft device for measuring temperature in clouds is described. Its sensor is a resistance thermometer made of platinum-coated tungsten wire 5 mm long and 2.5 mu m in diameter. The sensor is located on a rotatable vane behind a thin rod aimed at protecting it against the impact of cloud droplets, which according to limited experience gathered until now seems to be sufficiently effective as an antiwetting protection for the speeds of motorgliders. Contrary to the massive housings usually adopted in other constructions, the rod creates only negligible disturbances in the thermodynamic properties of the ambient air. The time constant of the sensor is of the order 10(-4) s, which permits measurements of temperature id clouds with a resolution of a few centimeters, depending on aircraft velocity. The thermometer was tested in a wind tunnel, and on an Ogar motorglider and a Do-228 aircraft. Its present version performs fairly well at low airspeeds of up to about 40 m s(-1). For faster aircraft further improvements seem necessary. The paper presents a detailed description of the instrument, discussion of test results, and examples of centimeter-scale features of temperature fields in clouds measured with the thermometer.

Preliminary results of the temperature measurements in Cumulus mediocris clouds with a new ultra-fast airborne thermometer installed on a motor-glider are presented. The time constant of the thermometer is about 2 × 10−4 s but due to limitations of the recorder the data were conditioned with the use of a 300 Hz low-pass filter and sampled with 999 Hz rate, giving spatial resolution of 3 cm at 30 m/s cruising speed. Records taken whilst penetrating various parts of Cumulus clouds on 23 and 25 August 1994 in the vicinity of Warsaw show a number of unexpected features such as single and multiple sharp jumps of temperature over layers only a few centimetres thick. Measurements of this kind are throwing a new light on the process of mixing in convective clouds.

A new instrument for measuring the temperature of air in clouds from an aircraft is described. The instrument uses a very fine thermometric sensor located behind a thin rod that protects the sensor from direct contact with cloud droplets. Because the airflow is less disturbed than in more massive protective housings, the accuracy and speed of the sensor are relatively less affected than in other types of airborne contact thermometers. Details of the construction of a prototype unit and results of preliminary flight and wind tunnel tests are given.

Aircraft observations of stack and cooling tower plumes taken at a big power plant are compared with corresponding outputs of one-dimensional plume model ALINA, yielding certain improvements to the entrainment parametrization and dynamics of the model. Some observations of plume-plume and plume-environment interactions are reported.

A mechanism of rainfall augmentation in the lee of big (~ 5000 MWe) power plants due to washout of liquid water content of cooling tower plumes by raindrops, has been studied by means of a one-dimensional plume model. The relative rainfall increase beneath the plume was estimated for various meteorological conditions and total outputs of cooling tower system. Usually the rainfall increase, doesn't exceed 20–40% and extends not more than 5–7 km from the power plant. In favourable conditions rainfall may however increase even 70 % or more over a large area. In such cases the amount of additionally deposited water may several times exceed the total emission from cooling towers. The total, yearly or seasonal increase of rainfall, which is much smaller due to variability of the wind direction, depends on local climatological conditions.

The mechanism of maintenance of cold steady downdrafts in convective clouds is investigated in a numerical experiment using a one-dimensional steady state model of the updraft-downdraft interaction. It is established that usually the downdraft cannot be effectively chilled unless it is fed with small droplets from the updraft by at least two simultaneously acting mechanisms: entrainment of the updraft air and fallout of drizzle-size droplets through the inclined updraft-downdraft interface. DOI: 10.1111/j.2153-3490.1980.tb00978.x