Cloud Motion Winds Research Articles

Spatial scale dependency of errors in satellite cloud track winds

Abstract Low level satellite cloud motion winds (CMWs) have been generated from METEOSAT data for Intensive Observing Periods (IOPs) during the FRONTS87 experiment of the Mesoscale Frontal Dynamics Project. CMWs were produced for a range of template sizes, ranging from 4∗4 infra-red pixels to 32∗32 infra-red pixels (note that in the area of interest an infra-red pixel is approximately 5∗10 km: in addition to templates comprising equal numbers of pixels on each side, winds were generated using templates comprising twice as many pixels in the E-W direction as in the N-S). During the IOPs the wind field was also observed using, among other facilities, dropsondes. The satellite winds were then compared with an objective analysis of the dropsonde data, which was averaged to the scale of the satellite winds in question. The relationship between template geometry (in infra-red pixels) and rms vector error (in m/s) is shown in Table 1. The large error for small template sizes occurs because noise in the radiances significantly affects the position of the peak in the correlation surface. The increase of error with template size at large template sizes occurs because the satellite wind is not representative of the wind field in the whole of the template. On the largest scale, satellite winds may represent the translation of synoptic scale weather systems rather than meteorological wind fields.

Salellite observations for global modelling

Since 1969 passive infrared and microwave radiometers have been used to provide vertical temperature and moisture profiles for Numerical Weather Prediction. The forecast impact in the Southern Hemisphere are generally large and positive, however in the Northern Hemispheric, where there is more conventional meteorological observations, the impact of the satellite profiles varies depending on the synoptic situation and has been both positive and negative. The major research effort is directed towards better use of satellite radiance measurements. Cloud Motion Winds also provide a large source of additional meteorological observations, especially in the tropical areas where there are few other upper level wind measurements. The impact of these data is generally positive in the Tropics and the Southern Hemisphere, however some problems have been found in the Northern Hemisphere. There has been a large attempt at ECMWF to improve the quality control of cloud track winds.

Verification of cloud motion winds at global scale

Cloud motion wind data from geostationary satellites are used at global scale for numerical weather prediction by the meteorological community. This paper presents verification activities for these data at international level and results from studies at EUMETSAT. Wind data originating from the different geostationary satellites have different qualities. Improvement of the global cloud wind data set seems necessary to improve their overall impact on numerical weather forecasting at global scale. EUMETSAT proposes the setting up of an international working group to coordinate further efforts between groups active in the field of cloud motion wind extraction.

Operational production of winds from cloud motions

Operational production of NOAA cloud motion winds is reviewed. A new technique to improve the height assignment of the cloud motion winds is described and statistical verification of satellite wind information is discussed. Higher quality satellite wind data are a valuable tool to improved descriptions of global atmospheric circulation, which is an important constituent in studies of climate and global change.

Cloud motion winds from METEOSAT: performance of an operational system

Wind fields have been estimated from the displacement of clouds in successive METEOSAT infrared (IR: 10.5–12.5 μm) images for more than a decade. The derivation of cloud motion winds (hereafter: CMW) workss fully automatically, man-machine interaction is only performed as the very last step of manual quality control. At present about 3000 wind vectors per day are produced with four production runs and disseminated via the Global Telecommunication System (GTS) mainly for use in the analysis for numerical weather prediction. In this paper the improvements to the CMW derivation from METEOSAT images are described. In particular the height assignment of a wind vector and radiance filtering techniques preceding the cloud tracking have ameliorated the errors in METEOSAT winds significantly. The low speed bias of high level CMWs (< 400 hPa) in comparison with radiosonde winds has been reduced by more than 60% since 1987. This has significantly enhanced the usefulness of CMWs for the global analysis/forecast systems and hence their potential for climate diagnostic studies.

Observations of mid-troposhheric circulation from water vapour radiances of polar orbiting satellites

Water vapour tracers can provide useful information on winds at ≈ 500mb by observing the 6·7μ radiances. This fills the data gap in the cloud motion winds provided by conventional meteorological geostationary satellites. There is no geostationary satellite at present over the Indian Ocean with 6·7μ imaging capability to provide mid-tropospheric winds. The potentials of 6·7μ radiances, available from polar orbiting satellites, for mid-tropospheric circulation features have been examined in this study. Tiros-N satellite data of May 1979 and ECMWF level-IIIb wind data were analysed to relate the radiances with the streamlines. We find that the radiances of 6·7μ from orbiting satellites agree well with the wind field.

Cloud observations from METEOSAT and the inference of winds

At the European Space Operations Centre (ESOC) the raw image data from the geostationary meteorological satellite METEOSAT are received, preprocessed and operational products are derived. Most products directly or indirectly make use of cloud information retrieved by a multispectral image analysis. The purpose of the paper is to describe the cloud retrieval and the subsequent use of the cloud information in the derivation of other products. In particular we discuss the production of cloud motion winds (CMW) based on an automatic cloud tracking and the estimation of the outgoing longwave radiation (OLR) from a condensed data set which is the direct output of the multispectral analysis. Results are presented for the monthly mean OLR and the longwave cloud radiative forcing for July 1983.

A bias in the satellite‐observed low‐level cloud motion winds over the central tropical Pacific

Island wind data from the central tropical Pacific are compared with satellite‐observed low‐level cloud motion winds over the same area during 1976–1980. The purpose of the comparison is to investigate the effect of gaps in the satellite wind series. It is found that the distribution of the gaps is such that the resulting time series is biased from the complete time series because of undersampling of the strongest westerly and northerly wind anomalies. This bias is strong enough, at least in this geographical region, to cause difficulties in estimating accurate mean fields from the cloud motion winds alone. The gaps leading to this bias occur on time scales of days to weeks and are most common during the northern hemisphere winter. Further, the gaps can be attributed to the presence of heavy, high cloud cover typical of strong convective events. The bias follows from the fact that the wind anomalies are also strongest at these times.

The Effect of the Arbitrary Level Assignment of Satellite Cloud Motion Wind Vectors on Wind Analyses in the Pre-thunderstorm Environment

The impact of satellite-derived cloud motion vectors on SESAME rawinsonde wind fields was studied in two separate cases. The effect of wind and moisture gradients on the arbitrary assignment of the satellite data is assessed to coordinate surfaces in a severe storm environment marked by strong vertical wind shear. Objective analyses of SESAME rawinsonde winds and combined winds are produced and differences between these two analyzed fields are used to make an assessment of coordinate level choice. It is shown that the standard method of arbitrarily assigning wind vectors to a low level coordinate surface yields systematic differences between the rawinsonde and combined wind analyses. Arbitrary assignment of cloud motions to the 0.9 sigma surface produces smaller differences than assignment to the 825 mb pressure surface. Systematic differences occur near moisture discontinuities and in regions of horizontal and vertical wind shears. The differences between the combined and SESAME wind fields are made smallest by vertically interpolating cloud motions to either a pressure or sigma surface.

Hurricane Debby—An Illustration of the Complementary Nature of VAS Soundings and Cloud and Water Vapor Motion Winds

The utility of VISSR Atmospheric Sounder (VAS) temperature and moisture soundings and cloud and water vapor motion winds in defining a storm and its surroundings at subsynoptic scales has been examined using a numerical analysis and prognosis system. It is shown that the VAS temperature and moisture data, which specify temperature and moisture well in cloud-free areas, are complemented by cloud and water vapor motion data generated in the cloudy areas. The cloud and water vapor 'winds' provide thermal gradient information for interpolating the soundings across cloudy regions. The loss of analysis integrity due to the reduction of VAS sounding density in the cloudy regions associated with synoptic activity is ameliorated by using cloud and water vapor motion winds. The improvement in numerical forecasts resulting from the addition of these data to the numerical analysis is recorded.

Improved Cloud Motion Wind Vector and Altitude Assignment Using VAS

Abstract A CO2 cloud tracking technique to determine simultaneous heights and velocities of cloud motion winds is presented. Using animated CO2 channel imagery from VAS, multi-level cloud situations are separated into high, middle and low level cloud motion wind vectors by the CO2 slicing method. The VAS CO2 channel radiometric values are used in the CO2 absorption method to assign quantitative heights to the cloud vectors; cloud top pressures are determined from the ratio of the deviations in cloud produced radiances and the corresponding clear air values for three CO2 channels in a radiative transfer equation formulation. Two case studies are presented that show CO2 cloud-motion wind vectors to be in good agreement with radiosonde wind observations and CO2 cloud heights to be within a 50 mb rms deviation of radiosonde, bispectral and stereo height determinations.

A comparison of cloud motion and ship wind observations over the Indian Ocean for the year of FGGE

Cloud motions over the Indian Ocean were compared to ship observations for the FGGE year. The statistics of this comparison show seasonal changes in the cloud-ship relationship as well as geographical and wind-pattern-dependent fetch history changes. Most of these changes follow simple boundary-layer relationships governed by friction and temperature advection. The most significant result is the improvement of the cloud-ship directional shear with wind speed. The mean veering angle between cloud and ship measurements decreased at higher wind speeds along with scatter of the shearing angle. This implies that the ability of cloud motion measurements to indicate the wind stress on the ocean improves for the important situations when the winds are strong.

Some Statistical Characteristics of Cloud Motion Winds Measured over the Indian Ocean during the Summer Monsoon

Abstract Cloud motions over the Indian Ocean for May–July 1979 were used to obtain spatial auto correlations of the deviations of the wind components from local means. Best correlations were associated with u′, low altitude clouds and alongwind displacements. Worst correlations arose from v′, high clouds and crosswind displacements. The crosswind anisotropy was ∼15%. All correlations were 0.49 or greater at 5° separation or less.

U.S. FGGE special data sets and special effort

Three topics are considered, all resulting from the desire to maximize the utilization and overall quality of the FGGE data sets. First, the production of Level III-b oceanographic and meteorological analyses in the FGGE format has been undertaken by the Fleet Numerical Oceanography Center. The oceanographic data, in particular the sub-surface oceanographic data, is a unique addition to the FGGE data base. Next, the FGGE Level II-b data set is the most comprehensive set of synoptic-scale meteorological data ever assembled to date. Under development is a program aimed at compacting and restructuring the Level II-b data according to observing systems, so that the data will be more readily usable and more economical for users interested in smaller and/or different portions of the total FGGE Level II-b data set. Lastly, as a result of the United States pre-FGGE Data Systems Tests, a need was indicated for a special effort to edit and enhance satellite soundings and cloud motion wind data during FGGE. A formal program to carry out such an effort has been established jointly by NASA, NOAA, and the University of Wisconsin.

A “Special Effort” to Provide Improved Sounding and Cloud-Motion Wind Data for FGGE

Enhancement and editing of high-density cloud motion wind assessments and research satellite soundings have been necessary to improve the quality of data used in The Global Weather Experiment. Editing operations are conducted by a man-computer interactive data access system. Editing will focus on such inputs as non-US satellite data, NOAA operational sounding and wind data sets, wind data from the Indian Ocean satellite, dropwindsonde data, and tropical mesoscale wind data. Improved techniques for deriving cloud heights and higher resolution sounding in meteorologically active areas are principal parts of the data enhancement program.

A Comparison of Cloud Motion Winds from ATS 6 Images with Coinciding SMS 1 Winds

A methodology is developed for accurate measurement of cloud motion winds from the geosynchronous ATS 6 image data. Attitude changes between consecutive images (as a function of scan-line number) are accounted for in wind computations through measurement of the earth-edge displacements between the successive infrared images. Also, an image matching procedure is used to remove obvious and distracting image distortions. The availability of SMS imagery coinciding with ATS 6 imagery makes SMS an excellent reference against which the quality of ATS 6 winds can be tested. The resulting winds inferred from cloud displacement measurements taken from a sequence of the corrected images are found to agree better than 2 m/sec rms with winds measured from coincident SMS 1 imagery.

Wind Estimates from Cloud Motions: Results from Phases I, II and II of anIn SituAircraft Verification Experiment

An experiment is in progress to verify geostationary-satellite-derived cloud-motion wind estimates by in-situ aircraft wind-velocity measurements. One or more low-level aircraft equipped with Inertial Navigation Systems (INS) were used to define the vertical extent and horizontal motion of a cloud and to measure the ambient wind field. A high-level aircraft, also equipped with an INS, took photographs to describe the horizontal extent of the cloud field and to measure cloud motion. To date the experiment has been conducted over tropical oceans and in the western Gulf of Mexico. A total of 60 h have been spent tracking some 40 tropical cumulus and five cirrus clouds. Results for tropical cumulus clouds indicate excellent agreement between the cloud motion and the wind at cloud base. The magnitude of the vector difference between the cloud motion and the cloud-base wind is less than 1.3 m/s for 67% of the cases with track lengths of 1 h or longer. Similarly, the vector differences between the cloud motion and the wind at sub-cloud (150 m), mid-cloud, and cloud-top levels are 1.5, 3.6 and 7.0 m/s, respectively. The cirrus cloud motions agreed best with the mean wind in the cloud layer with a vector difference of about 1.6 m/s.

Wind Estimates from Cloud Motions: Phase 1 of anin situAircraft Verification Experiment

An initial experiment was conducted to verify geostationary satellite derived cloud motion wind estimates with in situ aircraft wind velocity measurements. Case histories of one-half hour to two hours were obtained for 3-10km diameter cumulus cloud systems on 6 days. Also, one cirrus cloud case was obtained. In most cases the clouds were discrete enough that both the cloud motion and the ambient wind could be measured with the same aircraft Inertial Navigation System (INS). Since the INS drift error is the same for both the cloud motion and wind measurements, the drift error subtracts out of the relative motion determinations. The magnitude of the vector difference between the cloud motion and the ambient wind at the cloud base averaged 1.2 m/sec. The wind vector at higher levels in the cloud layer differed by about 3 m/sec to 5 m/sec from the cloud motion vector.

EOLE Balloon and ATS Cloud Motion Comparisons

Comparisons are made between winds obtained from EOLE balloons and winds determined from cloud motions as observed from the Applications Technology Satellites, ATS-1 and ATS-3. The results show that the mean winds determined from EOLE balloons are greater in speed than mean winds derived from cloud motion observations. Inspection of the total set of comparisons lead one to conclude that the cloud motion winds in the 20–50°S latitude band apply to a level lower than 200 mb. The mean differences become smaller toward the equator and the results further suggest that the cloud motion winds apply to altitudes higher than 200 mb equatorward of 20°S.