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Olli Turpeinen

Abstract

Cloud interactions and merging process in pairs of moderate-sized convective cells am studied using radar data with 5 min resolution from day 261 of GATE, in connection with a three-dimensional cloud model. The radar data indicate that most of the clouds are so-called parallel cells, in that the two clouds lie along a line parallel to the wind shear vector. This vector, obtained by subtracting the lower level wind from the upper level wind, is from the north in the layer up to 3 km. The two elements of the parallel pairs of cells appear simultaneously in half of the cases. Within the simultaneous cells, the northern echo, called the upshear cell, tends to be stronger than the southern echo, the downshear cell. If, however, the two cells do not appear simultaneously, the downshear cell usually develops earlier and also becomes stronger than the upshear cell.

A number of numerical simulations initiated with a pair of impulses with varying spacing, intensity and timing are carried out. The numerical results are in fair agreement with the radar observations. Except for the parallel upshear cell, all the cells are suppressed. The suppression can be attributed to the circulation of the adjacent cell forcing the inner downdrafts to develop inside the lateral boundaries of the clouds. In contrast, the upshear cell behaves like an isolated one, due to the increased moisture flux from the direction of the downshear cell. The sensitivity tests on the varying timing and intensity of the two impulses show that neither the use of non-simultaneous nor non-identical impulses promote merging. On the contrary, the minimum edge-to-edge separation between the echoes is larger than that in the simulations with simultaneous and identical impulses.

Merging is found to have a considerable influence on cloud development. Both the radar observations and numerical simulations show a substantial increase in the maximum area, maximum echo top and maximum reflectivity factor of the echoes as a result of the merging process. The numerical experiments indicate that the perturbation pressure structure caused by precipitation, downdrafts and the formation of a cloud bridge, a vertically thin cloudy area connecting the neighboring clouds, is crucial to trigger echo merging.

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Olli Turpeinen

Abstract

No abstract available.

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Olli M. Turpeinen

Abstract

A frontal development observed during the CASP (Canadian Atlantic Storm Project) project is used to investigate the dependence of the spinup time of the divergent circulation on (i) the initial humidity held, (ii) the vertical structure of the latent-heating profile, and (iii) the precipitation rates used in the diabatic initialization. Five 12-hour simulations using the Canadian regional finite-element (RFE) model with diabatic initialization are carried out: one control and four test simulation with varying initial conditions. One experiment uses the analyzed relative humidity in lieu of the satellite-inferred humidity enhancement, two simulations use rain rates reduced to half and zero, and in a fourth experiment the condensation scheme of the model is replaced by the moist adiabatic approach (generation friction) to determine the initial rain rates.

The results indicate that in spite of the diabatic initialization, the spinup times remain long (6–9 hours) if no humidity enhancement is applied where latent heat is released. The accuracy of the specified rain rates is not as crucial as the use of the satellite-inferred humidity: a reduction of the rain rates by a factor of 2 only results in a modest increase of the spinup time. The identification of rain areas is, however, essential, since the total absence of precipitation (i.e., an adiabatic initialization with a humidity enhancement) leads to a 9-hour long spinup. The condensation scheme used in the initialization does not seem to be of prime importance, as the shortening of the spinup process remains practically unchanged regardless of whether the model condensation scheme or the generation function is used.

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Olli Turpeinen
and
Man Kong Yau

Abstract

An analysis of 5 min resolution Quadra data on day 261 of GATE, (0953–1451 GMT) is made to yield statistics of maximum area, echo top, lifetime and maximum reflectivity factor in medium-sized convective cells. The results, obtained by tracking 140 echoes throughout their lifetime, indicate that the maximum area is log-normally distributed, 90% of the echoes being smaller than 40 km2 and existing less than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBZ, respectively. Further stratification of the data according to minimum edge to edge separation (d) reveals that merging cells (d = 0) have an average lifetime three times longer and a maximum area five times larger than isolated ones (d > 7 km). For a fixed maximum area, however, echo parameters generally decrease with decreasing d.

A fully three-dimensional cloud model including precipitation processes is used to simulate the development of an isolated and two adjacent cells. Comparison of modeled and observed echo parameters indicates a fair degree of realism in the simulations. The computed maximum reflectivity factor, however, is considerably higher than that of the observations because of the unrealistic drop-size distribution assumed in the model. Results of two cloud simulations suggest that the alignment of the clouds in relation to the wind-shear vector is an important factor in addition to d in determining the intensity of cloud development. The upshear cell of the parallel clouds, even with a small d value, behaves similarly to an isolated one. The suppression experienced by adjacent cells is attributed to the reduced low-level moisture convergence.

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Johannes Schmetz
and
Olli M. Turpeinen

Abstract

A retrieval method is described for estimating a mean column value of the upper tropospheric relative humidity (UTH) from radiance measurements in the 6.3 μm channel of the geostationary satellite METEOSAT. The physical retrieval method is based on an efficient radiative transfer scheme which uses the temperature forecast profiles from the European Centre for Medium Range Weather Forecasts (ECMWF) as ancillary data. Theoretical radiances for the given temperature profile and a set of fixed upper tropospheric humidities are employed to- relate the observed radiance to a mean humidity for a layer between 600 and 300 hPa. The retrieval is confined to areas with neither medium-nor high-level clouds.

A calibration procedure of the 6.3 μm channel is described which uses the radiative transfer scheme with measured radiosonde profiles of temperature and humidity and collocated satellite measurements. An example of the UTH product and a comparison with radiosondes is presented. An estimate of the error of the UTH is obtained from a sensitivity test of the radiation scheme to errors in the input profiles. Both the sensitivity and the comparison with radiosondes yield absolute error estimates for the UTH of 10%–15%.

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Olli M. Turpeinen
and
Johannes Schmetz

Abstract

The purpose of this note is to “validate” the upper tropospheric humidity (UTH) operationally extracted from the 6.3 μm channel data of METEOSAT. The validation is carded out by comparing the satellite data with observed humidifies from the conventional network of radiosondes. The validation is not an absolute error assessment of the UTH, but rather an intercomparison, because the upper-tropospheric radiosonde humidities may not be very accurate.

The results for three latitude belts indicate that the estimated humidity in the upper troposphere shows a fairly high linear correlation (coefficients 0.64–0.89) with the observed humidity and that the slope of the regression line ranges from 0.82–0.89. The rms-error of the UTH is generally less than 10%. The UTH tends to underestimate the observed humidity by about 4%. This relative bias is most pronounced in the midlatitude belts.

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Olli M. Turpeinen
and
A. A. Diallo

Abstract

Rainfall is estimated in Burkina Faso for a full year using the ESOC precipitation index (EPI), a statistical cloud indexing method based on satellite data from METEOSAT. The EPI is converted into rainfall with the linear regression calculated between the EPI and the observed rainfall from a dense network of rain gages. Only one regression line based on the largest possible sample is used. The purpose of the paper is to assess the accuracies of the yearly and seasonal rain estimates to find out whether a single EPI-rainfall relation can be applied.

The results show that the yearly precipitation can be estimated to a high degree of accuracy. On the other hand, the precision of the seasonal estimates exhibits large fluctuations. While the dry season estimates are reliable, the transition between the dry and rainy season is characterized by a considerable overestimation, caused by the abundance of cold, nonprecipitating cirrus on the northern side of the intertropical convergence zone. During the rainy season the method suffers from a slight underestimation.

To resolve the major problem, that of nonprecipitating cirrus, a lower temperature threshold of 220 K instead of 235 K is applied in the determination of the EPI. The rain estimates for the transition period do improve slightly, but the gain is offset by the deterioration of the rain estimates made for the whole year and the rainy season.

The results suggest that the rain estimates made with a single EPI-rainfall relation are useful, but that they could be improved with some type of seasonal adjustments.

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Wassila Thiao
and
Olli M. Turpeinen

Abstract

Diurnal variations of cold cloudiness during a 2-year period (October 1985–September 1987) over the tropical arm covered by Meteosat (from 23°N to 23°S and from 60°E to 60°W) are studied using a simple precipitation index based on infrared data with a unique blackbody temperature threshold of 235 K. The index is calculated every three hours and accumulated over 5-day periods.

The results indicate that the cold cloudiness undergoes a pronounced diurnal cycle only over the continent during the rainy season, with an afternoon maximum between 1500 and 2100 LST and a morning minimum between 0600 and 1200 LST. Over flat areas, diurnal variations are weaker than in the mountains. Over water surfaces, the diurnal variations of cold cloudiness are irregular, but along the coasts, morning maxima (0600–1200 LST) are the most frequent.

The present study shows the potential utility of the easily manageable precipitation-index dataset in cloud climatology studies, as the results reported here are in agreement with earlier studies based on more complex datasets.

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Olli M. Turpeinen
,
Azzouz Abidi
, and
Wahid Belhouane

Abstract

A validation of the ESOC (European Space 0Perations Centre) Precipitation Index (EPI) is carried out by comparing satellite data with observed rainfall in five African countries to determine the ability of estimating accumulated precipitation independently of the area considered. In the formulation of the EPI it is assumed that most of the tropical rain originates from deep and cold clouds. The scheme is a cloud indexing method based on the infrared channel, additionally including a stratification of data into three classes according to the Upper Tropospheric Humidity (UTH) obtained from METEOSAT 6.3 μm channel.

The results indicate that rainfall can be well estimated in the tropical area while more sophisticated methods are required for the subtropics. The stratification of the data according to the UTH constitutes an improvement which is particularly significant away from the Intertropical Convergence Zone (ITCZ).

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Olli M. Turpeinen
,
Louis Garand
,
Robert Benoit
, and
Michel Roch

Abstract

The usefulness of numerical weather prediction models in very short-range forecasting is limited by the spinup problem, resulting in an underestimation of both the divergent wind component and the precipitation.

To alleviate the spinup problem, latent-heating profiles were directly assimilated into the Canadian regional finite-element (RFE) model. The estimates of latent heating were based on the precipitation rates inferred from GOES infrared and visible imagery. The latent heating was distributed in the vertical according to the stratiform condensation scheme of the model, but the heating rates were normalized to correspond to the satellite-inferred rain rates. The initial relative humidity field was enhanced to 95% between sigma-level 0.875 and the cloud top wherever the probability of precipitation, derived from satellite imagery, was larger than 40%.

The results of a case study from the Canadian Atlantic Storms Program (CASP) indicated that the spinup time of the vertical motion, initially of the order of 9 hours, could be practically eliminated. The forecast precipitation rates in the frontal zone agreed closely with Nimbus-7 SMMR microwave observations as early as 1–2 hours after the initialization.

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