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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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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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