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David W. Martin
and
Dhirendra N. Sikdar

Abstract

We describe the behavior, dynamics and setting of three cloud clusters. The clusters occurred in the western Atlantic Ocean between 21 and 24 July 1969. Wind and cloud analyses made from intensive ship and satellite observations of the Barbados Oceanographic and Meteorological Experiment (BOMEX) were supplemented by analyses of thermodynamic structure (temperature, moisture and stability) from Martin and Sikdar (1975).

Each of the clusters moved west to northwest. In each cluster high-energy near-surface air was ingested on the front or right flank and vented in the upper troposphere toward the right flank or rear. The clusters were distinguished by a range of speeds (6–15 m s−1), maximum areas (30 000–200 000 km2) and lifetimes (<1 to ∼3 days). Downdrafts in the first cluster were weak. The second cluster was without significant downdrafts as it formed, but gradually assumed the appearance of tropical squall lines. Massive squall-line downdrafts were observed in the third cluster. Conditions favoring deep convection were high absolute moisture content of the subcloud layer, relatively high moisture content in the middle troposphere, a weak trade inversion, large-scale 950 mb convergence, and cyclonic or weakly anticyclonic 950 mb relative vorticity. Deep convection tended to parallel centers of cyclonic vorticity at 950 mb. Downdrafts were stronger where there was a distinct wind speed maximum in the middle troposphere.

The clusters occurred with a persistent westward moving cloud wave: one toward the apex, and two at the base close to the Intertropical Cloud Band (ITCB). Surface θ e was high within the cloud wave, and 950 mb relative vorticity was mostly cyclonic. The trailing edge of the cloud wave marked a surge in the northeast trades. The cloud wave was linked with a layer of warm, dry Saharan air between 650 and 850 mb. Baroclinicity across the front of the Saharan air supported a 20 m s−1 east-southeasterly jet at 650 mb. There was a ridge over the trailing edge of the cloud wave, and a trough over the cloud wave, 200–500 km downstream. Air advanced relative to the wave, sinking as it approached the ridge and rising in passing from ridge to trough. In this case the strongest controls on deep convection were exercised from the middle troposphere.

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Soon-Ung Park
and
Dhirendra N. Sikdar

Abstract

A well-organized severe storm complex on 30 May 1976 developed in the southeastern part of the mesonetwork of the National Severe Storms Laboratory (NSSL) in central Oklahoma and merged, in its mature stage, with a band of thunderstorms along a stationary front. Rawinsonde observations at nine stations within the mesonetwork, supplemented by satellite time lapse images, have been analyzed to depict the kinematic and thermodynamic structure of the storm and its near environment. An objective analysis technique is used to describe the evolving storm structure.

Among the significant features the analysis revealed, the low-level horizontal convergence was confined to the southeast prior to the appearance of the first cloud in the satellite images, and a well-defined mixed layer, capped by a strong stable layer, developed with northwest to southeast horizontal temperature and moisture, gradients. The height of the mixed layer increased with time in the dry northwest side, while it decreased in the southeast part accompanied by a developing low-level southerly jet. Then, the differential mixed-layer growth became accelerated. The low-level southerly jet pushed the moist air into the strong stable layer above a shallow mixed layer, and brought about the destruction of the stable layer. The mesoscale convergence and the heat flux from the superadiabatic layer in the low levels in the southeast region caused a rapid growth of the mixed layer to the lifting condensation level, releasing potential instability, and storm development ensued. The southerly air transported further northwest along the sloping mixed-layer top lost its momentum in the strong stable layer and descended with the air coming from the northwest at middle levels. In the cloud-free region, the strength of the inversion layer was maintained by the growth of the mixed layer and the differential mesoscale sinking enhanced by nearby active convection.

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Michael R. Howland
and
Dhirendra N. Sikdar

Abstract

Moisture budgets are calculated for premonsoon and monsoon onset conditions in the northeastern Arabian Sea during summer 1979 from kinematic analysis of aircraft dropsonde, ship and island radiosonde, and satellite-derived winds. Dramatic changes are observed between the premonsoon and monsoon onset mean kinematic and moisture fields. Specific humidity increased as much as 5 g kg−1 in much of the middle troposphere between 29 May and 17 June 1979. This is apparently due to deep convection during the monsoon onset period and mid-level advection of moisture during the premonsoon period. Flux of moisture through the budget boundaries is comparable to previous estimates for the Arabian Sea. It is shown that the loss of moisture through cirrus outflow accounts for only 1–3% of the total budget flux. Evaporation from the sea surface is 3 to 4 times higher during the onset period and was greatest south of 12°N. Maps of precipitation as a residual of the moisture budget computations agree remarkably well with convective features seen in satellite imagery. During the monsoon onset period, rainfall averaged about 1 mm h−1 over the entire budget area. In order to test the validity of the combined data base and moisture budget computation two independent estimates of precipitation were made using a Krishnamurti et al. parameterization scheme and the Stout et al. satellite technique. Both showed good agreement to the budget results.

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David W. Martin
and
Dhirendra N. Sikdar

Abstract

Ground based data from the Barbados Oceanographic and Meteorological Experiment (BOMEX) have been combined with satellite data in a study of the time changing cloud and thermodynamic structure of cloud clusters and their environment. Digital images from the third Applications Technology Satellite (ATS-3) were analyzed as “movie loop” sequences on a computer controlled image storage, display, and processing device called McIDAS. Three-to six-hourly soundings from the five BOMEX ships were corrected for radiation and lag errors in moisture, then analyzed in time sections of temperature anomaly, relative humidity, and equivalent potential temperature. The integration of satellite and surface sounding data was accomplished through plexiglas models in which time section strips were treated as space sections. Extended meridional and quasi-zonal space sections of temperature anomaly, relative humidity, and equivalent potential temperature then were constructed from the models.

The time sections reveal a complex stratification of the tropical troposphere, with multiple interwoven layers superimposed on a basic pattern of surface mixed and cloud layers, a trade inversion layer, and an upper tropospheric layer. These layers weakened and sometimes vanished in the vicinity of cloud clusters and the Intertropical Cloud Band (ITCB). Shallower convective systems and patterns were also reflected in this laminar structure, but mainly at lower levels. ATS picture-sequences covered three clusters: two along the ITCB and one to the north of the ITCB. The first of the ITCB clusters contained moderate convection with little organization as it traversed the ship array; the second had distinct squall characteristics. The northern cluster formed in two stages just east of the array: an arc of congestus cloud gradually increasing in area, then explosive development of cumulonimbi. This cluster and the squall cluster formed along the leading edge of a middle tropospheric layer of Saharan origin. Cluster formation occurred close to but not within the areas of greatest parcel instability, where deep convection apparently was inhibited by a strong trade inversion and a very dry mid-troposphere.

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John S. Stout
,
David W. Martin
, and
Dhirendra N. Sikdar

Abstract

A method of estimating GATE rainfall from either visible or infrared images of geosynchronous satellites is described. Rain is estimated from cumulonimbus cloud area by the equation R = a 0 A + a 1 dA/dt, where R is volumetric rainfall (m3 s−1), A cloud area (m2), t time (s), and a 0 and a 1, are constants. Rainfall, calculated from 5.3 cm ship radar, and cloud area are measured from clouds in the tropical North Atlantic. The constants a 0 and a 1 are fit to these measurements by the least-squares method. Hourly estimates by the infrared version of this technique correlate well (correlation coefficient of 0.84) with rain totals derived from composited radar for an area of 105 km2. The accuracy of this method is described and compared to that of another technique using geosynchronous satellite images. We conclude that this technique provides useful estimates of tropical oceanic rainfall on a convective scale.

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David Suchman
,
David W. Martin
, and
Dhirendra N. Sikdar

Abstract

A technique is described for inferring vertical mass circulations within and around cloud clusters in the tropics. Following Sikdar and Suomi (1972), we model deep convection in terms of three layers—two active and one passive. An inflow layer extends from the sea surface to the top of the trade inversion; an outflow layer extends through the depth of the cumulonimbus anvil. High-density satellite cloud tracer winds define flow for the two active layers. (The intermediate layer is not observed; in that sense it is considered to be passive.) Using the divergence field computed from the satellite winds, vertical velocities are calculated through the top of the inflow layer, and the bottom of the outflow layer; vertical mass transports follow immediately.

This simple model is applied to two mature disturbances from the Global Atmospheric, Research Program Atlantic Tropical Experiment (GATE). Vertical velocities and mass transports were estimated for three times, at 3 h intervals. Vertical velocities on the scale of the clusters averaged 2–18 cm s−1, with highest values in the outflow layer. For both GATE clusters, maximum ascent appeared to occur several hours earlier at low levels than at high levels. Vertical velocity was smaller on the larger cluster circulation scale, but still positive leading to rather large upward transport of mass outside the clusters. Cluster-scale mass transports ranged from 15 to almost 35 mb h−1. An exception was inflow level transport for the more mature of the GATE clusters, which became slightly negative, implying a large middle tropospheric transport of mass into the cluster. Centers of divergence and upward mass transport were well matched to convective activity in the satellite pictures, even reflecting changes in position and intensity.

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Dhirendra N. Sikdar
,
Robert E. Schlesinger
, and
Charles E. Anderson

Abstract

The time variation of mass-integrated liquid water and latent heat release for a severe thunderstorm in marked vertical wind shear is investigated for an actual Oklahoma storm and for a two-dimensional numerical modeling experiment.

For the actual storm, an approximate continuity equation for liquid water variation is used together with profiles of radar reflectivity. Empirical relationships are utilized to determine the rainfall rate (flux) and liquid water content from the radar reflectivity profiles. Estimates of total storm water mass are obtained from the reflectivity profiles for the volume swept by the radar beam in the vertical. The downdraft evaporation rate is parameterized on the basis of a previous study which estimated this quantity as a residual in the continuity equation for liquid water mass.

To estimate latent heat release and total liquid water mass from the numerical model, the two-dimensional cloud is extended to a three-dimensional region whose horizontal cross sections are ellipses approximating typical observed PPI radar echo shapes. At each level, horizontal averages of relevant integrands are assumed equal to those in the model plane.

It is found on the basis of these analyses that at maturity, the actual storm and the model storm exhibit comparable magnitudes with respect to both latent heat release and liquid water content.

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Cecilia Girz Griffith
,
William Lee Woodley
,
Pamela G. Grube
,
David W. Martin
,
John Stout
, and
Dhirendra N. Sikdar

Abstract

A diagnostic method to estimate rainfall over large space and time scales by the use of geosynchronous visible or infrared satellite imagery has been derived and tested. Based on the finding that arms of active convection and rainfall in the tropics are brighter or colder on the satellite visible or infrared photographs than inactive regions, ATS-3 and SMS/GOES images were calibrated with gage-adjusted 10 cm radar data over south Florida. The resulting empirical relationships require a time sequence of cloud area, measured from the satellite images at a specified threshold brightness or temperature to calculate rain volume over a given period.

Satellite rain estimates were made for two areas in south Florida that differ in size by an order of magnitude (1.3×104km2 vs 1.1×105km2) and verified by a combined system of gages and radar. Contrary to our expectations, the rain estimates for the smaller area agreed better with the raingage-radar groundtruth than the satellite rain estimates for the larger area. As expected, the accuracy of the rain estimates is a function of the period of rain estimation. The error and scatter of the hourly estimates are relatively large but both decrease as the estimates are accumulated with time. For periods of 6–9 h the mean absolute errors are factors of 1.50 and 1.90 using the visible and infrared imagery, respectively.

Additional tests for which groundtruth was available were also made to determine the applicability of this scheme to tropical areas other than the region of derivation. These include nine days over portions of Venezuela, five days over Honduras during the passage of Hurricane Fifi, and one day each of Hurricane Belle (1976) over the East Coast of the United States and Hurricane Agnes over Florida.

The potential of this method as a forecasting tool for hurricane-caused flooding is investigated in a study of selected Atlantic hurricanes between 1969 and 1977. Storms were ranked by satellite-calculated total daily rain volume and daily area-averaged rain depth. Relatively wet storms could be distinguished from the relatively dry storms. No correlation was found between volumetric storm rain output or area-averaged rainfall and storm intensity, which suggests that the location of latent heat releases and not its magnitude determines storm intensity.

Computer automation of the technique for real time and diagnostic estimates is discussed briefly. Satellite-inferred rains for the Big Thompson (Colorado) flood of 1976 and the Atlantic Ocean during GATE (summer 1974) are presented as examples of the real time and diagnostic computerized schemes, respectively.

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