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Arnold Gruber

Abstract

The daily variation of cloudiness, as measured by the Synchronous Meteorological Satellite (SMS-1), was studied for the GATE A/B area (outer hexagon). The amplitude of the diurnal variation was found to be more pronounced on convectively enhanced days than undisturbed days. A primary maximum in both upper and total cloudiness was observed at 1800 GMT (about 1630 local time) and a secondary maximum at 0900 GMT (0730 local time) was observed in the total cloudiness. The existence of a semidiurnal oscillation in cloudiness suggests the influence of the semidiurnal solar atmospheric tide on the cloudiness. The lack of a nighttime maximum in cloudiness implies no nighttime maximum in precipitation, in contrast to other observations of maritime precipitation.

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Arnold Gruber

Abstract

In order to assess the role played by convective processes in the vertical transfer of energy, the mean structure and energy budget over the Florida Peninsula have been studied, when the convective scale is the dominant scale of motion present.

The study utilized 0000 and 1200 GMT radiosonde data for the period 1957-65 and the convectively dominated period was identified as June, July and August. Energy budget computations show that the Florida Peninsula provides a considerable amount of latent and sensible energy across the air-surface interface. Convective processes make a significant contribution to the vertical transport of energy. The sea-breeze circulation is found to play an important role in the physical processes taking place. When the entire tropospheric volume is considered, it is found that there is a net expert of energy.

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Arnold Gruber

Abstract

The wavenumber–frequency spectra of the 200 mb wind field from 28.7°N to 28.7°S were examined for the 128–day period from mid-May to mid-September of 1970 and 1971. Evidence was presented that supported the existence of Rossby and mixed Rossby–gravity waves. Westward–propagating waves, with periods of about 10 days and wavenumber 10, centered at about 24°N were also observed. These waves appear to he associated with the tropical upper troughs.

There were indications of equatorward transfers of energy from the mid-latitudes at periods of 9–10 and about 5 days.

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Arnold Gruber

Abstract

An attempt to estimate rainfall in convectively active regions using Kuo's parameterization scheme has been made. The precipitation in this model is given by P = lQ 1t, where P is the precipitation per unit time, l the fraction of a synoptic area covered by deep active convection, Q 1 the mount of condensation heating according to moist adiabatic ascent, and Δt a time parameter related to the precipitating lifetime of the convective elements.

Investigation of the above equation when P, l and Q 1 were available indicated that an appropriate time parameter was 30 min and that the main contribution to the precipitation comes from the l parameter.

A case is presented where l is obtained from satellite observations. The resulting precipitation estimate appears quite reasonable. The potential for estimating precipitation over the tropical means is pointed out.

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Arnold Gruber

Abstract

The relationship between streamline pattern and momentum transport has been examined. It is shown, for divergent flow, that it is possible for a streamline pattern with troughs and ridges oriented northeast-southwest to have zonally averaged momentum transports associated with it which are directed equatorward in the Northern Hemisphere.

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Arnold Gruber

Abstract

The wavenumber-frequency spectra of satellite-observed brightness have been examined for the period 1 February 1967 through 29 February 1968 for the latitude belt 20N to 20S. It was found that the quasi-stationary modes and low wavenumbers contain most of the power. The propagating wave activity was located primarily in the 5–15N latitude zone. Perturbations with periods of 12.5 days and wavenumber 5 and about 6 days and wavenumber 9 were prominent. These are consistent with Rossby waves and easterly waves. Since the propagating brightness spectra are due to propagating clouds, the results indicate a traveling heat source as being important in the generation and maintenance of those waves. There was no indication in the brightness spectra of periods and wavelengths consistent with Kelvin waves. However, a tropospheric heat source is not ruled out for those waves.

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Brant Liebmann and Arnold Gruber

Abstract

The annual variation of the diurnal cycle of outgoing longwave radiation (OLR) is examined. Our results are based on the climatological amplitude and phase of the first diurnal harmonic for each month. The diurnal harmonic was extracted from a composite daily cycle from several polar orbiting satellites that flew in different years with ten different equator crossing times. We compute a “diurnal vector standard deviation” which is the square root of the sum of the variances of both components of the 12 climatological monthly diurnal vectors. This allows contributions from both phase and amplitude changes of the diurnal vector.

A map of the diurnal vector standard deviation is presented. The values over land are an order of magnitude larger than over the ocean. The maxima are located over the seasonally migrating monsoons and over the midlatitude semi-arid zones. In midlatitudes the large standard deviation results from an increased daily cycle of insolation during summer and from clouds associated with midlatitude storms which reduce the diurnal cycle during winter. In the tropical monsoon regions a large variability of the diurnal cycle results from a larger daily cycle of cloudiness during the wet season than in the dry season. At some locations over the monsoons, however, the diurnal amplitude is actually a minimum during the wet summer season. We believe the minimum is caused by the pervasive cloudiness in the most convective regions. In the midlatitudes and during the dry season in the tropics, the maximum emission generally occurs between 1200 and 1400 local time. During the rainy season it occurs between 0600 and 0900.

We hypothesize that there should be a spatial relationship between the diurnal cycle variability and the standard deviation of the 12 climatological monthly means of OLR, and we compare maps of the two quantities The large-scale features are in broad agreement and the correlation between the two maps is marginally statistically significant. A detailed comparison, however, reveals that the diurnal vector standard deviation is of much smaller scale than the standard deviation of OLR. We attribute the regional structure of the diurnal cycle variability to varying geography, vegetation, and available moisture. Some of the small-scale structure, however, undoubtedly arises because the diurnal cycle involves day-night differences which are inherently more noisy than the OLR field itself.

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Arnold Gruber and Jay S. Winston

A brief description of an earth radiation budget data set, as determined from NOAA operational spacecraft, is presented. The data are continuous from June 1974 through February 1978. Some samples of the mapped outputs are shown, and information on the availability of these data is provided.

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Arnold Gruber and Carmella Davis Watkins

Abstract

A statistical evaluation of satellite soundings from TIROS-N and NOAA-6 is presented. Collocated satellite-radiosonde data were collected by season from July 1979 through May 1980 for clear and cloudy retrievals. In addition to RMS and mean error statistics, vertical correlations, ratio of satellite variance to radiosonde variance, and land minus sea differences for 30–60°N, 30–60°S and 30°N–30°S were examined. The results indicate that the mean and RMS differences are larger for cloudy retrieval paths than for clear retrieval paths. The largest RMS errors occur in the 1000–850 mb layer and in the tropopause region. Vertical correlations are highest between adjacent layers.

The ratio of satellite variance to radiosonde variance is considerably higher than reported in Phillips et at. (1979), with clear retrievals exhibiting a higher ratio than cloudy retrievals. Land minus sea differences suggest that there is a continental influence in the coefficient data base used to generate cloudy retrievals.

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Mamoudou B. Ba and Arnold Gruber

Abstract

A multispectral approach is used to optimize the identification of raining clouds located at a given altitude estimated from the cloud-top temperature. The approach combines information from five channels on the National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellite (GOES): visible (0.65 μm), near infrared (3.9 μm), water vapor (6.7 μm), and window channels (11 and 12 μm). The screening of nonraining clouds includes the use of spatial gradient of cloud-top temperature for cirrus clouds (this screening is applied at all times) and the effective radius of cloud-top particles derived from the measurements at 3.9 μm during daytime. During nighttime, only clouds colder than 230 K are considered for the screening; during daytime, all clouds having a visible reflectance greater than 0.40 are considered for the screening, and a threshold of 15 μm in droplet effective radius is used as a low boundary of raining clouds. A GOES rain rate for each indicated raining cloud group referenced by its cloud-top temperature is obtained by the product of probability of rain (Pb) and mean rain rate (RRmean) and is adjusted by a moisture factor that is designed to modulate the evaporation effects on rain below cloud base for different moisture environments. The calibration of the algorithm for constants Pb and RRmean is obtained using collocated instantaneous satellite and radar data and hourly gauge-adjusted radar products collected during 17 days in June and July 1998. A comparison of the combined visible and a temperature threshold of 230 K (e.g., previous infrared/visible algorithms) with the combined visible and a threshold of 15 μm demonstrates that the latter improves the detection of rain from warm clouds without lowering the skill of the algorithm. The quantitative validation shows that the algorithm performs well at daily and monthly scales. At monthly scales, a comparison with GOES Precipitation Index (GPI) shows that GOES Multispectral Rainfall Algorithm's performance against gauges is much better for September and October 1999.

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