The tropical cyclone rainfall climatological study performed for the North Pacific was extended to the North Atlantic. Similar to the North Pacific tropical cyclone study, mean monthly rainfall within 444 km of the center of the North Atlantic tropical cyclones (i.e., that reached storm stage and greater) was estimated from passive microwave satellite observations during an 11-yr period. These satellite-observed rainfall estimates were used to assess the impact of tropical cyclone rainfall in altering the geographical, seasonal, and interannual distribution of the North Atlantic total rainfall during June-November when tropical cyclones were most abundant. The main results from this study indicate 1) that tropical cyclones contribute, respectively, 4%, 3%, and 4% to the western, eastern, and entire North Atlantic; 2) similar to that observed in the North Pacific, the maximum in North Atlantic tropical cyclone rainfall is approximately 5 degrees -10 degrees poleward (depending on longitude) of the maximum nontropical cyclone rainfall; 3) tropical cyclones contribute regionally a maximum of 30% of the total rainfall northeast of Puerto Rico, within a region near 15 degreesN, 55 degreesW, and off the west coast of Africa; 4) there is no lag between the months with maximum tropical cyclone rainfall and nontropical cyclone rainfall in the western North Atlantic, whereas in the eastern North Atlantic, maximum tropical cyclone rainfall precedes maximum nontropical cyclone rainfall; 5) like the North Pacific, North Atlantic tropical cyclones of hurricane intensity generate the greatest amount of rainfall in the higher latitudes; and 6) warm El Nino-Southern Oscillation events inhibit tropical cyclone rainfall.

Abstract The distribution and intensity of total (i.e., combined stratified and convective processes) rain rate/latent heat release (LHR) were derived for Tropical Cyclone Paka during the period 9–21 December 1997 from the F-10, F-11, F-13, and F-14 Defense Meteorological Satellite Special Sensor Microwave Imager and the Tropical Rainfall Measuring Mission Microwave Imager observations. These observations were frequent enough to capture three episodes of inner-core convective bursts and a convective rainband cycle that preceded periods of rapid intensification. During these periods of convective bursts, satellite sensors revealed that the rain rates/LHR 1) increased within the inner-core region, 2) were mainly convectively generated (nearly a 65% contribution), 3) propagated inward, 4) extended upward within the mid- and upper troposphere, and 5) became electrically charged. These factors may have increased the areal mean ascending motion in the mid- and upper-troposphere eyewall region, creating greater ...

The distribution and intensity of tropical cyclone precipitation has been known to have a large influence on the intensification and maintenance of the system. Therefore, monitoring the tropical cyclone convective rainband cycle and the large-scale environmental forcing mechanisms that initiate and maintain the tropical cyclone convective rainbands may aid in better understanding and predicting tropical cyclone intensification.To demonstrate how the evolution of the tropical cyclone precipitation can be monitored, the frequent Special Sensor Microwave/Imager (SSM/I) observations of precipitation from Typhoon Bobbie (June 1992) were used to help better delineate Bobbie's convective rainband cycle. Bobbie's SSM/I-observed convective rainband cycle was then related to the tropical cyclone's intensity change. To obtain a better understanding of how Bobbie's convective rainbands were initiated and maintained, total precipitable water (TPW) over the ocean regions, mean monthly sea surface temperatures (SSTs), and analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) model were examined. The SSM/I TPW helped to substantiate the ECMWF-analyzed regions of dry and moist air that were interacting with the system's circulation, while the mean monthly SSTs were used to determine whether the western North Pacific, where Bobbie traversed, was warm enough to allow for sufficient energy flux to support convection. The ECMWF model was employed to examine the environmental forcing mechanisms that may have initiated and maintained Bobbie's convective rainbands, such as mean vertical wind shear, environmental tropospheric water vapor nux and divergence, and upper-tropospheric eddy relative angular momentum flux convergence.Results from the analyses of Typhoon Bobbie suggested the following: 1) The SSM/I observations bf Bobbie's precipitation were able to detect and monitor convective rainband cycles that were similar to those observed with land-based and aircraft radar, in situ measurements, and SSM/I observations of western North Atlantic tropical cyclones. 2) The evolution of Bobbie's intensity coincided with the SSM/I-observed convective rainband cycles. 3) The SSM/I observations of the TPW over nonraining ocean regions were able to substantiate the ECMWF-analyzed moist and dry regions that were interacting with Bobbie's circulation. 4) In regions of warm SSTs and weak vertical wind shear, the enhancement of the precipitation in Bobbie's inner-core convective rainbands coincided with the inward convergence of upper-tropospheric eddy relative angular momentum, while the initialization of Bobbie's outer-core convective rainbands appeared to coincide with the large horizontal convergence of moisture, 5) The dissipation of rain in the inner-core convective rainbands appeared to be associated with inward propagation of newly formed outer convective rainbands, strong vertical wind shear(above 10 m s(-1)), and cool SSTs (below 26 degrees C).

Special Sensor Microwave/Imager(SSM/I) observations were used to examine spatial and temporal changes in the precipitation characteristics for western North Pacific tropical cyclones that reached storm stage or greater during 1987-92. The second version of the Goddard scattering algorithm, that employed the 85-GHz brightness temperatures to measure rain rate, provided an analysis of the tropical cyclone precipitation distribution in greater detail, while the numerous SSM/I observations helped to better define the relationship between the tropical cyclones' spatial and temporal distribution of precipitation and the systems intensity, intensity change, radiational forcing, and mean monthly sea surface temperatures (SSTs). The two SSM/Is flown since 1992 also helped to provide a more detailed analysis of the evolution of the tropical cyclone inner-core diabatic heating.Similar to the SSM/I-observed 1987-89 western North Atlantic tropical cyclones, the SSM/I observations of the western North Pacific tropical cyclones revealed that the more intense systems had higher rain rates and greater areal distribution of rain. In addition, the heaviest rain rates were found nearer to the center of all the tropical cyclones. However, western North Pacific typhoons were found to have heavier azimuthally averaged rain rates and a greater contribution from the heavier rain within the inner core (i.e., within ill km of the center) than the western North Atlantic hurricanes.The SSM/I observations of the western North Pacific tropical cyclones also suggested the following: 1) there appears to be a diurnal variation in the tropical cyclone precipitation (i.e., morning maximum and an evening minimum) except in the inner-core regions of systems that are at storm stage and greater; 2) the maximum rain rate that a tropical cyclone can produce in the inner-core region is dictated by SSTs with maximum rain rates occurring at SSTs greater than 29 degrees C; 3) the large changes in the tropical cyclone inner-core rain rate (latent heat release) help to initiate and maintain periods of tropical cyclone intensification; and 4) the intensity of these tropical cyclones become more responsive to rain-rate changes as the tropical cyclones become more intense.

The intensity, spatial, and temporal changes in precipitation were examined in three North Atlantic hurricanes during 1989 (Dean, Gabrielle, and Hugo) using precipitation estimates made from Special Sensor Microwave/Imager (SSM/I) measurements. In addition, analyses from a barotropic hurricane forecast model and the European Centre for Medium-Range Weather Forecast model were used to examine the relationship between the evolution of the precipitation in these tropical cyclones and external forcing. The external forcing parameters examined were 1) mean climatological sea surface temperatures, 2) vertical wind shear, 3) environmental tropospheric water vapor flux, and 4) upper-tropospheric eddy relative angular momentum flux convergence.The analyses revealed that 1) the SSM/I precipitation estimates were able to delineate and monitor convective ring cycles similar to those observed with land-based and aircraft radar and in situ measurements; 2) tropical cyclone intensification was observed to occur when these convective rings propagated into the inner core of these systems (within 111 km of the center) and when the precipitation rates increased; 3) tropical cyclone weakening was observed to occur when these inner-core convective rings dissipated; 4) the inward propagation of the outer convective rings coincided with the dissipation of the inner convective rings when they came within 55 km of each other; 5) in regions with the combined warm sea surface temperatures (above 26-degrees-C) and low vertical wind shear (less than 5 m s-1), convective rings outside the region of strong lower-tropospheric inertial stability could be initiated by strong surges of tropospheric moisture, while convective rings inside the region of strong lower-tropospheric inertial stability could be enhanced by upper-tropospheric eddy relative angular momentum flux convergence.

The Nimbus-7 Total Ozone Mapping spectrometer (TOMS) was used to map the distribution of total Ozone in and around western Atlantic tropical cyclones from 1979 to 1982. It was found that the TOMS-observed total Ozone distribution within the subtropics during the tropical cyclone seasonal correlated well with the tropopause topoghraphy, similar to earlier middle-latitudinal observations. This relationship made it possible to use TOMS to monitor the propagation of upper-tropospheric subtropical transient waves and the mutual adjustment between the tropical cyclone and the upper-tropospheric waves during their interaction. These total ozone patterns reflected the three-dimensional upper-tropospheric transport processes that were conducive for storm intensification and weakening. It was also found from satellite observations and numerical model simulations that modification of the environmental distribution of total ozone by the tropical cyclones was primarily caused by the secondary circulation associated with the tropical cyclone's outflow jet and the intrusion of stratospheric air in the eyes of tropical cyclones.

A statistical method has been developed using satellite, climatological, and persistence data to predict tropical cyclone position 12, 24, 48 and 72 h after initial observation. The satellite measurements were infrared window channel (11.0 μm) equivalent blackbody temperatures (TBB), which gave representations (through the cloud and surface temperature fields) of the structure of the cyclones and the circulation features surrounding them. There were 197 individual measurements of TBB for each cyclone observation. Algorithms have been prepared using digital data from a single satellite image, 14 climatological and persistence type variables, and a combination of these data sources. The algorithms were developed using a unique statistical procedure based on an eigenvector preprocessing and the use of independent tests for screening decisions. Independent testing of these algorithms showed that the average error made by the algorithms developed from the single satellite observation were comparable to the 48 h Joint Typhoon Warning Center (JTWC) forecast and were approximately 10% better for 72 h forecasts. Forecasts using only the climatological and persistence variables were about 20% worse than JTWC for 24 h forecasts and 10% worse for 48 and 72 h forecasts. When both satellite and nonsatellite variables were included, the performance was comparable to JTWC's for the 24 and 48 h forecasts and approximately 25% better than JTWC's for the 72 h forecasts. The performance of the objective algorithms for various partitions was analyzed. It is shown that both the satellite and nonsatellite variables make significant and unique contributions.

In light of previous theoretical calculations, an empirical-statistical analysis using satellite multifrequency dual polarized passive microwave data to detect rainfall areas over land was performed. The addition of information from a lower frequency channel (18.0 or 10.7 GHz) was shown to improve the discrimination of rain from wet ground achieved by using a single frequency dual polarized (37 GHz) channel alone. The algorithm was developed and independently tested using data from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR). Horizontally and vertically polarized brightness temperature pairs (TH, TV) at 37, 18, 10.7 GHz were sampled for rain areas over land (determined from ground-based radar), wet ground areas (adjacent and upwind from rain areas determined from radar), and dry land regions (areas where rain had not fallen during a previous 24 h period) over the central and eastern United States. Surface thermodynamic temperatures were both above and below 15°C. An examination of the data from each separate channel indicated that the probability (using the F test) for the mean vectors of any two populations being identical is less than 0.01 for classes sampled with surface thermodynamic temperatures ⩾15°C except for the rain over land and wet ground classes observed with the SMMR 37 GHz channel. For the classes sampled with surface thermodynamic temperatures <15°C, none of the classes were significantly different. Since most of the categories were significantly different for the warmer (≥15°C) land surface cases, a Fisher linear discriminant classifier was then developed for each channel and independently tested. The results from one test case showed that for areas of large-scale heavy rainfall, the lower frequency SMMR channels were better able to delineate rain from wet ground than the 37 GHz channel. However, in areas of light rain and/or where the rain area did not fill the lower frequency instantaneous field of view these channels were not able to differentiate, rain from wet ground.

Rapid scan visible images from the Visible Infrared Spin Scan Radiometer sensor on board SMS-2 and GOES-1 were used to derive high resolution upper and lower tropospheric environmental wind fields around three western Atlantic tropical cyclones (1975-78). These wind fields were used to derive upper and lower tropospheric areal mean relative vorticity and their differences, the net relative angular momentum balance and upper tropospheric mass outflow. These kinematic parameters were shown by studies using composite rawinsonde data to be strongly related to tropical cyclone formation and intensity changes. Also, the role of forced synoptic scale subsidence in tropical cyclone formation was examined. The studies showed that satellite-derived lower and upper tropospheric wind fields can be used to monitor and possibly predict tropical cyclone formation and intensity changes. These kinematic analyses showed that future changes in tropical cyclone intensity are mainly related to the spin-up of the storms by the net horizontal transport of relative angular momentum caused by convergence of cyclonic vorticity in the lower troposphere and to a lesser extent the divergence of anticyclone vorticity in the upper troposphere.

A relationship between maximum winds and satellite-measured equivalent blackbody temperatures near tropical cyclones is investigated with data from both the Atlantic and western North Pacific areas. This investigation revealed not only a significant correlation between satellite-derived equivalent blackbody temperatures and maximum winds but also a strong lag relationship between these temperatures and maximum winds. From this latter relationship a regression technique was developed to forecast 24 h changes of the maximum winds for weak (maximum winds ≤ 65 kt) and strong (maximum winds > 65 kt) tropical cyclones by utilizing the equivalent blackbody temperatures around the storm alone, and together with changes in maximum winds during the preceding 24 h and the current maximum winds. Testing of these equations with independent data showed that the mean errors of forecasts made by the equations are lower than the errors in forecasts made by persistence techniques.

During the 1975, 1976 and 1977 North Atlantic hurricane seasons, NOAA's National Environmental Satellite Service (NESS) and NASA's Goddard Space Flight Center (GSFC) conducted a cooperative program to determine the best resolution and frequency now available from satellite images for deriving winds to study and forecast tropical cyclones. Rapid-scan images were obtained in 1975 at 7.5 min interval from SMS 2 for Hurricane Eloise on 22 September and of tropical cyclone Caroline on 28, 29 and 30 August; in 1976 at 3 min intervals from GOES 1 for tropical storms Belle on 5 August and Holly on 25 October; and in 1977 at 3 min intervals Corn GOES 1 for tropical cyclone Anita on 30 and 31 August and 1 September. Cloud motions were derived from these images using the Atmospheric and Oceanographic Information Processing System (AOIPS) at GSFC. Winds that were derived from the movement of upper (∼200 mb) and lower tropospheric (∼900 mb) level clouds using rapid scan data were compared with the 15 and 30 min interval data. This was done using visible images having 1, 2, 4 and 8 km resolution for the areas within 650 km of the storm center for the 1975 and 1976 tropical cyclones. Greater than 10 (5) times as many clouds could be tracked to obtain winds at both levels using 3 and 7.5 min rapid-scan images as when using 30 min (15 min) interval images. In addition, by using the frequent images, it was possible to track a few bright areas within the central dense overcast which appeared to be moving with the winds at low levels. For Hurricanes Eloise and Caroline the winds that were derived by tracking these bright areas within the central dense overcast had speeds differing in the mean by only 2.5 m s −1 from the wind speed measured by aircraft flying at ∼0.5 km above the surface in the same quadrant 4 h later. Full-resolution visible images (1 km) were needed to track slow moving low-level cloud elements, since on a degraded resolution image, subpixel movement would introduce additive inaccuracies to the wind measurements. Rapid-scan full-resolution GOES 1 data for tropical cyclone Anita (1977) provided representative wind fields only outside the central dense overcast at the lower tropospheric level. For this area aircraft-measured wind speeds differed in the mean again by only 2.5 m s−1.

At 37 GHz, the frequency at which the Nimbus 6 Electrically Scanning Microwave Radiometer (ESMR 6) measures upwelling radiance, it has been shown theoretically that the atmospheric scattering and the relative independence on electromagnetic polarization of the radiances emerging from hydrometeors make it possible to monitor remotely active rainfall over land. In order to verify experimentally these theoretical findings and to develop an algorithm to monitor rainfall over land, the digitized ESMR 6 measurements were examined statistically. Horizontally and vertically polarized brightness temperature pairs (TH,TV) from ESMR 6 were sampled for areas of rainfall over land as determined from the rain recording stations and the WSR 57 radar, and areas of wet and dry ground (whose thermodynamic temperatures were greater than 5°C) over the southeastern United States. These three categories of brightness temperatures were found to be significantly different in the sense that the chances that the mean vectors of any two populations coincided were less than 1 in 100. Since these categories were significantly different, classification algorithms were then developed. Three decision rules were examined: the Fisher linear classifier, the Bayesian quadratic classifier, and a non-parametric linear classifier. The Bayesian algorithm was found to perform best, particularly at a higher confidence level. An independent test case analysis showed that a rainfall area delineated by the Bayesian classifier coincided well with the synoptic-scale rainfall area mapped by ground recording rain data and radar echoes.

Statistical analysis is performed by first sampling three categories of Nimbus 6 ESMR brightness temperatures (representing rain over land, wet land surfaces without rain, and dry land surfaces), then testing these populations for uniqueness. A classification algorithm to delineate rain over land is developed. It is found that synoptic-scale rainfall over land, where surface thermodynamic temperatures are greater than 5 C and the vegetation is bereft of dew, can indeed be delineated despite the large ESMR-6 instantaneous field of view. However, some ambiguity exists in distinguishing between rainfall areas and wet land surfaces.

Results are presented for a study program designed to develop a better means of obtaining wind data needed by hurricane forecasters and to determine the optimum space and time resolution of satellite data used to obtain such winds. The discussion covers cloud tracking with an image display and manipulation system and analysis of IR imagery of Hurricane Eloise. It is shown that greater resolution in time and space of satellite imagery made it possible to derive up to six times as many low-level winds in a hurricane case, to eliminate 'bad winds' caused by mistaking cloud growth for cloud motion, and to remove ambiguities that would have caused difficulty in tracking clouds using only the imagery taken at 30-min intervals. It is suggested to use short-interval imagery (less than 10 min) for wind determination for future research in areas of tropical cyclones.