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David A. Short

Abstract

Analyses of the frequency of rain occurrence over the equatorial Atlantic Ocean from two sources are compared: a nineteenth-century journal publication based on ship's logbook entries, and a 3-yr average, 1998–2000, of observations from the precipitation radar aboard the Tropical Rainfall Measuring Mission satellite observatory. The sources agree remarkably well on the position and shape of the equatorial maximum, with a correlation coefficient of 0.99. However, the magnitudes differ by about a factor of 2, with the modern estimate being lower. This disparity is likely to be attributable to characteristics of the observing systems. The radar sensitivity and scanning characteristics combine to underestimate rain occurrence. The precise nature of the nineteenth-century sources are not documented; however, they almost certainly have been incorporated into the Comprehensive Ocean–Atmosphere Data Set (COADS).

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David A. Short and Kenji Nakamura

Abstract

Probability distributions of measured radar reflectivity from the precipitation radar (PR) on board the Tropical Rainfall Measuring Mission (TRMM) satellite show a small, counterintuitive increase in the midrange, 20–34 dBZ, when comparing data from periods before and after the orbit altitude was boosted in August 2001. Data from two 2-yr time periods, 1999–2000 (preboost) and 2002–03 (postboost), show statistically significant differences of 2%–3% at altitudes of 2, 4, and 10 km and for path-averaged reflectivity.

The bivariate Gaussian function, used to model idealized radar response functions, has mathematical properties that indicate an increase in field-of-view (FOV) size associated with an increase in satellite altitude can be expected to result in a narrowing of observed dBZ distributions, with a resulting increase in midrange values. Numerical simulations with echo areas much smaller and larger than the TRMM PR FOV before (4.3 km) and after (5.0 km) boost are used to demonstrate basic characteristics of the observed and expected distribution changes.

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Ali Tokay and David A. Short

Abstract

An analysis of temporal variations in gamma parameters of raindrop spectra is presented utilizing surface-based observations from the Tropical Ocean Global Atmosphere Couple Ocean-Atmosphere Experiment. An observed dramatic change in the N 0 parameter, found to occur during rainfall events with little change in rainfall rate, is suggestive of a transition from rain of convective origin to rain originating from the stratiform portion of tropical systems. An empirical stratiform-convective classification method based on N 0 and R (rainfall rate) is presented. Properties of the drop size spectra from the stratiform classification are consistent with micro-physical processes occurring within an aggregation/melting layer aloft, which produces more large raindrops and fewer small to medium size raindrops than rain from the convective classification, at the same rainfall rate. The occurrence of precipitation was found to be 74% (stratiform) and 26% (convective), but total rainfall, on the other hand, was 32% and 68%, respectively. Case studies of the tropical systems studied here indicate that heavy convective showers are generally followed by longer intervals of lighter rain from the stratiform portion of the cloud systems. Differences in the shapes of the frequency distributions of the integral rainfall parameters (i.e., liquid water content, rainfall rate, and radar reflectivity) suggest that the lognormal distribution applies to some, but not all cases. The analysis shows that almost all the precipitation with a radar reflectivity above 40 dBZ falls within the convective classification. Regarding radar reflectivity versus rainfall rate relationships, the exponent is lower and the intercept is higher in the tropical stratiform classification than in the tropical convective classification. Collision and evaporation rates, which are important for cloud-modeling studies, indicate substantial variation at different rainfall rates and between the two types.

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David A. Short and Kenji Nakamura

Abstract

Observations from the precipitation radar aboard the Tropical Rainfall Measuring Mission satellite provide the first opportunity to map vertical structure properties of rain over the entire Tropics and subtropics. Storm height histograms reveal a distinct bimodal distribution over the oceans with the lowest mode near 2 km and the upper mode at 5 km. The low mode is the dominant feature over regions previously associated with precipitating marine stratocumulus/stratus and trade wind cumulus. In those regions a lognormal distribution fits the observed storm height distributions quite well, and a strong correlation exists between conditional mean rainfall rate and storm height. In addition, the low mode appears within the major tropical convergence zones associated with significant precipitation, and in those regions a mixed lognormal distribution is used to separate the storm height distribution into two parts: shallow and deep. In this exploratory analysis, the correlation between rainfall intensity and storm height is used in combination with the mixed lognormal distribution to estimate that shallow precipitation composes approximately 20% of the total precipitation over tropical oceans during both El Niño and La Niña conditions.

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John L. Stanford and David A. Short

Abstract

Global microwave brightness temperature measurements are analyzed to investigate the range of meridional wavelengths 2000–3000 km where spectral studies reveal larger than expected variance. The data, from the TIROS-N Microwave Sounding Unit, are sensitive to lower stratospheric temperatures (30–150 mb). The results reveal striking temperature anomalies with short meridional wavelengths (2000–3000 km) and long zonal wavelengths (zonal wavenumbers 1–4). The anomalies, with amplitudes ∼1–2 K, extend from the equatorial region to at least as high as 70°N and 70°S during January 1979. The features exhibit slow eastward movement or else are nearly stationary for several days. In the Northern Hemisphere, comparison with NMC data reveals that the strongest features tend to be associated with major jet streams.

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David A. Short, Kunio Shimizu, and Benjamin Kedem

Abstract

Optimization of the threshold method, achieved by determination of the threshold that maximizes the correlation between an area-average rain-rate moment and the area coverage of rain rates exceeding the threshold, is demonstrated empirically and theoretically. Empirical results for a sequence of GATE radar snapshots show optimal thresholds of 5 and 27 mm h−1 for the first and second moments, respectively. Theoretical optimization of the threshold method by the maximum-likelihood approach of Kedem and Pavlopoulos predicts optimal thresholds near 5 and 26 mm h−1 for lognormally distributed rain rates with GATE-like parameters. The agreement between theory and observations suggests that the optimal threshold can be understood as arising due to sampling variations, from snapshot to snapshot, of a parent rain-rate distribution. Optimal thresholds for gamma and inverse Gaussian distributions are also derived and compared

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David A. Short and Robert F. Cahalan

Abstract

The interannual variability (IAV) in monthly averaged outgoing infrared radiation (IR, from the NOAA polar orbiting satellites) is observed to be larger during summer than during winter over the north Pacific Ocean. A statistical analysis of the daily observations shows the daily variance to be similar during both seasons while the autocorrelation function is quite different. This leads to a seasonal difference in estimates of the climatic noise level, i.e., the variances expected in summer and winter monthly averages due to the number of effectively independent samples in each average. Because of a less vigorous tropospheric circulation, monthly means of IR during summer are affected by the passage of fewer synoptic-scale disturbances and their attendant cloudiness. Fewer independent samples imply a larger variance in the time averages. While the observed IAV is less in winter, the ratio of the observed IAV to the climatic noise level is larger, suggesting that signals of climatic variability in outgoing IR may be more readily diagnosed during winter in this region. The climatic noise level in monthly averaged IR and cloudiness is also estimated for two other climatic regimes—the quiescent subtropical north Pacific and the ITCZ in the western Pacific.

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Dennis L. Hartmann and David A. Short

Abstract

The potential of zonal asymmetry as a sensitivity mechanism for the earth's climate is investigated through a simple extension of an energy balance climate model. The results of this model are compared with the reconstruction of climate during the last major glaciation. The comparison suggests that it is quite possible that the last glaciation was in near equilibrium with the present solar constant and that zonal asymmetry plays a very important role in the transitions between glacial and interglacial conditions. On the basis of these inferences, the planetary wave structure during summer and the transition seasons would appear to be very important for climatic change.

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David A. Short and John M. Wallace

Abstract

Outgoing infrared radiation (IR) values inferred from radiance measurements in the water vapor window (10.5–12.5 μm) taken at approximately 0900 and 2100 LT by scanning radiometers aboard the polar orbiting NOAA satellites are compared in order to determine whether a significant diurnal variation exists. Data with a resolution of 2.5° in latitude and longitude are mapped for latitudes between 50°N and 50°S for two seasons. June–August 1975 and December 1975-February 1976. On the basis of these maps it is possible to identify broad categories of geographical regions which display similar patterns of 12 h differences in outgoing IR. For several of these categories composite histograms are constructed in order to investigate the morning to evening difference in outgoing IR for ranges of values corresponding to high clouds, middle clouds and low clouds.

Over land regions and coastal waters it is found that the 12 h differences are dominated by middle and high clouds, likely of convective origin. The difference patterns are consistent with widely held views concerning the modulation of convection by diurnally oscillating, thermally driven boundary-layer circulations.

Over the tropical oceans the pattern of morning to evening differences in outgoing IR shows remarkable geographical consistency. There is evidence of a morning bias in high cloudiness (possibly of convective origin) and low cloudiness (stratus, stratocumulus and trade wind cumulus), and an evening bias in middle cloudiness. Because of the compensation between these three cloud regimes the net morning to evening differences in outgoing IR are rather small except over the eastern oceans where the low cloud regime predominates and tends to produce a weak but statistically significant evening maximum.

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Dennis L. Hartmann and David A. Short

Abstract

Daily observations of albedo and outgoing terrestrial radiation derived from NOAA Scanning Radiometer measurements are used to relate cloudiness variations to regional features of the general circulation and to estimate the relative importance of the albedo and infrared effects of clouds on the net radiation balance of the earth on a regional basis. The results indicate that there are clear relationships between the variability in outgoing IR and features of the atmospheric circulation, which appear to be linked to changes in cloudiness. A method requiring only measurements of planetary albedo and total outgoing IR is devised to evaluate the relative importance of the albedo and IR effects of the current distribution of cloud for the net radiation balance of the earth. The results obtained from this method suggest that globally the effect on the radiation balance of the high albedo of clouds is two or more times greater than the effect of clouds in reducing outgoing IR, so that an increase in the fractional area of the current distribution of cloud would tend to cool the earth. In addition, very large geographical variations in the radiative effects of cloud are displayed and related to circulation features.

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