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Timothy D. Mitchell and Joanne Camp

Abstract

The Conway–Maxwell–Poisson distribution improves the precision with which seasonal counts of tropical cyclones may be modeled. Conventionally the Poisson is used, which assumes that the formation and transit of tropical cyclones is the result of a Poisson process, such that their frequency distribution has equal mean and variance (“equi-dispersion”). However, earlier studies of observed records have sometimes found overdispersion, where the variance exceeds the mean, indicating that tropical cyclones are clustered in particular years. The evidence presented here demonstrates that at least some of this overdispersion arises from observational inhomogeneities. Once this is removed, and particularly near the coasts, there is evidence for equi-dispersion or underdispersion. To more accurately model numbers of tropical cyclones, we investigate the use of the Conway–Maxwell–Poisson as an alternative to the Poisson that represents any dispersion characteristic. An example is given for East China where using it improves the skill of a prototype seasonal forecast of tropical cyclone landfall.

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D. M. O'Brien and R. M. Mitchell

Abstract

This paper examines the feasibility of remotely sensing cloud-top pressure from observations of reflected sunlight at frequencies in the A band of absorption of oxygen (13 070 cm−1, 765 nm). The data are assumed to consist of several channels within the A band and one reference channel outside the A band. The principal difficulty is that the reflected radiance is attenuated by absorption by O2 not only in the atmosphere above the cloud, but also along photon trajectories within the cloud. The extent of the extra absorption is unknown a priori because the microphysics structure of the cloud is unknown. This paper investigates the possibility of simultaneously retrieving both cloud-top pressure and parameters that determine the distribution of photon pathlength within the cloud. Estimates are derived for the errors in the retrieved parameters induced by instrumental noise and uncertainties in the profiles of temperature and aerosol extinction. The study investigates the sensitivity of the errors to spectral resolution and to both the number and placement of the spectral channels. Accuracies of 5 hPa appear possible over optically thick clouds with an instrument with high spectral resolution (∼1 cm−1).

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D. M. O'Brien and R. M. Mitchell

Abstract

This paper investigates the feasibility of recovering both the tropospheric aerosol loading and the surface wind speed from satellite measurements of the radiance within cloud free regions of sun glint over the ocean surface. The method relies on the marked distinction between the spatial variation of the surface reflected radiance and the atmospheric radiance. Analysis of AVHRR channel 2 (720–980 nm) data from a 128 km × 8 km test site at 17°S and 162°E indicates that the retrieval of aerosol loading and wind speed is insensitive to the size of the dataset, once a certain minimum size has been reached. Extensive numerical simulations reveal zones over the oceans where it is possible to retrieve the aerosol loading and surface wind speed with accuracy better than 10%. The recovery of wind speed to this accuracy is feasible over a somewhat larger area than for the aerosol loading.

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Mitchell D. Goldberg and Larry M. McMillin

Abstract

Deep-layer mean temperatures from Microwave Sounding Unit (MSU) observations have been used by scientists to study trends and interannual variations of tropospheric and lower-stratospheric temperature. The spatial resolution of MSU deep-layer mean temperatures is rather poor for studying trends in localized regions. A method is developed in which infrared observations from the High-resolution InfraRed Sounder (HIRS) is used in combination with MSU to derive deep-layer mean temperatures with improved vertical and horizontal resolution. Even though the relationship between infrared radiance and temperature is not linear, the layer associated with the mean temperature is shown to be well defined with a small airmass dependency that is similar to MSU’s airmass dependency. Preliminary validation of HIRS–MSU-derived layer mean temperatures with radiosonde layer mean temperatures show similar precision when compared to MSU-only derived temperatures.

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Mitchell D. Goldberg and Henry E. Fleming

Abstract

An algorithm for generating deep-layer mean temperatures from satellite-observed microwave observations is presented. Unlike traditional temperature retrieval methods, this algorithm does not require a first guess temperature of the ambient atmosphere. By eliminating the first guess a potentially systematic source of error has been removed. The algorithm is expected to yield long-term records that are suitable for detecting small changes in climate.

The atmospheric contribution to the deep-layer mean temperature is given by the averaging kernel. The algorithm computes the coefficients that will best approximate a desired averaging kernel from a linear combination of the satellite radiometer's weighting functions. The coefficients are then applied to the measurements to yield the deep-layer mean temperature. Three constraints were used in deriving the algorithm: 1) the sum of the coefficients must be one, 2) the noise of the product is minimized, and 3) the shape of the approximated averaging kernel is well behaved. Note that a trade-off between constraints 2 and 3 is unavoidable.

The algorithm can also be used to combine measurements from a future sensor [i.e., the 20-channel Advanced Microwave Sounding Unit (AMSU)] to yield the same averaging kernel as that based on an earlier sensor [i.e., the 4-channel Microwave Sounding Unit (MSU)]. This will allow a time series of deep-layer mean temperatures based on MSU measurements to be continued with AMSU measurements. The AMSU is expected to replace the MSU in 1996.

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R. M. Mitchell and D. M. O'Brien

Abstract

This paper investigates the feasibility of satellite remote sensing of surface pressure using radiometric measurements in the oxygen A band. It is shown out the primary sources of error are the uncertainties in the temperature profile, the surface reflection coefficient, and the aerosol loading. The impact of each of these errors upon the surface pressure is estimated. Pressure measurements to better than 2 hPa accuracy appear possible in principle over a wide range of surface reflectances if the temperature profile is known to an accuracy of 1 K, if the aerosol loading is known to an accuracy of 10%, and if instrumental noise can be limited to 0.1%. However, the calculations of this paper suggest that the radiometer must sample over extremely narrow spectral windows (1 cm−1) if this accuracy is to be achieved.

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C. D. Hewitt and J. F. B. Mitchell

Abstract

A simulation of the climate for 6 kyr BP, using the Hadley Centre's atmospheric GCM with prescribed SSTs is described. The control simulation successfully reproduces the large-scale features of the present-day climate and has realistic atmospheric interannual variability. The anomaly simulation for 6 kyr BP produces a climate with an enhanced Northern Hemisphere seasonal cycle, and, in particular, a strengthened African-Asian summer monsoon. Integrated over the full annual cycle, the land surface of the southern Tropics dries while the northern Tropics get wetter, and the high northern latitudes also dry. The model simulates large regional interdecadal differences in the response at 6 kyr BP highlighting the need to allow for and account for variability on long, that is, at least decadal, timescales. The authors describe the consequences of part of the experimental design employed, whereby the SSTs for the 6 kyr BP simulation are the same as in the control as recommended by the Paleoclimate Modelling Intercomparison Project, in particular, the potential importance of ocean and sea ice feedbacks.

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Henry E. Fleming, David S. Crosby, and Mitchell D. Goldberg

Abstract

Layer-mean virtual temperatures retrieved from satellite measurements are more accurate than retrievals at specific pressure not only because an averaging process is involved, but also because of advantages in the retrieval process. In this note, a “retrieval efficiency” is derived to express this advantage over simple averaging as a function of layer thickness. The efficiency is examined for two common cases of retrieval initial guess: a statistical sample mean and a forecast profile obtained from a numerical prediction model. The advantage of the layer-mean retrieval clearly is demonstrated in both cases.

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Cheng-Zhi Zou, Mei Gao, and Mitchell D. Goldberg

Abstract

The Microwave Sounding Unit (MSU) onboard the National Oceanic and Atmospheric Administration polar-orbiting satellites measures the atmospheric temperature from the surface to the lower stratosphere under all weather conditions, excluding precipitation. Although designed primarily for monitoring weather processes, the MSU observations have been extensively used for detecting climate trends, and calibration errors are a major source of uncertainty. To reduce this uncertainty, an intercalibration method based on the simultaneous nadir overpass (SNO) matchups for the MSU instruments on satellites NOAA-10, -11, -12, and -14 was developed. Due to orbital geometry, the SNO matchups are confined to the polar regions, where the brightness temperature range is slightly smaller than the global range. Nevertheless, the resulting calibration coefficients are applied globally to the entire life cycle of an MSU satellite.

Such intercalibration reduces intersatellite biases by an order of magnitude compared to prelaunch calibration and, thus, results in well-merged time series for the MSU channels 2, 3, and 4, which respectively represent the deep layer temperature of the midtroposphere (T2), tropopause (T3), and lower stratosphere (T4). Focusing on the global atmosphere over ocean surfaces, trends for the SNO-calibrated T2, T3, and T4 are, respectively, 0.21 ± 0.07, 0.08 ± 0.08, and −0.38 ± 0.27 K decade−1 from 1987 to 2006. These trends are independent of the number of limb-corrected footprints used in the dataset, and trend differences are marginal for varying bias correction techniques for merging the overlapping satellites on top of the SNO calibration.

The spatial pattern of the trends reveals the tropical midtroposphere to have warmed at a rate of 0.28 ± 0.19 K decade−1, while the Arctic atmosphere warmed 2 to 3 times faster than the global average. The troposphere and lower stratosphere, however, cooled across the southern Indian and Atlantic Oceans adjacent to the Antarctic continent. To remove the stratospheric cooling effect in T2, channel trends from T2 and T3 (T23) and T2 and T4 (T24) were combined. The trend patterns for T23 and T24 are in close agreement, suggesting internal consistencies for the trend patterns of the three channels.

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Mitchell D. Goldberg, David S. Crosby, and Lihang Zhou

Abstract

The Advanced Microwave Sounding Unit-A (AMSU-A) is the first of a new generation of polar-orbiting cross-track microwave sounders operated by the National Oceanic and Atmospheric Administration. A feature of a cross-track sounder is that the measurements vary with scan angle because of the change in the optical pathlength between the earth and the satellite. This feature is called the limb effect and can be as much as 30 K. One approach to this problem is to limb adjust the measurements to a fixed view angle. This approach was used for the older series of Microwave Sounding Units. Limb adjusting is important for climate applications and regression retrieval algorithms. This paper describes and evaluates several limb adjustment procedures. The recommended procedure uses a combined physical and statistical technique. The limb adjusted measurements were compared with computed radiances from radiosondes and National Centers for Environmental Prediction models. The model error was found to be less than the instrument noise for most of the temperature sounding channels. The error in the window channels was small relative to the observed range of these channels. Limb adjusted fields appear to be smooth. Statistical tests of the distributions of the adjusted measurements at each scan angle show them to be very similar.

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