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I. R. Paluch
,
C. A. Knight
, and
L. J. Miller

Abstract

Measurements of cloud droplet size spectra from a forward scattering spectrometer probe (FSSP) in developing cumulus show an excellent correlation between the calculated radar reflectivity factor and cloud liquid water content at constant altitude, if the data are restricted to the early stage of cumulus development before coalescence growth becomes significant. The correlation is examined using droplet data from the FSSP both with and without the correction for the nonuniformities in the laser beam field of the FSSP. The close correlation between liquid water content and reflectivity factor suggests that sensitive, relatively low wavelength radar can be useful for mapping liquid water content in the early stages of cumulus development. The analysis reveals several ambiguities that require further study, including the altitude-dependent parameter in the empirical correlation, the value of which appears anomalous. The relation between liquid water content and reflectivity factor provides indirect evidence that aircraft samples collected over 100-m distances represent averages of nonuniform mixtures of cloud and entrained air, and the observed relation can be explained in terms of a simple model of a mixed region that is locally homogeneous but nonuniformly mixed over 100-m aircraft sampling distances. An “equivalent mixed interface” 50–100 m long, with linearly increasing mixing proportions, can reproduce the observed liquid water-reflectivity factor relationship.

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J. P. Thomas
,
R. J. Knight
,
H. K. Roscoe
,
J. Turner
, and
C. Symon

Abstract

Satellite radiometer measurements of global sea surface temperature (SST) with an accuracy of 0.3 K are required for climate change monitoring. In order to validate that this accuracy can be achieved, in situ measurements of sea surface radiance must be made during satellite overpasses. In the past decade attempts have been made to design self-calibrating, infrared radiometers for measuring SST from research ships, and some commercially manufactured models are now available. The British Antarctic Survey deployed one such radiometer on board the royal research ship Bransfield between October 1991 and May 1992. Its purpose was to measure SST within the Along Track Scanning Radiometer (ATSR) swath when the ERS-1 satellite passed over the ship. The ship radiometer was claimed to have an accuracy of ±0.1 K but this had not been verified under realistic measurement conditions. An evaluation of the radiometer's accuracy was therefore carried out during a voyage from the British Isles to Antarctica. At intervals throughout the voyage the temperature of well-stirred seawater in a tank on the deck of the ship was measured using both the radiometer and thermometers, which were accurate to ±0.1 K. These measurements revealed that the radiometer values of water temperature were more than 1.5 K warmer than the values given by the thermometers. The cause of this offset was thought to be incorrect calibration of platinum resistance thermometers within the instrument, and an empirical correction was derived. When the correction was applied the rms difference between the thermometer and the radiometer measurements of the temperature of the seawater in the tank was 0.1 K using the radiometer's 11-µm channel. The rms difference using the 12-µm channel was 0.14 K, which was larger because of an unidentified beat signal that affected this channel. These results, therefore, showed that this radiometer was capable of making SST measurements that were accurate enough to validate the ATSR SST and also to carry out useful investigations of the ocean skin effect.

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D. R. Fereday
,
J. R. Knight
,
A. A. Scaife
,
C. K. Folland
, and
A. Philipp

Abstract

Observed atmospheric circulation over the North Atlantic–European (NAE) region is examined using cluster analysis. A clustering algorithm incorporating a “simulated annealing” methodology is employed to improve on solutions found by the conventional k-means technique. Clustering is applied to daily mean sea level pressure (MSLP) fields to derive a set of circulation types for six 2-month seasons. A measure of the quality of this clustering is defined to reflect the average similarity of the fields in a cluster to each other. It is shown that a range of classifications can be produced for which this measure is almost identical but which partition the days quite differently. This lack of a unique set of circulation types suggests that distinct weather regimes in NAE circulation do not exist or are very weak. It is also shown that the stability of the clustering solution to removal of data is not maximized by a suitable choice of the number of clusters. Indeed, there does not appear to be any robust way of choosing an optimum number of circulation types. Despite the apparent lack of preferred circulation types, cluster analysis can usefully be applied to generate a set of patterns that fully characterize the different circulation types appearing in each season. These patterns can then be used to analyze NAE climate variability. Ten clusters per season are chosen to ensure that a range of distinct circulation types that span the variability is produced. Using this classification, the effect of forcing of NAE circulation by tropical Pacific sea surface temperature (SST) anomalies is analyzed. This shows a significant influence of SST in this region on certain circulation types in almost all seasons. A tendency for a negative correlation between El Niño and an anomaly pattern resembling the positive winter North Atlantic Oscillation (NAO) emerges in a number of seasons. A notable exception is November–December, which shows the opposite relationship, with positive NAO-like patterns correlated with El Niño.

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A. Verhoef
,
A. Diaz-Espejo
,
J. R. Knight
,
L. Villagarcía
, and
J. E. Fernández

Abstract

Data for water vapor adsorption and evaporation are presented for a bare soil (sandy loam, clay content 15%) in a southern Spanish olive grove. Water losses and gains were measured using eight high-precision minilysimeters, placed around an olive tree, which had been irrigated until the soil reached field capacity (∼0.22 m3 m−3). They were subsequently left to dry for 10 days. A pair of lysimeters was situated at each of the main points of the compass (N, E, S, W), at a distance of 1 m (the inner set of lysimeters; ILS) and 2 m (the outer set of lysimeters; OLS), respectively, from the tree trunk.

Distinct periods of moisture loss (evaporation) and moisture gain (vapor adsorption) could be distinguished for each day. Vapor adsorption often started just after noon and generally lasted until the (early) evening. Values of up to 0.7 mm of adsorbed water per day were measured. Adsorption was generally largest for the OLS (up to 100% more on a daily basis), and increased during the dry down. This was mainly the result of lower OLS surface soil moisture contents (period-average absolute difference ∼0.005 m3 m−3), as illustrated using various analyses employing a set of micrometeorological equations describing the exchange of water vapor between bare soil and the atmosphere. These analyses also showed that the amount of water vapor adsorbed by soils is very sensitive to changes in atmospheric forcing and surface variables. The use of empirical equations to estimate vapor adsorption is therefore not recommended.

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Anthony R. Lupo
,
Joseph J. Nocera
,
Lance F. Bosart
,
Eric G. Hoffman
, and
David J. Knight

Abstract

This paper examines the climatological, large-scale, and synoptic-scale aspects of South American cold surges using NCEP–NCAR gridded reanalyses for the 1992–96 period. Three common cold surge types are identified on the basis of a thickness (1000–850 hPa) criteria: type 1—a transient surge associated with weak anticyclone development east of the Andes in the absence of ridging aloft, type 2—a strong and persistent surge associated with dynamic anticyclogenesis aloft and strong surface anticyclone development east of the Andes, and type 3—a surge east of the Brazilian coastal mountains. Cold surges are most common during the winter and spring (Jun–Nov), accounting for 189 of the 256 events (74%).

Case studies of two events (19–22 Jul 1992 and 12–14 Apr 1993) are conducted from both a conventional isobaric and a potential vorticity (PV) perspective. The upper-air flow pattern in the July 1992 type 2 case is characterized by the presence of a strong ridge–trough couplet, which amplifies and becomes quasi-stationary, allowing for a deep layer of equatorward flow over South America. Dynamically, this flow pattern favors the development of a very strong surface anticyclone to the east of the Andes in response to a combination of differential anticyclonic vorticity advection, low-level cold advection, and, equivalently, positive PV advection. Because of the associated cold air damming east of the Andes, modified cool air is transported into the western part of Amazonia. Cold air damming east of the Brazilian coastal mountains is associated with the transition of the July 1992 type 2 surge into a type 3 surge.

The cold surge of April 1993 is examined as a rare event that does not fit the above classification. It is characterized by explosive cyclogenesis close to the coast of Argentina. Unlike the representative type 2 cold surge of July 1992, which tends to occur in association with southwesterly flow aloft, the April 1993 cold surge occurs beneath westerly and northwesterly flow aloft. Cold air penetration into lower latitudes is restricted because the geostrophic wind has a component directed away from the Andes equatorward of the cyclone. The dynamical forcing mechanisms associated with the April 1993 event are of smaller scale than those of the much more common surges typified by the July 1992 event.

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J. Ballard
,
C. W. P. Palmer
,
P. E. Morris
,
T. J. Nightingale
,
J. Firth
,
R. J. Knight
,
P. Venters
,
R. J. Wells
, and
J. G. Whitney

Abstract

The Improved Stratospheric and Mesospheric Sounder (ISAMS) is an infrared spectroradiometer that formed part of the science instrument payload of the Upper Atmosphere Research Satellite. An essential part of the success of ISAMS in orbit was a program of prelaunch calibration and characterization of many aspects of the instrument's performance. A brief description of ISAMS is followed by a detailed discussion of the calibration and characterization methodology, the facilities used in this program, and the results from the spectral and radiometric measurements. The results are discussed in terms of factors affecting the in-flight performance of ISAMS, particularly the spectral response of the measurement channels, the radiometric linearity, stray radiations and their dependence on the line of sight view, signal-to-noise ratios, and the sensitivity of the in-flight radiometric calibration to anticipated changes to the thermal environment within ISAMS. Some of the “lessons learned” are discussed with reference to the ISAMS design and the design of future instruments and test facilities.

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C. W. P. Palmer
,
P. Venters
,
R. J. Knight
,
J. Ballard
,
T. J. Nightingale
, and
P. E. Morris

Abstract

In Part I the Improved Stratospheric and Mesospheric Sounder instrument and its calibration facility were described, and the results of the radiometric and spectral calibrations were presented. In Part II the remaining prelaunch calibrations are discussed.

The optical calibrations include the procedure used to coalign the different spectral channels and the mapping of the resulting instantaneous fields of view. These maps revealed detailed stray light processes within the instrument. The remaining optical calibration is of angular motion of the instrument scan mirror. The pressure modulator calibrations consist of the filling procedures and algorithm validation, which allow the gas conditions within the modulator to be determined from the instrument telemetry. Some overall conclusions to the calibration process are drawn.

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W. E. Eichinger
,
H. E. Holder
,
R. Knight
,
J. Nichols
,
D. I. Cooper
,
L. E. Hipps
,
W. P. Kustas
, and
J. H. Prueger

Abstract

The Soil Moisture–Atmosphere Coupling Experiment (SMACEX) was conducted in the Walnut Creek watershed near Ames, Iowa, over the period from 15 June to 11 July 2002. A main focus of SMACEX is the investigation of the interactions between the atmospheric boundary layer, surface moisture, and canopy. A vertically staring elastic lidar was used to provide a high-time-resolution continuous record of the boundary layer height at the edge between a soybean and cornfield. The height and thickness of the entrainment zone are used to estimate the surface sensible heat flux using the Batchvarova–Gryning boundary layer model. Flux estimates made over 6 days are compared to conventional eddy correlation measurements. The calculated values of the sensible heat flux were found to be well correlated (R 2 = 0.79, with a slope of 0.95) when compared to eddy correlation measurements in the area. The standard error of the flux estimates was 21.4 W m−2 (31% rms difference between this method and surface measurements), which is somewhat higher than a predicted uncertainty of 16%. The major sources of error were from the estimates of the vertical potential temperature gradient and an assumption that the entrainment parameter A was equal to the ratio of the entrainment flux and the surface heat flux.

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Duncan Ackerley
,
Ben B. B. Booth
,
Sylvia H. E. Knight
,
Eleanor J. Highwood
,
David J. Frame
,
Myles R. Allen
, and
David P. Rowell

Abstract

A full understanding of the causes of the severe drought seen in the Sahel in the latter part of the twentieth-century remains elusive some 25 yr after the height of the event. Previous studies have suggested that this drying trend may be explained by either decadal modes of natural variability or by human-driven emissions (primarily aerosols), but these studies lacked a sufficiently large number of models to attribute one cause over the other. In this paper, signatures of both aerosol and greenhouse gas changes on Sahel rainfall are illustrated. These idealized responses are used to interpret the results of historical Sahel rainfall changes from two very large ensembles of fully coupled climate models, which both sample uncertainties arising from internal variability and model formulation. The sizes of these ensembles enable the relative role of human-driven changes and natural variability on historic Sahel rainfall to be assessed. The paper demonstrates that historic aerosol changes are likely to explain most of the underlying 1940–80 drying signal and a notable proportion of the more pronounced 1950–80 drying.

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B. Guenther
,
W. Barnes
,
E. Knight
,
J. Barker
,
J. Harnden
,
R. Weber
,
M. Roberto
,
G. Godden
,
H. Montgomery
, and
P. Abel

Abstract

The Moderate-Resolution Imaging Spectroradiometer (MODIS) is a key instrument on the NASA Earth Observing System. It is a multispectral sensor that will be used to track long-term global change in the land, atmosphere, and ocean components of the earth. Major advances are being made with MODIS over previous sensors in the form of improved on-orbit sensor characterization and calibration using a system of onboard calibrators. This article describes those calibrators and provides an early estimate of the expected accuracy for the MODIS calibrated datasets resulting from the use of these calibrators. The focus of the paper is the calibration approach that is being implemented at-launch for the top-of-the-atmosphere data products.

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