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Steven C. Sherwood
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Steven C. Sherwood

Abstract

A retrieval of ice crystal size near the tops of active deep cumulonimbus clouds (Cb) throughout the Tropics over a 12-yr period is presented based on radiances from the 3.7-μm channel of the Advanced Very High Resolution Radiometer (AVHRR). Effective diameters (D e) are 10%–20% smaller over land than ocean. Downwind of continents, crystals are smallest when low-level, offshore transport is strongest. Other regional, seasonal, interannual, and long-term variations are also found. These are compared with variations of the Total Ozone Mapping Spectroradiometer (TOMS) retrieved tropospheric aerosol and with variations of convective intensity and amount in an effort to identify potential causes by statistical association. Ice particles prove to be smaller when aerosol amounts are greater and when convection is more intense, but appear unrelated to convective rate of occurrence. Aerosols appear to be the most important influence on seasonal and longer timescales, with a consistent ∼20% decrease in Cb crystal effective diameter per unit increase in TOMS aerosol index in regions of biomass burning. In the Sahel region of Africa, where dust and burning both contribute to TOMS retrievals, this sensitivity is closer to ∼10%. The variety of signals makes the possibility of accidental statistical association unlikely, although it cannot be ruled out. Based on these numbers and on geographic maps of D e, open biomass burning appears to be more important than urban sources of aerosol in influencing Cb microphysics. Dust generated near burning sites may also contribute to the observed aerosol influence on D e.

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Steven C. Sherwood

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A simple four-cell model of the tropical atmosphere in equilibrium with its boundaries is introduced, which can support a variable diabatic circulation and prognostic temperature and humidity profiles. The model is used to predict atmospheric perturbations away from the observed base state. Prognostic variables include radiation, surface fluxes, and dynamic transports, with temperature and water vapor levels determined by conservation constraints. The model includes a specially developed water vapor scheme that performs favorably compared with observations. The model is used to simulate the local and nonlocal sensitivity of the tropical maritime atmosphere to changes in surface temperature and other boundary conditions at very large horizontal scales. The main findings are as follows. (i) The sensitivity of boundary layer convergence to sea surface temperature (SST) variations depends on the behavior of convective heating over cooler regions and may be overestimated by heuristic models that ignore or oversimplify thermodynamic and radiative constraints. (ii) The maintenance of humidity equilibrium over weakly convective areas is modulated by local radiative feedback. (iii) Evaporation feedbacks on SST may be overestimated by heuristic arguments that do not carefully treat atmospheric water transport. An explanation for the constant–relative humidity behavior of general circulation models under climate changes is also offered based on the results.

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Steven C. Sherwood

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Analyzed wind fields are used to perform a simple advection of moisture by the large-scale circulation in three dimensions at 2.5° resolution. The unresolved moisture sink Q 2 due to convection is neglected, except in regions of strong ascent where it is used to enforce a 90% relative humidity ceiling, as determined from sounding and geostationary satellite observations. The result is a simulation of water vapor that agrees quantitatively with satellite (Special Sensor Microwave Water Vapor) and sounding observations over the tropical oceans, in both arid and moist regions, to within 10% relative humidity or better from 700 to 300 mb inclusively. Horizontal transport into arid regions from convective regions is accomplished by large coherent structures. Implications of the results for the role of convection in maintaining the observed humidity distribution, and for the interpretation of observed correlations between cloud cover and vapor, are discussed.

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Steven C. Sherwood

Abstract

All instrumental climate records are affected by instrumentation changes and variations in sampling over time. While much attention has been paid to the problem of detecting “change points” in time series, little has been paid to the statistical properties of climate signals that result after adjusting (“homogenizing”) the data—or to the effects of the irregular sampling and serial correlation exhibited by real climate records. These issues were examined here by simulating multistation datasets. Simple homogenization methods, which remove apparent artifacts and then calculate trends, tended to remove some of the real signal. That problem became severe when change-point times were not known a priori, leading to significant underestimation of real and/or artificial trends. A key cause is false detection of change points, even with nominally strict significance testing, due to serial correlation in the data. One conclusion is that trends in previously homogenized radiosonde datasets should be viewed with caution.

Two-phase regression reduced but did not resolve this problem. A new approach is proposed in which trends, change points, and natural variability are estimated simultaneously. This is accomplished here for the case of incomplete data from a fixed station network by an adaptation of the “iterative universal Kriging” method, which converges to maximum-likelihood parameters by iterative imputation of missing values. With careful implementation this method’s trend estimates had low random errors and were nearly unbiased in these tests. It is argued that error-free detection of change points is neither realistic nor necessary, and that success should be measured instead by the integrity of climate signals.

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Steven C. Sherwood

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Conditions leading to convective outbreak in the Tropics are investigated by multivariate analysis of sounding and satellite data from the tropical western Pacific area. Circumstances that make the prediction problem difficult are discussed and addressed by applying linear “error-in-variables” and nonlinear statistical simulation techniques to a large dataset.

Low- to midtropospheric moisture is identified as the dominant factor regulating convective outbreak in this region. Based on the results it is argued that such moisture is particularly important in regulating spontaneous convective outbreak, but instability and near-surface wind speed probably play some role in allowing previous shallow or midtopped cumulus activity to deepen. Mesoscale-mean convective available potential energy sufficient for convection is found to exist almost 90% of the time.

Quantitative estimates of noise in the data are obtained and accounted for in reaching these conclusions. The results imply that large-scale mean fields alone may not contain enough information to determine the behavior of convection except probabilistically. Both types of statistical model predict that even under favorable mesoscale-mean conditions, convection is typically only 20%–30% likely to break out during a given 3-h period.

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Steven C. Sherwood

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An argument is made that the concepts of conditional instability and conditional symmetric instability need to be revisited. Confusion in the profession has led to two extant definitions of conditional instability that are superficially similar but fundamentally inconsistent. Only one definition corresponds to a real instability. Further, spillover of this confusion into the analysis of slantwise instability mechanisms appears to be producing inappropriate diagnosis of conditional symmetric instability. Concepts from hydrodynamic stability theory are helpful in discussing the situation.

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Steven C. Sherwood

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The water vapor budget of the free troposphere of the maritime Tropics is investigated using radiosonde observations, analyzed fields, and satellite observations, with particular attention paid to regions free of organized convection. In these arid regions, time-average drying by subsidence must be balanced by moistening via horizontal advection from convective areas and via vertical turbulent transport from below. It is found that for at least 25% of the maritime Tropics, 80% ± 10% of this source above 700 mb is by horizontal advection. The remainder comes from vertical convective transport (scales <250 km), with a pronounced local maximum at 500 mb. The regions for which this is true are characterized by pentad outgoing longwave radiation >270 W m−2 and may be said to exist out of equilibrium with the surface as regards moisture. Transport from below makes a significant contribution between 700 and 800 mb, despite the usual presence of an inversion below these levels, but is difficult to quantify accurately. The convective transport convergence is estimated as a residual from large-scale budgets and directly from sounding time series by an independent method, which shows a narrow maximum at 500 mb.

Half of the paper addresses the question of data accuracy, including sounding and analyzed data, as it pertains to the question at hand. It is concluded that the moisture budgets from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are of useful accuracy despite some significant mean descrepancies between the analyses and sounding observations in convective areas. The budget is found to be similar to that of a general circulation model based on the ECMWF forecasting model. Humidity measurements from operational soundings appear responsive below 300 mb, but then abruptly become unresponsive.

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Steven C. Sherwood and Ralph Wahrlich

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Using a compositing technique, the temporal progression of tropical convective systems and the mean atmospheric state in their vicinity is constructed from a time series of geostationary satellite and operational rawinsonde data. The technique establishes the stage in the life cycle of convection prevailing at a given place and time, by a simple objective method using time series of satellite brightness temperature (T b) histograms collected from a region surrounding the site. Soundings are classified according to their placement in the convective life cycle, and composites formed that represent the areal-mean state of the convecting atmosphere at each stage, for several scales of horizontal averaging.

The temporal structure found here for the mesoscale-mean atmosphere closely resembles existing observations of the horizontal structure of a tropical squall line, albeit with certain reductions in amplitude and stabilization rate. This supports the generality of previous findings that the physical mechanisms documented for squall line systems are characteristic of other forms of tropical convection, and quantifies their imprint on thermodynamic mean fields at large spatial scales. The results show an instability decay time during convection of about 3 h at the 120-km horizontal scale. This time grows with scale, as does the duration of the mature stage of convection, which is of similar magnitude. The results also show a column-integrated loss of water vapor and moist static energy following convection. These results may be of use in model validation and theoretical treatments of convective interaction with dynamics.

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Penelope Maher and Steven C. Sherwood

Abstract

Precipitation is influenced by multiple large-scale natural processes. Many of these large-scale precipitation “drivers” are not independent of one another, which complicates attribution. Moreover, it is unclear whether natural interannual drivers alone can explain the observed longer-term precipitation trends or account for projected precipitation changes with global warming seen in climate models. Separating the main interannual drivers from processes that may prevail on longer time scales, such as a poleward circulation shift or increased specific humidity, is essential for an improved understanding of precipitation variability and for making longer-term predictions.

In this study, an objective approach to disentangle multiple sources of large-scale variability is applied to Australian precipitation. This approach uses a multivariate linear independence model, involving multiple linear regressions to produce a partial correlation matrix, which directly links variables using significance thresholds to avoid overfitting. This is applied to regional winter precipitation in Australia as a test case, using the ECMWF Interim Re-Analysis (ERA-Interim) and Australian Water Availability Project datasets. Traditional drivers and several drivers associated with the width of the tropics are assessed.

The results show that the web of interactions implied by correlations can be simplified using this multivariate linear independence model approach: the total number of apparent precipitation drivers was reduced in each region studied, compared to correlations meeting the same statistical significance. Results show that the edge of the tropics directly influences regional precipitation in Australia and also has an indirect influence, through the interaction of the subtropical ridge and atmospheric blocking. These results provide observational evidence that changes associated with an expansion of the tropics reduce precipitation in subtropical Australia.

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