Abstract

Seasonal variations of liquid and ice water paths (LWP and IWP) in nonprecipitating clouds over oceans are estimated for 4 months by combining the International Satellite Cloud Climatology Project (ISCCP) and Special Sensor Microwave/Imager (SSM/I) data. The ISCCP data are used to separate clear/cloudy skies and warm/cold clouds and to determine cloud optical thickness, cloud-top temperature, and sea surface temperature. SSM/I data are used to separate precipitating and nonprecipitating clouds and to determine LWP. About 93% of all clouds are nonprecipitating clouds, and about half of nonprecipitating clouds are warm (cloud-top temperature > 0°C). The average LWP for warm nonprecipitating clouds is about 6 mg cm⁻². The values of total water path obtained from the ISCCP values of optical thickness for cold nonprecipitating clouds are larger than the LWP values from SSM/I, which the authors explain in terms of IWP. The average IWP for cold nonprecipitating clouds is about 7 mg cm⁻², with LWP being about 5 Mg cm⁻². Tropical and cold hemisphere clouds have higher IWP values (around 10 mg cm⁻²) than those in warm hemispheres; where LWP values for warm nonprecipitating clouds vary little with latitude or season. Ice fractions, IWP/(LWP + IWP), in cold nonprecipitating clouds increase systematically with decreasing cloud-top temperatures, reaching 50% at about −15°C but ranging from about −5° to −10°C in the northern midlatitudes in autumn and the Tropics year-round to about −25°C in the southern midlatitudes in summer. The ratio of IWP to LWP in cold nonprecipitating clouds reaches almost 3 in the northern midlatitudes in autumn and falls as low as 0.6 in the southern midlatitudes in spring-summer. Combining warm and cold nonprecipitating clouds gives a global ratio of IWP to LWP that is about 0.7 over oceans.

Abstract

An algorithm for extending one-dimensional, forward-in-time, upstream-biased, flux-form transport schemes (e.g., the van Leer scheme and the piecewise parabolic method) to multidimensions is proposed. A method is also proposed to extend the resulting Eulerian multidimensional flux-form scheme to arbitrarily long time steps. Because of similarities to the semi-Lagrangian approach of extending time steps, the scheme is called flux-form semi-Lagrangian (FFSL). The FFSL scheme can be easily and efficiently implemented on the sphere. Idealized tests as well as realistic three-dimensional global transport simulations using winds from data assimilation systems are demonstrated. Stability is analyzed with a von Neuman approach as well as empirically on the 2D Cartesian plane. The resulting algorithm is conservative and upstream biased. In addition, it contains monotonicity constraints and conserves tracer correlations, therefore representing the physical characteristics of constituent transport.

Abstract

Sea level rise results from several contributing physical processes, including ocean thermal expansion and glacier and ice sheet mass loss. Future projections of sea level remain highly uncertain due to several sources of aleatory and epistemic uncertainty. Quantifying different sources of sea level rise involves considering possible pathways of future radiative forcing and integrating models of different sea level rise processes. The probabilistic hazard analysis strategy has been proposed for combining sea level rise prediction models and climate forcing scenarios to examine sea level rise prediction uncertainty and the sources of this uncertainty. In this study we carry out an illustrative probabilistic sea level rise hazard analysis using ensembles of sea level rise predictions and emissions scenarios from the literature. This illustrative analysis allows us to estimate the probability that sea level rise will exceed a specified threshold at a given location and time and highlights how sea level rise uncertainty is sensitive to scenario inputs and sea level rise projection modeling choices. Probabilistic hazard is depicted for Earth using sea level rise hazard maps. We also demonstrate how hazard deaggregation can help us quantify the relative contributions of sea level rise sources, prediction models, and climate forcing scenarios to sea level rise hazard. The ice sheet contribution to sea level rise has a large impact on probabilistic projection of sea level rise due to the disagreements between current ice sheet models related to differences in modeling ice sheet instability.

Abstract

Numerical experiments on the wind-driven ocean circulation in a closed basin show that mesoscale eddiescan appear spontaneously during the integration of the equations of motion for a baroclinic ocean. For somevalues of the basic parameters governing the flow, the solutions reach a steady state while for other valuesfinite-amplitude eddies remain a part of the final statistically steady state. In the eddying cases the solutionscan be regarded as a mean flow upon which is superimposed a set of eddies which propagate westward at afew kilometers per day. The eddies typically have horizontal wavelengths of a few hundred kilometers.

Analyses of die energetics show the eddies to be generated by the process of baroclinic instability. Thepotential energy of the mean flow is released to supply energy to the eddies. The computed Reynoldsstresses, while small compared to the terms in the geostrophic balance of the mean momentum equations, dohave a strong influence on the mean circulation and, in fact, the deep mean circulation is driven entirelyby the eddies. If the flow were steady, there would be no flow in the deep layer in this model. Finally, thecomputed curl of the Reynolds stresses shows that the vorticity balance of the mean flow is strongly affected by the presence of mesoscale eddies.

In the first part of this report we describe the two-layer model and discuss its numerical formulation.Then the results of a preliminary eddy experiment are discussed in detail, showing the spontaneous growthof baroclinic eddies and describing the final statistical steady state that occurs. Energetic analyses andvorticity balances show the important role played by the eddies in determining the character of the oceanicgeneral circulation.

Part II of this paper will discuss a variety of experiments which explore the dependence of results on thebasic parameters and boundary conditions governing the model. In particular the dependence of resultson wind stress magnitude and distribution, lateral viscosity coefficient, basin size, and boundary conditions(free slip and no slip) will be examined.

Abstract

The response of a stratified atmosphere to local heating is a common element in several problems in mesoscale dynamics. To investigate this response, a time-dependent linearized problem is solved analytically for an elevated, local heat source turned on as a pulse in a stratified, moving fluid. The thermally induced circulation in the vicinity of the drifting disturbance is qualitatively similar to that of a cumulus cloud in mean wind. The updraft at the center of this cloud is surrounded by the compensating downdrafts at early times even if that air has also been heated. Once the updraft at the drifting center weakens, upward motion begins in the adjacent regions. An integration of the pulse solution yields the response to steady heating, turned on at t = 0. As steady state is approached, this solution exhibits a region of positive displacement moving downstream while negative displacement develop near the stationary heat source. The solution offers an explanation to a curious negative phase relationship between heating and displacement and the lack of a true steady state noted by other authors. It is suggested that the nature of this response may help to explain three problems in mesoscale dynamics: cloud interaction, heat island/orographic rain, and the squall line.

Abstract

In this investigation the wind-driven ocean circulation theories are extended to include mesoscale eddiesas an integral part of the general circulation of the ocean. A two-layer numerical model of ocean circulationin a simple, rectangular basin driven by a steady wind stress is used for this purpose. The equations ofmotion are integrated as an initial value problem until the solutions reach either a steady state or, in thecase of an ocean in which eddies have appeared spontaneously as a result of baroclinic instability, a statistically steady state.

Part I of this study discussed the formulation of the numerical model and presented results from a preliminary numerical experiment. Energetic analyses showed that eddies result from baroclinic instability during the spin-up of the ocean from rest and that, in the final statistically steady state, the eddy momentum and buoyancy fluxes played an important role in establishing the mean circulation. In the particular caseexamined there, the region of eddy generation was in the westward return flow and not in the strong boundary jets.

In this part of the study, results from ten additional experiments are examined to understand, in a limitedway, how eddy generation and the resulting eddy statistics depend upon the basic parameters describing themodel ocean. In particular, the dependence of results on the coefficient of lateral viscosity, the wind stressamplitude, the wind stress distribution (one and two gyres), the basin size, and the boundary conditions(slip and no-slip) are discussed. Results show a wide range of model behavior under the conditions examined,but the common result is that the mean circulation of eddying oceans is importantly altered, one mighteven say largely determined, by the statistical nature of the eddy field.

Abstract

A new U.S. Climate Reference Network (USCRN) was officially and nationally commissioned by the Department of Commerce and the National Oceanic and Atmospheric Administration in 2004. During a 1-yr side-by-side field comparison of USCRN temperatures and temperatures measured by a maximum–minimum temperature system (MMTS), analyses of hourly data show that the MMTS temperature performed with biases: 1) a systematic bias–ambient-temperature-dependent bias and 2) an ambient-solar-radiation- and ambient-wind-speed-dependent bias. Magnitudes of these two biases ranged from a few tenths of a degree to over 1°C compared to the USCRN temperatures. The hourly average temperatures for the USCRN were the dependent variables in the development of two statistical models that remove the biases due to ambient temperature, ambient solar radiation, and ambient wind speed in the MMTS. The model performance was examined, and the results show that the adjusted MMTS data were substantially improved with respect to both systematic bias and the bias associated with ambient solar radiation and ambient wind speed. In addition, the results indicate that the historical temperature datasets prior to the MMTS era need to be further investigated to produce long-term homogenous times series of area-average temperature.

Abstract

A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-divergence flow structure; 6) the evolution of a traveling 700 mb shortwave; 7) the development of a rear-inflow jet; 8) the leading convective rainfall followed by a transition zone and trailing stratiform precipitation; 9) the observed configuration of front-to-rear relative flow at both upper and lower levels separated by the rear-to-front flow at midlevels; 10) the simulation of “onion-shaped” soundings; 11) the splitting of the wake low; 12) the maintenance and intensification of a mesovortex; 13) the distribution and magnitude of convective and stratiform rainfall; and 14) the diurnal cycle of the planetary boundary layer.

One of the encouraging results is that the model accurately simulates the rear-inflow jet as verified against Doppler windprofiler data after the 18-h integration from essentially synoptic-scale initial conditions. The results confirm the previously proposed hypothesis that the wake low develops hydrostatically as a consequence of adiabatic warming by descending flow entering the squall line within the rear-inflow jet The observed “onion-shaped” soundings are a manifestation of the warming and drying of air within the descending rear inflow jet. It is found that the present wake low is not a transient meso-β scale phenomenon, but has a time scale of more than 50% of the squall line lifetime. Another finding is that the present mesovortex is not produced by latent heat release associated with the squall line but was in existence prior to the model initialization time. The vortex appears to have a significant effect on the distribution of the rainfall associated with the squall line and on the intensity of the rear-inflow jet. Other mesoscale circulation features are also documented in this paper.

This study, along with previous investigations using the model, indicates that the meso-β scale structure and evolution of MCSs under certain synoptic-scale environmental conditions can be well simulated using the standard network observations if compatible grid resolution, reasonable model physics and initial conditions are utilized.

Abstract

In 2004 a new aspirated surface air temperature system was officially deployed nationally in the U.S. Climate Reference Network (USCRN) commissioned by the National Oceanic and Atmospheric Administration. The primary goal of the USCRN is to provide future long-term and high-quality homogeneous observations of surface air temperature and precipitation that can be coupled to past long-term observations for the detection and attribution of present and future climate change. In this paper two precision air temperature systems are included for evaluating the new USCRN air temperature system based on a 1-yr side-by-side field comparison. The measurement errors of the USCRN temperature sensor are systematically analyzed, and the components of error attributable to the datalogger, lead wires, fixed resistors, and the temperature coefficient of the resistors are presented. Although the current configuration is adequate, a more desirable configuration of USCRN temperature sensor coupled with the datalogger is proposed as a means of further reducing the uncertainty for the USCRN temperature measurement.

Abstract

Aerosols can influence cloud radiative effects and, thus, may alter interpretation of how Earth’s radiative budget responds to climate forcing. Three different ensemble experiments from the same climate model with different greenhouse gas and aerosol scenarios are used to analyze the role of aerosols in climate feedbacks and their spread across initial condition ensembles of transient climate simulations. The standard deviation of global feedback parameters across ensemble members is low, typically 0.02 W m⁻² K⁻¹. Feedbacks from high (8.5 W m⁻²) and moderate (4.5 W m⁻²) year 2100 forcing cases are nearly identical. An aerosol kernel is introduced to remove effects of aerosol cloud interactions that alias into cloud feedbacks. Adjusted cloud feedbacks indicate an “aerosol feedback” resulting from changes to climate that increase sea-salt emissions, mostly in the Southern Ocean. Ensemble simulations also indicate higher tropical cloud feedbacks with higher aerosol loading. These effects contribute to a difference in cloud feedbacks of nearly 50% between ensembles of the same model. These two effects are also seen in aquaplanet simulations with varying fixed drop number. Thus aerosols can be a significant modifier of cloud feedbacks, and different representations of aerosols and their interactions with clouds may contribute to multimodel spread in climate feedbacks and climate sensitivity in multimodel archives.