If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization

. In hydrological climate-change impact studies, regional climate models (RCMs) are commonly used to transfer large-scale global climate model (GCM) data to smaller scales and to provide more detailed regional information. Due to systematic and random model errors, however, RCM simulations often show considerable deviations from observations. This has led to the development of a number of correction approaches that rely on the assumption that RCM errors do not change over time. It is in principle not possible to test whether this underlying assumption of error stationarity is actually fulfilled for future climate conditions. In this study, however, we demonstrate that it is possible to evaluate how well correction methods perform for conditions different from those used for calibration with the relatively simple differential split-sample test. For five Swedish catchments, precipitation and temperature simulations from 15 different RCMs driven by ERA40 (the 40 yr reanalysis product of the European Centre for Medium-Range Weather Forecasts (ECMWF)) were corrected with different commonly used bias correction methods. We then performed differential split-sample tests by dividing the data series into cold and warm respective dry and wet years. This enabled us to cross-evaluate the performance of different correction procedures under systematically varying climate conditions. The differential split-sample test identified major differences in the ability of the applied correction methods to reduce model errors and to cope with non-stationary biases. More advanced correction methods performed better, whereas large deviations remained for climate model simulations corrected with simpler approaches. Therefore, we question the use of simple correction methods such as the widely used delta-change approach and linear transformation for RCM-based climate-change impact studies. Instead, we recommend using higher-skill correction methods such as distribution mapping.

Is bias correction of Regional Climate Model (RCM) simulations possible for non-stationary conditions?

Although much attention has been paid to investigating and controlling air pollution in China, the trends of air-pollutant concentrations on a national scale have remained unclear. Here, we quantitatively investigated the variation of air pollutants in China using long-term comprehensive data sets from 2013 to 2017, during which Chinese government made major efforts to reduce anthropogenic emission in polluted regions. Our results show a significant decreasing trend in the PM2.5 concentration in heavily polluted regions of eastern China, with an annual decrease of ∼7% compared with measurements in 2013. The measured decreased concentrations of SO2, NO2 and CO (a proxy for anthropogenic volatile organic compounds) could explain a large fraction of the decreased PM2.5 concentrations in different regions. As a consequence, the heavily polluted days decreased significantly in corresponding regions. Concentrations of organic aerosol, nitrate, sulfate, ammonium and chloride measured in urban Beijing revealed a remarkable reduction from 2013 to 2017, connecting the decreases in aerosol precursors with corresponding chemical components closely. However, surface-ozone concentrations showed increasing trends in most urban stations from 2013 to 2017, which indicates stronger photochemical pollution. The boundary-layer height in capital cities of eastern China showed no significant trends over the Beijing-Tianjin-Hebei, Yangtze River Delta and Pearl River Delta regions from 2013 to 2017, which confirmed the reduction in anthropogenic emissions. Our results demonstrated that the Chinese government was successful in the reduction of particulate matter in urban areas from 2013 to 2017, although the ozone concentration has increased significantly, suggesting a more complex mechanism of improving Chinese air quality in the future.

/pdf/contrasting-trends-of-pm2-5-and-surface-ozone-concentrations-1vf1fdgnx5.pdf

Contrasting trends of PM2.5 and surface-ozone concentrations in China from 2013 to 2017

Influence of meteorological conditions on PM2.5 concentrations across China: A review of methodology and mechanism.

Aerosols have significant and complex impacts on regional climate in East Asia. Cloud‐aerosol‐precipitation interactions (CAPI) remain most challenging in climate studies. The quantitative understanding of CAPI requires good knowledge of aerosols, ranging from their formation, composition, transport, and their radiative, hygroscopic, and microphysical properties. A comprehensive review is presented here centered on the CAPI based chiefly, but not limited to, publications in the special section named EAST‐AIRcpc concerning (1) observations of aerosol loading and properties, (2) relationships between aerosols and meteorological variables affecting CAPI, (3) mechanisms behind CAPI, and (4) quantification of CAPI and their impact on climate. Heavy aerosol loading in East Asia has significant radiative effects by reducing surface radiation, increasing the air temperature, and lowering the boundary layer height. A key factor is aerosol absorption, which is particularly strong in central China. This absorption can have a wide range of impacts such as creating an imbalance of aerosol radiative forcing at the top and bottom of the atmosphere, leading to inconsistent retrievals of cloud variables from space‐borne and ground‐based instruments. Aerosol radiative forcing can delay or suppress the initiation and development of convective clouds whose microphysics can be further altered by the microphysical effect of aerosols. For the same cloud thickness, the likelihood of precipitation is influenced by aerosols: suppressing light rain and enhancing heavy rain, delaying but intensifying thunderstorms, and reducing the onset of isolated showers in most parts of China. Rainfall has become more inhomogeneous and more extreme in the heavily polluted urban regions.

East Asian Study of Tropospheric Aerosols and their Impact on Regional Clouds, Precipitation, and Climate (EAST-AIRCPC)

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019GL082666

Shift in the Temporal Trend of Boundary Layer Height in China Using Long-Term (1979–2016) Radiosonde Data

. Our knowledge is still poor regarding the response of the precipitation vertical
structure to aerosols, partly due to the ignorance of precipitation occurring
at different spatial scales. A total of 6 years of collocated ground-based PM 10 
and satellite-based (Tropical Rainfall Measuring Mission, TRMM) radar data, along with ERA-Interim reanalysis, are
used in this study to investigate the aerosol effects on three localized rain
regimes (shallow, stratiform, and convective rain) over the Pearl River Delta
region of China. A subjective analysis method is proposed to discriminate
between the localized and synoptic-scale precipitations based on weather
composite charts where daily averaged wind field at 850 hPa is overlaid with
the geopotential height at 500 hPa. In general, average rain rate tends to be
greater under polluted conditions than under clean conditions. But such
potential aerosol effects are regime dependent: as the atmosphere becomes
slightly polluted (PM 10≤38 µ g m −3 ), the top 1 % radar reflectivity ( Z ) for all regimes
initially increases, followed by continued increases and weak decreases for
convective and stratiform/shallow rain regimes, respectively. As the
atmosphere becomes much more polluted, such regime dependences of aerosol
effects are more significant. From a perspective of the vertical Z structure,
comparisons between polluted conditions (days with the highest third of
PM 10 concentration) and clean conditions (days with the lowest third of
PM 10 concentration) show that the convective rain regime exhibits a deeper
and stronger Z pattern, whereas a much shallower and weaker Z pattern is
observed for stratiform and shallow precipitation regimes. In particular, the
top height of the 30 dBZ rain echo increases by ∼29  %
( ∼1.27  km) for the convective regime, but decreases by
 ∼10.8  % ( ∼0.47  km) for the stratiform regime.
However, no noticeable changes are observed for the shallow precipitation regime.
Impacts of meteorological factors are further studied on both rain top height
(RTH) and the center of gravity of Z , including vertical velocity, vertical wind
shear, convection available potential energy, and vertically integrated
moisture flux divergence (MFD). The possible invigoration effect on convective
precipitation seems dependent on wind shear, in good agreement with previous
findings. Overall, the observed dependence of the precipitation vertical
structure on ground-based PM 10 supports the notion of aerosol
invigoration or suppression effect on cold or warm rain and adds new insights into
the nature of the complex interactions between aerosol and various localized
precipitation regimes.

/pdf/aerosol-induced-changes-in-the-vertical-structure-of-1y3f0n8ebu.pdf

Aerosol-induced changes in the vertical structure of precipitation: a perspective of TRMM precipitation radar

Key Points: • Develop an algorithm to track mesoscale convective systems globally using satellite infrared brightness temperature and precipitation d ata • Satellite-based tracking reproduces mesoscale conve tive system statistics derived from tracking using ground-based radar network data • Global mesoscale convective system characteristics and their regional and seasonal variabilities are presented

A Global High-Resolution Mesoscale Convective System Database Using Satellite-Derived Cloud Tops, Surface Precipitation, and Tracking

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019EA000620

On the Relationship Between Aerosol and Boundary Layer Height in Summer in China Under Different Thermodynamic Conditions

The local‐scale precipitation (LSP) is mainly driven by thermal convection. Here we reveal a decreasing trend in the summertime LSP frequency over both China and the United States by utilizing the hourly rain gauge data from 1981 to 2012. The contrasting aerosol trend likely contributes to this same declining trend of LSP in both countries. As aerosol optical depth (AOD) goes beyond the turning zone of 0.25–0.30, the impact of aerosol on precipitation changes from invigoration to suppression. The mean AOD is generally less and larger than this range and of opposite trends in China and United States, respectively, which likely accounts for the same declining trend of LSP hours in the two countries. The observed boomerang shape points to the importance of aerosol loading, which matters as much as, if not more than the AOD trend, thereby potentially serving as a constraint for climate model evaluation. Plain Language Summary Local‐scale precipitation (LSP) is an integral part of the freshwater cycle. Here, we show that summer LSP hours have significantly declined in the United States and China over the past three decades, a phenomenon that cannot be well explained by global warming. The relationship between LSP hours and aerosol loading is a boomerang shape; a turning zone exists for the shifting effect of aerosols from enhancing to suppressing rainfall as aerosol loading increases. China is above this zone with an increasing aerosol trend, and the United States is below it with a decreasing trend, but they have similar reductions in LSP hours. This disparate role of aerosols in the rainfall process requires holistic thinking about air pollution and climate change.

Dandan Chen

Papers

Shift in the Temporal Trend of Boundary Layer Height in China Using Long-Term (1979–2016) Radiosonde Data

Aerosol-induced changes in the vertical structure of precipitation: a perspective of TRMM precipitation radar

A Global High-Resolution Mesoscale Convective System Database Using Satellite-Derived Cloud Tops, Surface Precipitation, and Tracking

On the Relationship Between Aerosol and Boundary Layer Height in Summer in China Under Different Thermodynamic Conditions

Declining Summertime Local-Scale Precipitation Frequency Over China and the United States, 1981–2012: The Disparate Roles of Aerosols