Abstract

Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) radio occultation (RO) refractivity profiles in altostratus and nimbostratus clouds from 2007 to 2010 are first identified based on collocated CloudSat data. Vertical temperature profiles in these clouds are then retrieved from cloudy refractivity profiles. Contributions of cloud liquid water content and ice water content are also included in the retrieval algorithm. The temperature profiles and their lapse rates are compared with those from a standard GPS RO wet retrieval without including cloud effects. On average, the temperatures from cloudy retrieval are about 0.5–1.0 K warmer than the GPS RO wet retrieval, except for the altitudes near the nimbostratus base. The differences of temperature between the two methods are largest in summer and smallest in winter. The lapse rate in altostratus clouds is around 6.5°–7.5°C km⁻¹ and does not vary greatly with height. On the contrary, the lapse rate increases significantly with height in nimbostratus clouds, from about 2.5°–3.5°C km⁻¹ near the cloud base to about 5.0°–6.0°C km⁻¹ at cloud center and 6.5°–7.5°C km⁻¹ below the cloud top. Seasonal variability of lapse rate derived from the cloudy retrieval is larger than that derived from the wet retrieval. The lapse rate within clouds is smaller in summer and larger in winter. The mean lapse rate decreases with temperature in all seasons.

Abstract

Mathematical solutions accounting for the effects of liquid and ice clouds on the propagation of the GPS radio signals are first derived. The percentage contribution of ice water content (IWC) to the total refractivity increases linearly with the amount of IWC at a rate of 0.6 (g m⁻³)⁻¹. Measurements of coincident profiles of IWC from CloudSat in deep convection during 2007–10 are then used for estimating the ice-scattering effects on GPS radio occultation (RO) measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC). The percentage contribution of IWC to the total refractivity from CloudSat measurements is consistent with the theoretical model, reaching about 0.6% at 1 g m⁻³ IWC.

The GPS RO refractivity observations in deep convective clouds are found to be systematically greater than the refractivity calculated from the ECMWF analysis. The fractional N bias (GPS minus ECMWF) can be as high as 1.8% within deep convective clouds. Compared with ECMWF analysis, the GPS RO retrievals have a negative temperature bias and a positive water vapor bias, which is consistent with a positive bias in refractivity. The relative humidity calculated from GPS retrievals is usually as high as 80%–90% right above the 0°C temperature level in deep convection and is about 15%–30% higher than the ECMWF analysis. The majority of the data points in deep convection are located on the negative side of temperature differences and the positive side of relative humidity differences between GPS RO retrievals and ECMWF analysis.

Abstract

Tropical cyclone (TC) temperature and water vapor structures are essential atmospheric variables. In this study, global positioning system (GPS) radio occultation (RO) observations from the GPS RO mission named the Constellation Observing System for Meteorology, Ionosphere, and Climate and the Global Navigation Satellite System (GNSS) Receiver for Atmospheric Sounding on board both MetOp-A and MetOp-B satellites over the 9-yr period from 2007 to 2015 are used to generate a set of composite structures of temperature and water vapor fields within tropical depressions (TDs), tropical storms (TSs), and hurricanes (HUs) over the Atlantic Ocean and TDs, TSs, and typhoons (TYs) over the western Pacific Ocean. The composite TC structures are different over the two oceanic regions, reflecting different climatological environments. The warm cores for TCs over the western Pacific Ocean have higher altitudes and larger sizes than do those over the Atlantic Ocean for all storm categories. A radial variation of the warm-core temperature anomaly with descending altitude is seen, probably resulting from spiral cloud and rainband features. The large TC water vapor pressure anomalies, which are often more difficult to obtain than temperature anomalies, are located below the maximum warm-core temperature anomaly centers. Thus, the maximum values of the fractional water vapor pressure anomaly, defined as the anomaly divided by the environmental value, for TSs and HUs over the Atlantic Ocean (1.4% for TSs and 2.2% for HUs) are higher than those for TSs and TYs over the western Pacific Ocean (1.2% for TSs and 1.4% for TYs). These TC structures are obtained only after a quality control procedure is implemented, which consists of a range check that removes negative refractivity values and unrealistic temperature values, as well as a biweight check that removes data that deviate from the biweight mean by more than 3 times the biweight standard deviation. A limitation of the present study is an inability to resolve the TC inner-core structures because of a lack of sufficient RO profiles that collocate with TCs in their inner-core regions and the relatively coarse along-track resolutions of GPS RO data.

Abstract

The climate sensitivity of the Madden–Julian oscillation (MJO) is measured across a broad range of temperatures (1°–35°C) using a convection-permitting global climate model with homogenous sea surface temperatures. An MJO-like signal is found to be resilient in all simulations. These results are used to investigate two ideas related to the modern “moisture mode” view of MJO dynamics. The first hypothesis is that the MJO has dynamics analogous to a form of radiative convective self-aggregation in which longwave energy maintenance mechanisms shut down for SST ≪ 25°C. Inconsistent with this hypothesis, the explicitly simulated MJO survives cooling and retains leading moist static energy (MSE) budget terms associated with longwave destabilization even at SST < 10°C. Thus, if the MJO is a form of longwave-assisted self-aggregation, it is not one that is temperature critical, as is observed in some cases of radiative–convective equilibrium (RCE) self-aggregation. The second hypothesis is that the MJO is propagated by horizontal advection of column MSE. Inconsistent with this view, the simulated MJO survives reversal of meridional moisture gradients in the basic state and a striking role for horizontal MSE advection in its propagation energy budget cannot be detected. Rather, its eastward motion is balanced by vertical MSE advection reminiscent of gravity or Kelvin wave dynamics. These findings could suggest a tight relation between the MJO and classic equatorial waves, which would tend to challenge moisture mode views of MJO dynamics that assume horizontal moisture advection as the MJO’s propagator. The simulation suite provides new opportunities for testing predictions from MJO theory across a broad climate regime.

Abstract

The strength of the equinoctial Hadley circulation (HC) is investigated in idealized simulations conducted on an equatorial beta plane in which the zonal width of the domain is varied to either permit or suppress large-scale eddies. The presence of such eddies is found to amplify the HC by a factor of 2–3 in simulations with slab-ocean boundary conditions or with a simple representation of ocean heat transport. Additional simulations in which the eddy forcing is prescribed externally indicate that this amplification is primarily associated with large-scale eddy momentum fluxes rather than large-scale eddy heat fluxes. These results contrast with results from simulations with fixed distributions of sea surface temperature (SST), in which the HC strength has been found to be relatively insensitive to large-scale eddy momentum fluxes.

In both the interactive- and fixed-SST cases, the influence of nonlinear momentum advection by the mean flow complicates efforts to use the angular-momentum budget to constrain the HC strength. However, a strong relationship is found between the HC strength and a measure of the meridional gradient of boundary layer entropy, indicating a possible thermodynamic control on the HC strength. In simulations with interactive SSTs, meridional eddy momentum fluxes affect the boundary layer entropy by inducing a low-level frictional flow that reduces the ability of the HC to transport heat poleward. This allows for the maintenance of a large meridional entropy gradient in the presence of a strong HC. The results highlight the potential utility of a thermodynamic perspective for understanding the HC in flow regimes for which dynamical constraints may be difficult to apply.

Abstract

While several mechanisms have been suggested to account for the association of the Arctic and Antarctic Oscillations (AO/AAO) with atmospheric parameters, this paper focuses on the relationship with the atmospheric outgoing longwave radiation (OLR). The main objective of this paper is to demonstrate through AO/AAO composite analysis that the NCEP–NCAR reanalysis OLR agrees with the independent observations of the NASA Earth Radiation Budget Experiment (ERBE) broadband satellite instruments both in zonal averages and in geographically mapped space, and to verify AO/AAO characterized general circulations derived from models and analyses.

The results indicate several pronounced areas of storminess that are AO/AAO dependent. One is the well-known variation over the North Atlantic Ocean toward Europe. Also, several major areas are indicated in the tropical region—one in the Indian Ocean and the others in the west and central Pacific Ocean. In addition to demonstrating that the signals are statistically significant, also tested is the relationship of the features to other well-known tropical forcing mechanisms: the Madden–Julian oscillation (MJO) and the El Niño–La Niña sea surface temperature variations. It is shown that the tropical features do, in fact, have a strong relationship to the MJO, which indicates an additional tropical–extratropical interaction. With regard to the sea surface temperature, no correlation associated with the AO/AAO variability is seen.

These associations with the cloudiness and atmospheric radiation budget suggest that if there is to be improvement of numerical model forecasts to an extended time period that numerical model radiation physics will have to be taken into consideration and improved.

Abstract

Given a biased coupled model and the atmospheric and oceanic observing system, maintaining a balanced and coherent climate estimation is of critical importance for producing accurate climate analysis and prediction initialization. However, because of limitations of the observing system (e.g., most of the oceanic measurements are only available for the upper ocean), directly evaluating climate estimation with real observations is difficult. With two coupled models that are biased with respect to each other, a biased twin experiment is designed to simulate the problem. To do that, the atmospheric and oceanic observations drawn from one model based on the modern climate observing system are assimilated into the other. The model that produces observations serves as the truth and the degree by which an assimilation recovers the truth steadily and coherently is an assessment of the impact of the data constraint scheme on climate estimation. Given the assimilation model bias of warmer atmosphere and colder ocean, where the atmospheric-only (oceanic only) data constraint produces an overcooling (overwarming) ocean through the atmosphere–ocean interaction, the constraints with both atmospheric and oceanic data create a balanced and coherent ocean estimate as the observational model. Moreover, the consistent atmosphere–ocean constraint produces the most accurate estimate for North Atlantic Deep Water (NADW), whereas NADW is too strong (weak) if the system is only constrained by atmospheric (oceanic) data. These twin experiment results provide insights that consistent data constraints of multiple components are very important when a coupled model is combined with the climate observing system for climate estimation and prediction initialization.

Abstract

Passive microwave radiometer data over the ocean have been widely used, but data near coastlines or over lakes often cannot be used because of the large footprint with mixed signals from both land and water. For example, current standard Special Sensor Microwave Imager (SSM/I) products, including wind, water vapor, and precipitation, are typically unavailable within about 100 km of any coastline. This paper presents methods of correcting land-contaminated radiometer data in order to extract the coastal information. The land contamination signals are estimated, and then removed, using a representative antenna pattern convolved with a high-resolution land–water mask. This method is demonstrated using SSM/I data over the Great Lakes and validated with simulated data and buoy measurements. The land contamination is significantly reduced, and the wind speed retrieval is improved. This method is not restricted to SSM/I and wind retrievals alone; it can be applied more generally to microwave radiometer measurements in coastal regions for other retrieval purposes.

Abstract

The Asian monsoon and El Niño–Southern Oscillation (ENSO) are known to interact with each other. In this paper, four primary indices (the Indian monsoon rainfall index, the Webster and Yang monsoon index, the tropical-wide oscillation index, and the Southern Oscillation index) that characterize the temporal variation of these complex, chaotic and quasi-oscillatory phenomena are used to assess the action from the Asian monsoon to ENSO, that is, the linkage between the strong/weak monsoon and La Niña/El Niño. The evolution of the four previously documented indices and other auxiliary data over a 43-yr period is examined using the observed database and the reanalysis of the National Centers for Environmental Prediction. The Asian monsoon and ENSO intersect in a common area, namely, the warm pool in the western tropical Pacific. This region (e.g., 10°S–5°N, 110°–170°E) is located at the longitudinally central portion of the Walker circulation and also the equatorial end of the Indo-Pacific meridional overturning cell that is part of the zonal mean Hadley circulation. In recent decades, the connection between the monsoon and ENSO has changed considerably. This change is related to the atmospheric circulation over the entire North Pacific Ocean, which entered a new regime in about 1976. Before 1976, the correlations among the four primary indices, and those between the indices and the Niño-3 index of sea surface temperature, were strong. In recent decades, the ocean temperature in the entire North Pacific became considerably colder. The lower-tropospheric winds became simultaneously more cyclonic over the North Pacific. ENSO is now related to atmospheric fluctuations both in the Indian sector and in northeastern China. The western North Pacific monsoon in the vicinity of the Philippine Islands (9°–19°N, 139°–141°E) may play an important role together with the off-equatorial ocean heat content in a larger region (5°–15°N, 135°–170°E) in maintaining or even increasing ENSO activities.

Abstract

We use the best currently available in situ and satellite-derived surface and top-of-the-atmosphere (TOA) shortwave radiation observations to explore climatological annual cycles of fractional (i.e., normalized by incoming radiation at the TOA) atmospheric shortwave absorption $\tilde{a}$ on a global scale. The analysis reveals that $\tilde{a}$ is a rather regional feature where the reported nonexisting $\tilde{a}$ in Europe is an exception rather than the rule. In several regions, large and distinctively different $\tilde{a}$ are apparent. The magnitudes of $\tilde{a}$ reach values up to 10% in some regions, which is substantial given that the long-term global mean atmospheric shortwave absorption is roughly 23%. Water vapor and aerosols are identified as major drivers for $\tilde{a}$ while clouds seem to play only a minor role for $\tilde{a}$ . Regions with large annual cycles in aerosol emissions from biomass burning also show the largest $\tilde{a}$ . As biomass burning is generally related to human activities, $\tilde{a}$ is likely also anthropogenically intensified or forced in the respective regions. We also test if climate models are able to simulate the observed pattern of $\tilde{a}$ . In regions where $\tilde{a}$ is driven by the annual cycle of natural aerosols or water vapor, the models perform well. In regions with large $\tilde{a}$ induced by biomass-burning aerosols, the models’ performance is very limited.