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Da Yang and Andrew P. Ingersoll

Abstract

The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Despite its primary importance, a generally accepted theory that accounts for fundamental features of the MJO, including its propagation speed, planetary horizontal scale, multiscale features, and quadrupole structures, remains elusive. In this study, the authors use a shallow-water model to simulate the MJO. In this model, convection is parameterized as a short-duration localized mass source and is triggered when the layer thickness falls below a critical value. Radiation is parameterized as a steady uniform mass sink. The following MJO-like signals are observed in the simulations: 1) slow eastward-propagating large-scale disturbances, which show up as low-frequency, low-wavenumber features with eastward propagation in the spectral domain, 2) multiscale structures in the time–longitude (Hovmöller) domain, and 3) quadrupole vortex structures in the longitude–latitude (map view) domain. The authors propose that the simulated MJO signal is an interference pattern of westward and eastward inertia–gravity (WIG and EIG) waves. Its propagation speed is half of the speed difference between the WIG and EIG waves. The horizontal scale of its large-scale envelope is determined by the bandwidth of the excited waves, and the bandwidth is controlled by the number density of convection events. In this model, convection events trigger other convection events, thereby aggregating into large-scale structures, but there is no feedback of the large-scale structures onto the convection events. The results suggest that the MJO is not so much a low-frequency wave, in which convection acts as a quasi-equilibrium adjustment, but is more a pattern of high-frequency waves that interact directly with the convection.

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A. E. Dessler and P. Yang

Abstract

Thin cirrus clouds (with optical depths τ ≪ 1) play a potentially important role in the earth's atmosphere. However, their tenuous nature makes them difficult to detect, and as a result there are few quantitative, global analyses of them. The Moderate Resolution Imaging Spectrometer (MODIS) on board the Terra satellite has a channel at 1.375 μm that is specifically designed to detect these clouds, and can measure optical depths as low as 0.02 with an uncertainty factor of 2. During two 3-day periods from December 2000 and June 2001, about one-third of the pixels flagged as cloud free by the MODIS cloud mask are shown to contain detectible thin cirrus. These thin cirrus generally have optical depths below ∼0.05 and appear with greater frequency and optical depth near deep convection.

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Da Yang and Andrew P. Ingersoll

Abstract

The Madden–Julian oscillation (MJO), also known as the intraseasonal oscillation (ISO), is a planetary-scale mode of variation in the tropical Indian and western Pacific Oceans. Basic questions about the MJO are why it propagates eastward at ∼5 m s−1, why it lasts for intraseasonal time scales, and how it interacts with the fine structure that is embedded in it. This study will test the hypothesis that the MJO is not a wave but a wave packet—the interference pattern produced by a narrow frequency band of mixed Rossby–gravity (MRG) waves. As such, the MJO would propagate with the MRG group velocity, which is eastward at ∼5 m s−1. Simulation with a 3D model shows that MRG waves can be forced independently by relatively short-lived, eastward- and westward-moving disturbances, and the MRG wave packet can last long enough to form the intraseasonal variability. This hypothesis is consistent with the view that the MJO is episodic, with an irregular time interval between events rather than a periodic oscillation. The packet is defined as the horizontally smoothed variance of the MRG wave—the rectified MRG wave, which has features in common with the MJO. The two-dimensional Fourier analysis of the NOAA outgoing longwave radiation (OLR) dataset herein indicates that there is a statistically significant correlation between the MJO amplitude and wave packets of MRG waves but not equatorial Rossby waves or Kelvin waves, which are derived from the Matsuno shallow water theory. However, the biggest absolute value of the correlation coefficient is only 0.21, indicating that the wave packet hypothesis explains only a small fraction of the variance of the MJO in the OLR data.

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Yang Yang, M. Uddstrom, M. Revell, P. Andrews, and R. Turner

Abstract

The New Zealand Limited Area Model is used to investigate the impact of assimilating NOAA-15 and -16 Advanced Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (ATOVS) radiances on surface air temperature over Canterbury, New Zealand, for two föehn cases in January 2004. For both cases, the simulated westerly-northwesterly wind crossing the Southern Alps and descending in the lee (i.e., a föehn) was stronger with ATOVS data (pass 2) than without ATOVS data (pass 1). Also, for one case, the timing of the passage of a cold front over Canterbury was more accurately forecast in pass 2. The associated differences in the potential height ΔH and winds ΔV over South Island between pass 1 and pass 2 for both cases developed from small differences in the initial conditions. It is suggested the dynamical forcing of the Southern Alps contributes to the amplification of ΔH and ΔV. The enhanced ΔV led to stronger adiabatic descent in the lee (or a stronger föehn) with stronger adiabatic warming and surface diabatic heating in pass 2. Additionally, the later passage of the cold front in pass 2 during one case allowed a longer period of heating of the surface air ahead of the cold front. As a result, large well-organized differences in surface air temperature between pass 1 and pass 2 (ΔT of 4–10 K) occurred over Canterbury. Thus, the Southern Alps acted to amplify the impact of assimilating ATOVS radiances on simulated surface air temperature over Canterbury under föehn conditions. Verification with surface observations at five climate stations over Canterbury showed a positive impact of ATOVS radiance assimilation for the two cases.

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Fengge Su, Yang Hong, and Dennis P. Lettenmaier

Abstract

Satellite-based precipitation estimates with high spatial and temporal resolution and large areal coverage provide a potential alternative source of forcing data for hydrological models in regions where conventional in situ precipitation measurements are not readily available. The La Plata basin in South America provides a good example of a case where the use of satellite-derived precipitation could be beneficial. This study evaluates basinwide precipitation estimates from 9 yr (1998–2006) of Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA; 3B42 V.6) through comparison with available gauged data and the Variable Infiltration Capacity (VIC) semidistributed hydrology model applied to the La Plata basin. In general, the TMPA estimates agreed well with the gridded gauge data at monthly time scales, most likely because of the monthly adjustment to gauges performed in TMPA. The agreement between TMPA and gauge precipitation estimates was reduced at daily time scales, particularly for high rain rates. The TMPA-driven hydrologic model simulations were able to capture the daily flooding events and to represent low flows, although peak flows tended to be biased upward. There was a good agreement between TMPA-driven simulated flows in terms of their reproduction of seasonal and interannual streamflow variability. This analysis shows that TMPA has potential for hydrologic forecasting in data-sparse regions.

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S. Yang, X. Zou, and P. S. Ray

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.

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X. Zou, S. Yang, and P. S. Ray

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.

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A. J. Baran, P. N. Francis, and P. Yang

Abstract

The processes that contribute to the absorption of infrared radiation by atmospheric ice crystals are studied. The processes are separated into the geometric optics (i.e., refraction, internal, and external reflection) and above-edge (i.e., the capture of photons beyond the physical cross section of the particle via tunneling through an inertial barrier) contributions. The geometric optics and above-edge contributions to ice crystal absorption are compared and contrasted assuming ice spheres, randomly oriented hexagonal ice columns, and randomly oriented ice aggregates (i.e., systematically increasing particle complexity). The geometric optics and above-edge absorption coefficients have been calculated using the complex angular momentum approximation applied to the ice sphere, and a composite method has been used to calculate the two sets of absorption coefficients for the hexagonal ice column and ice aggregate based on the finite difference time domain and improved geometric optics methods. The impact of the geometric optics and above-edge contributions to retrieval of ice crystal effective size (r e) is studied for each particle geometry using aircraft-based downwelling radiometric measurements of cirrus at the wavelengths of 8.55 and 11.0 μm. The retrieved r e is compared with in situ measurements of crystal effective size. The profile averaged value of r e is estimated to be in the range 32–49 μm.

The retrieved r e assuming the ice sphere with the geometric optics contribution only is found to be 30.4 ± 14.2 μm, while with the above-edge contribution included, it is 15.5 ± 7.2 μm. The impact of the ice sphere above-edge contribution acts to reduce the retrieved r e by about half, and this results in the retrieved r e being about a factor of 2–3 less than the in situ measurements of r e. Interestingly, as the particle complexity increases from the ice sphere to the ice aggregate, the impact of the above-edge contribution on the retrieval of r e is found to systematically diminish. For the ice aggregate, the retrieved r e with the geometric optics contribution only is found to be 27.4 ± 4.3 μm. However, with the above-edge contribution included, it is found to be 28.6 ± 3.9 μm. Clearly, as particle complexity increases and particle symmetry decreases, the impact of the above-edge contribution on the retrieval of r e at the wavelengths of 8.55 and 11.0 μm is considerably diminished. However, in general the above-edge contribution should not be ignored and a full electromagnetic solution is still preferred in the resonance region. The findings also indicate that ice aggregates are a better representation of cirrus cloud midinfrared radiative properties than pristine solid hexagonal ice columns.

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Stephen P. Palm, Yuekui Yang, Vinay Kayetha, and Julien P. Nicolas

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Blowing snow is a frequent and widespread phenomenon over most of Antarctica. The transport and sublimation of blowing snow are important for the mass balance of the Antarctic ice sheet, and the latter is a major contributor to the hydrological cycle in high-latitude regions. Although much is known about blowing snow from surface observations, knowledge of the thermodynamic structure of deep (>50 m) blowing-snow layers is lacking. Here, dropsonde measurements are used to investigate the temperature, moisture, and wind structure of deep blowing-snow layers over Antarctica. The temperature lapse rate within the blowing-snow layer is at times close to dry adiabatic and is on average between dry and moist adiabatic. Initiation of blowing snow causes the surface temperature to increase to a degree proportional to the depth of the blowing-snow layer. The relative humidity with respect to ice is generally largest near the surface (but less than 100%) and decreases with height, reaching a minimum near the top of the layer. These findings are at odds with the generally accepted theory that blowing-snow sublimation will cool and eventually saturate the layer. The observations support the conclusion that high levels of wind-shear-induced turbulence cause mixing and entrainment of warmer air from above the blowing-snow layer, which suppresses humidity and produces the observed well-mixed temperature structure within the layer. The results may have important consequences for understanding the mass balance of the Antarctic ice sheet and the moisture budget of the atmosphere in high latitudes.

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Paloma Borque, Edward P. Luke, Pavlos Kollias, and Fan Yang

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Turbulence and drizzle-rate measurements from a large dataset of marine and continental low stratiform clouds are presented. Turbulence peaks at cloud base over land and near cloud top over the ocean. For both regions, eddy dissipation rate values of 10−5–10−2 m2 s−3 are observed. Surface-based measurements of cloud condensation nuclei number concentration N CCN and liquid water path (LWP) are used to estimate the precipitation susceptibility S 0. Results show that positive S 0 values are found at low turbulence, consistent with the principle that aerosols suppress precipitation formation, whereas S 0 is smaller, and can be negative, in a more turbulent environment. Under similar macrophysical conditions, especially for medium to high LWP, high (low) turbulence is likely to lessen (promote) the suppression effect of high N CCN on precipitation. Overall, the turbulent effect on S 0 is stronger in continental than marine stratiform clouds. These observational findings are consistent with recent analytical prediction for a turbulence-broadening effect on cloud droplet size distribution.

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