Dong L. Wu

Arctic low cloud changes as observed by MISR and CALIOP: Implication for the enhanced autumnal warming and sea ice loss

[1]xa0Retreat of Arctic sea ice extent has led to more evaporation over open water in summer and subsequent cloud changes in autumn. Studying recent satellite cloud data over the Arctic Ocean, we find that low (0.5–2 km) cloud cover in October has been increasing significantly during 2000–2010 over the Beaufort and East Siberian Sea (BESS). This change is consistent with the expected boundary layer cloud response to the increasing Arctic evaporation accumulated during summer. Because low clouds have a net warming effect at the surface, October cloud increases may be responsible for the enhanced autumnal warming in surface air temperature, which effectively prolong the melt season and lead to a positive feedback to Arctic sea ice loss. Thus, the new satellite observations provide a critical support for the hypothesized positive feedback involving interactions between boundary layer cloud, water vapor, temperature, and sea ice in the Arctic Ocean.

Global survey of concentric gravity waves in AIRS images and ECMWF analysis

Concentric gravity waves (CGWs) are atmospheric phenomena with ring-shape perturbations originating in the troposphere. They can propagate up to the ionosphere and thermosphere and dynamically couple the lower and upper atmosphere. In this study we developed a novel ring detection algorithm to extract CGWs from the Atmosphere Infrared Sounder (AIRS) radiance data and the European Center for Medium-Range Weather Forecasting (ECMWF) analysis temperature in the stratosphere to produce the first global maps of such phenomena. The algorithm is capable of estimating wave amplitude, wavelength, propagation direction, and source location. Both AIRS and ECMWF data show a significant diurnal variation in wave propagation direction and generation, in addition to strong seasonal variations in wavelength and amplitude. Occurrence of these ring waves is associated not only with tropical deep convections but also with summertime midlatitude convection, wintertime extratropical jets, and topography such as islands. The high-resolution ECMWF analysis data capture most of the CGW features, but the wave amplitude is significantly weaker than AIRS observations, showing few convectively generated CGWs.

Verification of air/surface humidity differences from AIRS and ERA‐Interim in support of turbulent flux estimation in the Arctic

Evaporation from the Arctic Ocean and its marginal seas is essential for air moisture, cloudiness, and precipitation, as well as for the associated feedbacks, which contribute to the Arctic amplification of climate warming. However, evaporation in the Arctic is still associated with large uncertainties. The Boisvert et al. (2013) moisture flux scheme (BMF13) is based on application of the Atmospheric Infrared Sounder (AIRS) data, which produces high-quality, global, daily atmospheric temperature and moisture profiles even in the presence of clouds. Comparing the results of BMF13 against the ERA-Interim reanalysis, we found differences up to 55u2009Wu2009m−2 in the surface latent heat flux in the Beaufort-East Siberian Seas (BESS). We found out that the quality of the input data for the BMF13 and ERA-Interim flux schemes was the main cause for the differences. Differences in the input data sets cause moisture flux estimates to differ up to 1.6u2009×u200910−2u2009gu2009m−2u2009s−1 (40u2009Wu2009m−2 latent heat flux) in the BESS region, when both data sets were applied to the BMF13 scheme. Thus, the input data sets, AIRS version 6 and ERA-Interim reanalysis, were compared with a variety of in situ data. In skin temperature ERA-Interim had twice as large an error as AIRS version 6, but smaller errors in air specific humidity. The results suggested that AIRS data and the BMF13 scheme are a good option to estimate the moisture flux in the Arctic. However, the differences detected demonstrate a need for more in situ measurements of air temperature and humidity in the Arctic.

Introducing multisensor satellite radiance‐based evaluation for regional Earth System modeling

Earth System modeling has become more complex, and its evaluation using satellite data has also become more difficult due to model and data diversity. Therefore, the fundamental methodology of using satellite direct measurements with instrumental simulators should be addressed especially for modeling community members lacking a solid background of radiative transfer and scattering theory. This manuscript introduces principles of multisatellite, multisensor radiance-based evaluation methods for a fully coupled regional Earth System model: NASA-Unified Weather Research and Forecasting (NU-WRF) model. We use a NU-WRF case study simulation over West Africa as an example of evaluating aerosol-cloud-precipitation-land processes with various satellite observations. NU-WRF-simulated geophysical parameters are converted to the satellite-observable raw radiance and backscatter under nearly consistent physics assumptions via the multisensor satellite simulator, the Goddard Satellite Data Simulator Unit. We present varied examples of simple yet robust methods that characterize forecast errors and model physics biases through the spatial and statistical interpretation of various satellite raw signals: infrared brightness temperature (Tb) for surface skin temperature and cloud top temperature, microwave Tb for precipitation ice and surface flooding, and radar and lidar backscatter for aerosol-cloud profiling simultaneously. Because raw satellite signals integrate many sources of geophysical information, we demonstrate user-defined thresholds and a simple statistical process to facilitate evaluations, including the infrared-microwave-based cloud types and lidar/radar-based profile classifications.

Variations of global gravity waves derived from 14 years of SABER temperature observations

The global gravity wave (GW) potential energy (PE) per unit mass is derived from SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperatures profiles over the past 14 years (2002-2015). Since the SABER data cover longer than one solar cycle, multivariate linear regression is applied to calculate the trend (means linear trend from 2002 to 2015) of global GW PE and the responses of global GW PE to solar activity, to QBO (quasi-biennial oscillation) and to ENSO (El Nino-Southern Oscillation). We find a significant positive trend of GW PE at around 50°N during July from 2002 to 2015, in agreement with ground-based radar observations at a similar latitude but from 1990 to 2010. Both the monthly and the deseasonalized trends of GW PE are significant near 50°S. Specifically, the deseasonalized trend of GW PE has a positive peak of 12-15% per decade at 40°S-50°S and below 60 km, which suggests that eddy diffusion is increasing in some places. A significant positive trend of GW PE near 50°S could be due to the strengthening of the polar stratospheric jets, as documented from MERRA (Modern Era Retrospective-analysis for Research and Applications) wind data. The response of GW PE to solar activity is negative in the lower and middle latitudes. The response of GW PE to QBO (as indicated by 30 hPa zonal winds over the equator) is negative in the tropical upper stratosphere and extends to higher latitudes at higher altitudes. The response of GW PE to ENSO (as indicated by the MEI index) is positive in the tropical upper stratosphere.

An investigation of the Arctic inversion using COSMIC RO observations

The stable temperature inversion over sea ice plays an important role in the surface climate of the Arctic Ocean through direct and indirect feedbacks. Although several studies have investigated Arctic inversion characteristics such as height, depth, and frequency, there are significant challenges for long-term climate monitoring mainly due to limited sampling over the ocean and/or poor resolution of available observations. This study investigates the Arctic temperature inversion during the cold season using the high-resolution refractivity profiles from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO). For the coldest and driest months in the Arctic, a reliable retrieval technique for boundary layer properties, such as inversion height and surface-based inversion (SBI) frequency, is developed. We find that these variables have a strong negative relationship over the Arctic Ocean and are well correlated in the time and space domain. The spatial patterns show a minimum inversion height (maximum SBI frequency) over the ice-covered Pacific sector of the ocean similar to that observed in past studies. Seasonal evolution of the inversion characteristics suggests a surface temperature control over the sea ice region, with the peak in SBI frequency occurring during the transition period from winter to spring. There is little diurnal variability in the mean inversion height during the cold season. Despite its limitations, the RO refractivity profile is found quite useful for monitoring the Arctic boundary layer, including interannual variability of inversion characteristics.

THz Limb Sounder (TLS) for Lower Thermospheric Wind, Oxygen Density, and Temperature

Neutral winds are one of the most critical measurements in the lower thermosphere and E region ionosphere (LTEI) for understanding complex electrodynamic processes and ion-neutral interactions. We are developing a high-sensitivity, low-power, noncryogenic 2.06 THz Schottky receiver to measure wind profiles at 100-140 km. The new technique, THz limb sounder (TLS), aims to measure LTEI winds by resolving the wind-induced Doppler shift of 2.06 THz atomic oxygen (OI) emissions. As a transition between fine structure levels in the ground electronic state, the OI emission is in local thermodynamic equilibrium(LTE) at altitudes up to 350km. This LTE property, together with day-and-night capability and small line-of-sight gradient, makes the OI limb sounding a very attractive technique for neutral wind observations. In addition to the wind measurement, TLS can also retrieve [OI] density and neutral temperature in the LTEI region. TLS leverages rapid advances in THz receiver technologies including subharmonically pumped (SHP)mixers and Schottky-diode-based power multipliers. Current SHP Schottky receivers have produced good sensitivity for THz frequencies at ambient environment temperatures (120-150 K), which are achievable through passively cooling in spaceflight. As an emerging technique, TLS can fill the critical data gaps in the LTEI neutral wind observations to enable detailed studies on the coupling and dynamo processes between charged and neutral molecules.

Horizontal winds derived from the polar mesospheric cloud images as observed by the CIPS instrument on the AIM satellite

A cloud pattern matching technique is applied to polar mesospheric cloud (PMC) images taken by the Cloud Imaging and Particle Size instrument (CIPS) to infer the wind velocities in the mesopause region. CIPS measurements are analyzed to detect patterns that repeat from one orbit to the next but are displaced in location; the displacement provides a measure of the wind velocity. Pattern matching is achieved by resampling the CIPS data to longitude and latitude grids with the grid-box size forced at ~5u2009km in both directions. The correlated patterns are searched within a geographic region referred to as a “frame” of ~500u2009km in longitudeu2009×u2009400u2009km in latitude. The histograms of the derived velocities indicate that easterly winds prevail, with a mean zonal wind of −20 to −15u2009m/s. Mean meridional winds are overall small, but in late summer the histogram indicated a poleward wind of ~20–30u2009m/s. The variability of CIPS cloud albedo on consecutive orbits is also examined at fixed geolocations. The statistical results suggest that ~86% of pairs underwent mean cloud albedo variation of < 50% on consecutive orbits, suggesting a moderate change. It is also found that the correlation of the cloud structures between two consecutive orbits at a fixed location is generally poor. These findings suggest that cloud patterns are subject to wind advection, but the cloud patches are more extended in size than the movement that occurs. Cloud voids are found to be more likely to remain at the same geolocations.

Increasing evaporation amounts seen in the Arctic between 2003 and 2013 from AIRS data: INCREASING EVAPORATION IN ARCTIC

The vertical moisture flux (i.e., evaporation) plays an important role in the Arctic energy budget, the water vapor feedback, and Arctic amplification. It is one of the most uncertain variables, especially in this “new Arctic” climate system, which is dominated by large ice-free ocean areas for a longer portion of the year. Moisture flux rates, produced using Atmospheric Infrared Sounder (AIRS) data, from the Arctic Ocean and surrounding seas were found to have increased between 2003 and 2013 by 7.2u2009×u200910−4u2009gu2009m−2u2009s−1 per year (equivalent to 1.79u2009Wu2009m−2 per year in latent heat). This is a 7% increase in the average moisture flux each year and a 0.12% increase in the yearly global ocean latent heat flux, with some months increasing more than others. The largest increases seen are in the Arctic coastal seas during the spring and fall where there has been a reduction in sea ice cover and an increase in sea surface temperatures. Increases in the moisture flux from the surface also correspond to increases in total atmospheric column water vapor and low-level clouds, especially in the central Arctic regions. Changes in the atmospheric water vapor in the surrounding seas (e.g., East Greenland) are most likely due to lower latitude transport of moisture rather than from the surface. Yearly, the moisture flux from the surface supplies about 10% of the total column atmosphere water vapor.