Andreas Giez

Role of entrainment in surface-atmosphere interactions over the boreal forest

We present a description of the evolution of the convective boundary layer (CBL) over the boreal forests of Saskatchewan and Manitoba, as observed by the National Center for Atmospheric Research (NCAR) Electra research aircraft during the 1994 Boreal Ecosystem-Atmosphere Study (BOREAS). All observations were made between 1530 and 2230 UT (0930–1630 local solar time (LST)). We show that the CBL flux divergence often led to drying of the CBL over the course of the day, with the greatest drying (approaching 0.5 g kg−1 h−1) observed in the morning, 1000–1200 LST, and decreasing over time to nearly no drying (0–0.1 g kg−1 h−1) by midafternoon (1500–1600 LST). The maximum warming (0.45 K h−1 ) also occurred in the morning and decreased slightly to about 0.4 Kh−1 by midafternoon. The CBL vapor pressure deficit (VPD) increased over the course of the day. A significant portion of this increase can be explained by the vertical flux divergence, though horizontal advection also appears to be important. We suggest a linkage among boundary layer growth, the vertical flux divergences, and boundary layer cloud formation, with cloud activity peaking at midday in response to rapid CBL growth, then decreasing somewhat later in the day in response to CBL warming and decreased growth. We also see evidence of feedback between increasing VPD and stomatal control. We use eddy-covariance flux measurements from the Electra to compute the virtual temperature entrainment ratio Ar. The computed mean value of 0.08±0.12 is somewhat lower than the commonly assumed value of 0.2, as well as with other estimates from BOREAS. This value is very sensitive to the determination of CBL depth. We find that Ar increases with an increasing jump in mean wind across the CBL top. The entrainment flux of water vapor is found to be most dependent on time of day (negative correlation). The ratio of entrainment to surface flux of water vapor is 1.57±0.25. Airborne lidar observations of the CBL top reveal a CBL top “thickness” that is smaller than would be expected from simple theory but consistent with past lidar observations. The normalized thickness is found to have a very consistent value h¯/h0-1/0.116±0.008, where 12 cases were examined. A new method of computing the variability of the CBL top is illustrated, and we show that this variance in the CBL depth also scales with the depth but that the value of this normalized variance differs substantially from the “thickness” defined in past literature.

Estimation of boundary layer humidity fluxes and statistics from airborne differential absorption lidar (DIAL)

The water vapor differential absorption lidar (DIAL) of the German Aerospace Research Establishment (DLR) was flown aboard the National Center for Atmospheric Research (NCAR) Electra research aircraft during the Boreal Ecosystem-Atmosphere Study (BOREAS). The downward looking lidar system measured two-dimensional fields of aerosol backscatter and water vapor mixing ratio in the convective boundary layer (CBL) and across the CBL top (zt). We show a case study of DIAL observations of vertical profiles of mean water vapor, water vapor variance, skewness, and integral scale in the CBL. In the entrainment zone (EZ) and down to about 0.3 zi the DIAL observations agree with in situ observations and mixed-layer similarity theory. Below, the water vapor optical depth becomes large and the DIAL signal-to-noise ratio degrades. Knowing the water vapor surface flux and the convective velocity scale w* from in situ aircraft measurements, we derive entrainment fluxes by applying the mixed-layer gradient (MLG) and mixed-layer variance (MLV) methods to DIAL mixing ratio gradient and variance profiles. Entrainment flux estimates are sensitive to our estimate of zt. They are shown to be rather insensitive to the input surface flux and to the DIAL data spatial resolution within the investigated range. The estimates break down above about 0.9 zt as the flux-gradient and flux-variance relationships were developed to describe the large-scale mixing in the mid-CBL. The agreement with in situ entrainment flux estimations is within 30% for the MLV method. On a flight leg with significant mesoscale variability the entrainment flux turns out to be 70% higher than the in situ value. This is in good agreement with the fact that large-eddy simulations (LES) of mean water vapor profiles and variances, upon which the MLG and MLV methods are based, do not include mesoscale variability. The additional water vapor variance from mesoscales may then lead to the overestimate of the flux. Deviations from the in situ observations may also be due to poor LES resolution of small-scale mixing in the EZ, similarly coarse resolution of the DIAL data, or a capping inversion in the LES model (8 K) which is significantly stronger than the observed inversion (3–4 K).

Water Vapor Flux Measurements from Ground-Based Vertically Pointed Water Vapor Differential Absorption and Doppler Lidars

Abstract For the first time, two lidar systems were used to measure the vertical water vapor flux in a convective boundary layer by means of eddy correlation. This was achieved by combining a water vapor differential absorption lidar and a heterodyne wind lidar in a ground-based experiment. The results prove that the combined lidar system can determine vertical flux profiles with a height resolution of approximately 100 m. Vertical averaging over a greater height interval reduces the error sufficiently that the changes in flux occurring throughout the day as a result of solar heating can be resolved. Horizontal and, for the first time, vertical integral scales were calculated from the lidar signals. The error analysis based on these results indicates that instrumental white noise and sampling error are the main sources of the statistical error in the flux measurement. Since the lidars measure simultaneously at many levels throughout the boundary layer, these errors can be reduced by vertical averaging to ...

ML-CIRRUS - The airborne experiment on natural cirrus and contrail cirrus with the high-altitude long-range research aircraft HALO

AbstractThe Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models.Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations.In spring 2014, HALO performed 16 f...

Static Pressure from Aircraft Trailing-Cone Measurements and Numerical Weather-Prediction Analysis

Accurate static-pressure measurements are a prerequisite for safe navigation and precise air-data measurements on aircraft. Pressure is also fundamental for wind and air temperature analysis in meteorology. Static-pressure measurement by aircraft is disturbed by aerodynamics and needs to be corrected using calibration. In this paper, a comparison has been made between static pressure measured by means of a trailing cone in the atmosphere behind two different jet aircraft at flight levels up to 450 and data from numerical weather predictions. The height is derived from differential Global Navigation Satellite System measurements. The Global Navigation Satellite System height is compared to numerical-weather-prediction geopotential height. The numerical-weather-prediction data were provided by the Integrated Forecast System of the European Centre for Medium-Range Weather Forecasts. When computing the geopotential with latitude-/height-dependent gravity, the pressure/height differences are −0.01±0.15  hPa an...

Comparison of Static Pressure from Aircraft Trailing Cone Measurements and Numerical Weather Prediction Analysis

Accurate static pressure measurements are a prerequisite for safe navigation and precise air data measurements on aircraft. Pressure is also fundamental to assess winds and air temperature and, hence, important for meteorology. The direct static pressure measurement by aircraft is disturbed by the aircraft aerodynamics and needs to be corrected using proper calibration. In this paper we compare static pressure measured by means of a trailing cone (TC) in the undisturbed atmosphere behind two different jet aircraft (Dassault FALCON 20E and Gulfstream 550 “HALO”) at flight levels (FL) from 40 to 450 during 6 flights on different days with data from numerical weather predictions (NWP). The height is derived from differential Global Navigation Satellite System (GNSS) measurements. The GNSS height is compared to NWP geopotential height. The NWP data were provided by the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The IFS model assumes constant gravity g. For constant g, the pressure differences (at same height) have mean values and standard deviations of 0.40±0.17 hPa for 159 individual measurements of 43±31 s duration each. The respective height differences (at same pressure) are -10±5 m on average over the same measurements. When computing the geopotential with latitude/height dependent gravity (which is 0.4 % smaller at FL 450 than at 0 km) the agreement becomes significantly better: -0.01±0.15 hPa for pressure, 0.6±2.8 m for height. This pressure accuracy implies NWP temperature errors <0.1 K on average below 10 km altitude. Standard deviations of random errors in the TC-NWP difference are 0.06 hPa and 1 m. The TC measurements provide a first quantification of the case-specific accuracy of NWP pressure geopotential relationships. The method of comparing operational pressure/GNSS measurements on aircraft with NWP analysis or predictions can be used to test the height keeping performance of aircraft after or during operation.

Measurement of the Vertical Water Vapor Flux in the Convective Boundary Layer Using a DIAL and a Coherent Wind Lidar

A ground—based vertically—pointing water vapor DIAL and a coherent wind lidar were used to determine the vertical flux of latent heat in a convective boundary layer by means of eddy correlation. Vertical profiles as well as the temporal evolution of the flux were derived from the data. Instrumental white noise and the sampling error represent the main error sources for this kind of measurement.

Airborne limb-imaging measurements of temperature, HNO 3 , O 3 , ClONO 2 , H 2 O and CFC-12 during the Arctic winter 2015/16: characterization, in-situ validation and comparison to Aura/MLS

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) was operated on board the German High Altitude and Long Range Research Aircraft (HALO) during the PGS (POLSTRACC/GWLCYCLE/SALSA) aircraft campaigns in the Arctic winter 2015/2016. Research flights were conducted from 17 December 2015 until 18 March 2016 within 25–87 N, 80W–30 E. From the GLORIA infrared limb-emission measurements, two-dimensional cross sections of temperature, HNO3, O3, ClONO2, H2O and CFC-12 are retrieved. During 15 scientific flights of the PGS campaigns the GLORIA instrument measured more than 15 000 atmospheric profiles at high spectral resolution. Dependent on flight altitude and tropospheric cloud cover, the profiles retrieved from the measurements typically range between 5 and 14 km, and vertical resolutions between 400 and 1000 m are achieved. The estimated total (random and systematic) 1σ errors are in the range of 1 to 2 K for temperature and 10 % to 20 % relative error for the discussed trace gases. Comparisons to in situ instruments deployed on board HALO have been performed. Over all flights of this campaign the median differences and median absolute deviations between in situ and GLORIA observations are−0.75K±0.88 K for temperature, −0.03ppbv± 0.85 ppbv for HNO3, −3.5ppbv± 116.8 ppbv for O3,−15.4pptv±102.8 pptv for ClONO2,−0.13ppmv± 0.63 ppmv for H2O and −19.8pptv± 46.9 pptv for CFC-12. Seventy-three percent of these differences are within twice the combined estimated errors of the cross-compared instruments. Events with larger deviations are explained by atmospheric variability and different sampling characteristics of the instruments. Additionally, comparisons of GLORIA HNO3 and O3 with measurements of the Aura Microwave Limb Sounder (MLS) instrument show highly consistent structures in trace gas distributions and illustrate the potential of the high-spectral-resolution limb-imaging GLORIA observations for resolving narrow mesoscale structures in the upper troposphere and lower stratosphere (UTLS).