Ground-based observations from two different radiometers are used to evaluate commonly used microwave/millimeter-wave propagation models at 150 GHz. This frequency has strong sensitivity to changes ...in precipitable water vapor (PWV) and cloud liquid water. The observations were collected near Hesselbach, Germany, as part of the Atmospheric Radiation Measurement program's support of the General Observing Period and the Convective and Orographic Precipitation Study. The observations from the two radiometers agree well with each other, with a slope of 0.993 and a mean bias of 0.12 K. The observations demonstrate that the relative sensitivity of the different absorption models to PWV in clear-sky conditions at 150 GHz is significant and that four models differ significantly from the observed brightness temperature. These models were modified to get agreement with the 150-GHz observations, where the PWV ranged from 0.35 to 2.88 cm. The models were modified by adjusting the strength of the foreign- and self-broadened water vapor continuum coefficients, where the magnitude was model dependent. In all cases, the adjustment to the two components of the water vapor continuum was in opposite directions (i.e., increasing the contribution from the foreign-broadened component while decreasing contribution from the self-broadened component or vice versa). While the original models had significant disagreements relative to each other, the resulting modified models show much better agreement relative to each other throughout the microwave spectrum. The modified models were evaluated using independent observations at 31.4 GHz.
An improved liquid water absorption model is developed for frequencies between 0.5 and 500 GHz. The empirical coefficients for this model were retrieved from a dataset that consists of both ...laboratory observations of the permittivity of liquid water (primarily at temperatures above 0 degree C) and field observations collected by microwave radiometers in three separate locations with observations at temperatures as low as -32 degree C. An optimal estimation framework is used to retrieve the model's coefficients. This framework shows that there is high information content in the observations for seven of the nine model coefficients, but that the uncertainties in all of the coefficients result in less than 15% uncertainty in the liquid water absorption coefficient for all temperatures between -32 degree and 0 degree C and frequencies between 23 and 225 GHz. Furthermore, this model is more consistent with both the laboratory and field observations over all frequencies and temperatures than other popular absorption models.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Boundary layer moisture variability at the Eastern North Atlantic
(ENA) site during marine conditions is examined at monthly and daily timescales using 5 years of ground-based observations and output ...from the European
Center for Medium range Weather Forecast (ECMWF) reanalysis model. The
annual cycle of the mixed-layer total water budgets is presented to estimate
the relative contribution of large-scale advection, local moisture tendency,
entrainment, and precipitation to balance the moistening due to surface
latent heat flux on monthly timescales. When marine conditions prevail,
advection of colder and dry air from the north acts as an important moisture
sink (∼ 50 % of the overall budget) during fall and winter
driving the seasonality of the budget. Entrainment and precipitation
contribute to the drying of the boundary layer (∼ 25 % and
∼ 15 % respectively), and the local change in moisture
contributes to a small residual part. On a daily temporal scale, moist and
dry mesoscale columns of vapor (∼ 10 km) are analyzed during
10 selected days of precipitating stratocumulus clouds. Adjacent moist and
dry columns present distinct mesoscale features that are strongly correlated
with clouds and precipitation. Dry columns adjacent to moist columns have
more frequent and stronger downdrafts immediately below the cloud base.
Moist columns have more frequent updrafts, stronger cloud-top cooling, and
higher liquid water path and precipitation compared to the dry columns. This
study highlights the complex interaction between large-scale and local
processes controlling the boundary layer moisture and the importance of
spatial distribution of vapor to support convection and precipitation.
Each atmospheric aerosol type has distinctive light-absorption characteristics related to its physical/chemical properties. Climate models treat black carbon as the main light-absorbing component of ...carbonaceous atmospheric aerosols, while absorption by some organic aerosols is also considered, particularly at ultraviolet wavelengths. Most absorbing aerosols are assumed to be < 1 mu m in diameter (sub-micron). Here we present results from a recent field study in India, primarily during the post-monsoon season (October-November), suggesting the presence of absorbing aerosols sized 1-10 mu m. Absorption due to super-micron-sized particles was nearly 30% greater than that due to smaller particles. Periods of increased absorption by larger particles ranged from a week to a month. Radiative forcing calculations under clear-sky conditions show that super-micron particles account for nearly 44% of the total aerosol forcing. The origin of the large aerosols is unknown, but meteorological conditions indicate that they are of local origin. Such economic and habitation conditions exist throughout much of the developing world. Hence, large absorbing particles could be an important component of the regional-scale atmospheric energy balance.
This paper presents a new neural network (NN) algorithm for real-time retrievals of low amounts of precipitable water vapor (PWV) and integrated liquid water from millimeter-wave ground-based ...observations. Measurements are collected by the 183.3-GHz G-band vapor radiometer (GVR) operating at the Atmospheric Radiation Measurement (ARM) Program Climate Research Facility, Barrow, AK. The NN provides the means to explore the nonlinear regime of the measurements and investigate the physical boundaries of the operability of the instrument. A methodology to compute individual error bars associated with the NN output is developed, and a detailed error analysis of the network output is provided. Through the error analysis, it is possible to isolate several components contributing to the overall retrieval errors and to analyze the dependence of the errors on the inputs. The network outputs and associated errors are then compared with results from a physical retrieval and with the ARM two-channel microwave radiometer (MWR) statistical retrieval. When the NN is trained with a seasonal training data set, the retrievals of water vapor yield results that are comparable to those obtained from a traditional physical retrieval, with a retrieval error percentage of ~5% when the PWV is between 2 and 10 mm, but with the advantages that the NN algorithm does not require vertical profiles of temperature and humidity as input and is significantly faster computationally. Liquid water path (LWP) retrievals from the NN have a significantly improved clear-sky bias (mean of ~2.4 g/m 2 ) and a retrieval error varying from 1 to about 10 g/m 2 when the PWV amount is between 1 and 10 mm. As an independent validation of the LWP retrieval, the longwave downwelling surface flux was computed and compared with observations. The comparison shows a significant improvement with respect to the MWR statistical retrievals, particularly for LWP amounts of less than 60 g/m 2 . This paper shows that the GVR alone can provide overall improved PWV and LWP retrievals when the PWV amount is less than 10 mm, and, when combined with the MWR, can provide improved retrievals over the whole water-vapor range.
HIGH AND DRY Shupe, Matthew D.; Turner, David D.; Walden, Von P. ...
Bulletin of the American Meteorological Society,
02/2013, Letnik:
94, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Cloud and atmospheric properties strongly influence the mass and energy budgets of the Greenland Ice Sheet (GIS). To address critical gaps in the understanding of these systems, a new suite of cloud- ...and atmosphere-observing instruments has been installed on the central GIS as part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) project. During the first 20 months in operation, this complementary suite of active and passive ground-based sensors and radiosondes has provided new and unique perspectives on important cloud–atmosphere properties.
High atop the GIS, the atmosphere is extremely dry and cold with strong near-surface static stability predominating throughout the year, particularly in winter. This low-level thermodynamic structure, coupled with frequent moisture inversions, conveys the importance of advection for local cloud and precipitation formation. Cloud liquid water is observed in all months of the year, even the particularly cold and dry winter, while annual cycle observations indicate that the largest atmospheric moisture amounts, cloud water contents, and snowfall occur in summer and under southwesterly flow. Many of the basic structural properties of clouds observed at Summit, Greenland, particularly for low-level stratiform clouds, are similar to their counterparts in other Arctic regions.
The ICECAPS observations and accompanying analyses will be used to improve the understanding of key cloud–atmosphere processes and the manner in which they interact with the GIS. Furthermore, they will facilitate model evaluation and development in this data-sparse but environmentally unique region.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
This paper presents a process-oriented evaluation of precipitating stratocumulus and its transition to cumulus in version 1 of the Energy Exascale Earth System Model (E3SMv1) using ...comprehensive case-study observations from a field campaign of the Atmospheric Radiation Measurement program (ARM). The E3SMv1 single-column model (SCM) of the marine boundary layer and its low clouds and precipitation are compared to observations including subcloud drizzle retrievals from a combination of Doppler radar and lidar backscatter measurements. The SCM is also compared to a large-eddy simulation (LES) of the same case. The combination of advanced remote sensing observations and LES is a powerful framework to evaluate the physical parameterizations of large-scale models. Given the observed large-scale environment, the E3SMv1 SCM realistically represents the evolution of clouds and boundary layer structure during the stratocumulus-to-cumulus transition. The model well simulates the liquid water path and its diurnal cycle in the stratocumulus period as well as the two-layer vertical thermodynamic structure and lower cloud fraction in the transition period. E3SMv1’s success in simulating the cloud in the stratocumulus period permitted examination of its precipitation processes. Here problems were identified with E3SMv1 producing an unrealistically small subcloud precipitation fraction, an unrealistic double peak in the vertical profiles of precipitation mass, and drizzle that evaporates too close to the surface. Further model diagnostics determined that these unrealistic characteristics resulted from an overly long microphysics time step and an unrealistic parameterization of the precipitation fraction. These results imply that careful consideration of these issues is needed in order to better simulate precipitation processes in marine stratocumulus.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The partition of cloud and drizzle water path in precipitating clouds plays a key role in determining the cloud lifetime and its evolution.
A technique to quantify cloud and drizzle water path by ...combining measurements from a three-channel microwave radiometer (23.8, 30, and
90 GHz) with those from a vertically pointing Doppler cloud radar and a ceilometer is presented. The technique is showcased using 1 d of
observations to derive precipitable water vapor, liquid water path, cloud water path, drizzle water path below the cloud base, and drizzle water
path above the cloud base in precipitating stratocumulus clouds. The resulting cloud and drizzle water path within the cloud are in good qualitative
agreement with the information extracted from the radar Doppler spectra. The technique is then applied to 10 d each of precipitating closed and
open cellular marine stratocumuli. In the closed-cell systems only ∼20 % of the available drizzle in the cloud falls below the cloud base,
compared to ∼40 % in the open-cell systems. In closed-cell systems precipitation is associated with radiative cooling at the cloud top
<-100Wm-2 and a liquid water path >200 g m−2. However, drizzle in the cloud begins to exist at weak radiative cooling
and liquid water path >∼150 g m−2. Our results collectively demonstrate that neglecting scattering effects for frequencies at and
above 90 GHz leads to overestimation of the total liquid water path of about 10 %–15 %, while their inclusion paves the path for
retrieving drizzle properties within the cloud.
Active remote sensing of marine boundary-layer clouds is challenging as drizzle drops often dominate the observed radar reflectivity. We present a new method to simultaneously retrieve cloud and ...drizzle vertical profiles in drizzling boundary-layer clouds using surface-based observations of radar reflectivity, lidar attenuated backscatter, and zenith radiances under conditions when precipitation does not reach the surface. Specifically, the vertical structure of droplet size and water content of both cloud and drizzle is characterised throughout the cloud. An ensemble optimal estimation approach provides full error statistics given the uncertainty in the observations. To evaluate the new method, we first perform retrievals using synthetic measurements from large-eddy simulation snapshots of cumulus under stratocumulus, where cloud water path is retrieved with an error of 31 g m-2. The method also performs well in non-drizzling clouds where no assumption of the cloud profile is required. We then apply the method to observations of marine stratocumulus obtained during the Atmospheric Radiation Measurement MAGIC deployment in the Northeast Pacific. Here, retrieved cloud water path agrees well with independent three-channel microwave radiometer retrievals, with a root mean square difference of 10-20 g m-2.
A field experiment was conducted in northern Chile at an altitude of 5.3 km to evaluate the accuracy of line‐by‐line radiative transfer models in regions of the spectrum that are typically opaque at ...sea level due to strong water vapor absorption. A suite of spectrally resolved radiance instruments collected simultaneous observations that, for the first time ever, spanned the entire terrestrial thermal spectrum (i.e., from 10 to 3000 cm−1, or 1000 to 3.3 μm). These radiance observations, together with collocated water vapor and temperature profiles, are used to provide an initial evaluation of the accuracy of water vapor absorption in the far‐infrared of two line‐by‐line radiative transfer models. These initial results suggest that the more recent of the two models is more accurate in the strongly absorbing water vapor pure rotation band. This result supports the validity of the Turner et al. (2012) study that demonstrated that the use of the more recent water vapor absorption model in climate simulations resulted in significant radiative and dynamical changes in the simulation relative to the older water vapor model.
Key PointsFirst ground‐based spectrally‐resolved observations of entire infrared spectrumExtremely dry conditions result in semi‐transparent regions in far‐infraredObservations used to evaluate line‐by‐line radiative transfer models
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