The objective is to determine if cloudy atmospheres absorb more shortwave radiation than predicted by state-of-the-art climate models.This experiment is developed fully in a separate report - "ARESE (ARM Enhanced Shortwave Experiment) Science Plan" by Francisco P. J. Valero, et. al. This document contains only a summary description of that science plan.
Two objectives are embedded in this hypothesis: (1) the direct measurement of the absorption of solar radiation by clear and cloudy atmospheres and the placement of bounds on these measurements and (2) the investigation of the possible causes of absorption in excess of model predictions.
Evidence from several experimental and theoretical investigations over the past four decades has shown that the magnitude of shortwave absorption by clouds is uncertain. There has been some evidence that absorption is in excess of that predicted by models. F. Valero and several other investigators have concluded that the absorption by the entire atmospheric column in the presence of clouds exceeds model predictions of absorption by perhaps 35 W/m/m (dayside average) over the Pacific warm pool. The relative error this presents in current theoretical estimates of solar absorption is large, considering that average clear-sky absorption in that region is about 100 W/m/m (dayside average). The absolute error appears to be small when compared to other terms in the energy budget, but that is misleading. Most of the solar radiation absorbed in the tropics goes toward heating the surface, the remainder, about 20%, helps drive the atmospheric circulation. Thus, what appears to be small errors in absorption by the atmosphere might have huge consequences in tropical atmospheric dynamics. Another consequence of the inadequacy of our understanding of solar absorption by clouds is the misinterpretation of remote sensing data used to infer cloud microphysical properties.
Relative to the ARM-UAV Science Team this hypothesis is under the auspices of the Radiative Fluxes Working Group. However, ARESE has its own separate science team chaired by John Vitko, Jr. from Sandia National Laboratories.
The experimental emphasis of ARESE focuses on the measurement of atmospheric column absorption through the acquisition of fluxes at different altitudes in the atmosphere and at the surface. This will be achieved by using satellite, aircraft, and ground observational platforms. The aircraft will cover the range from the tropopause to the low troposphere. Ground observations will be made from the ARM CART central and extended facilities which are part of the ARM SGP site. Radiances measured by both GOES and NOAA series satellites will be compared to fluxes measured by aircraft at the tropopause, and then used to retrieve TOA fluxes.The ARESE strategy involves the acquisition of radiometric data with multiple, coordinated aircraft and from the ground. The aircraft will fly tracks over the ground stations stacked at different altitudes. In this manner, it will be possible to obtain coeval measurements of radiative fluxes from which the absorption of radiation by the atmosphere can be estimated. Additionally, the aircraft sampling from the tropopause will be able to measure the reflectivity of the cloudy and clear atmospheres, and the surface observations will provide the radiative flux transmitted through the column. The top of the troposphere reflectivity and surface transmissivity values provide an additional indication of the magnitude of absorption by the atmospheric column.
The measurements at the tropopause will be made from a NASA ER-2 aircraft. Upper tropospheric measurements will be made from an ARM-UAV Egrett and lower tropospheric measurements from an ARM-UAV DHC-6 Twin Otter. All three aircraft will be equipped with identical Valero radiometers, as will be the ground sites.
The list of measurements to be made as part of the ARESE experiment are extensive and covered fully in the ARESE science plan. The list below is intended to be representative but not complete.Common to all three aircraft is a RAMS for the characterization of shortwave upwelling and downwelling fluxes at their respective flight altitudes. The components of a RAMS are:
a) zenith SBBR covering the spectral range from 0.3 to 4.0 µm,For ground based measurements a zenith RAMS system is used consisting of items a, c, and e above.
b) nadir SBBR covering the spectral range from 0.3 to 4.0 µm,
c) zenith broadband radiometer covering the spectral range from 0.3 to 0.75 µm,
d) nadir broadband radiometer covering the spectral range from 0.3 to 0.75 µm,
e) zenith TDDR (7 channels: 0.500, 0.865, 1.05, 1.25, 1.50, 1.65, and 1.75 µm), and
f) nadir TDDR (7 channels: 0.500, 0.865, 1.05, 1.25, 1.50, 1.65, and 1.75 µm).NASA ER-2:The ER-2 will measure upwelling and downwelling solar fluxes at the tropopause with a RAMS. Additional supporting instrumentation includes a LIDAR for atmospheric characterization and a MAS for satellite instrument calibration.
ARM-UAV Egrett:The Egrett will similarly measure upwelling and downwelling solar fluxes in the upper troposphere with a RAMS. Additional supporting instrumentation includes a CDL for atmospheric characterization above and below the aircraft, a nadir SSP for collection of spectrally resolved 0.4 to 4.0 µm radiances from features directly below the aircraft, and the MPIR for clouds image data in bands centered on 0.68, 0.88, 1.37, and 1.6 µm. The MPIR's cross track FOV will be plus or minus 40°.
ARM-UAV DHC-6:The DHC-6 will also measure upwelling and downwelling solar fluxes, but in the lower troposphere, with a RAMS. The additional onboard supporting instrumentation is a zenith SSP for collection of spectrally resolved 0.4 to 1.0 µm radiances from features directly above the aircraft.
Ground:The Lamont zenith RAMS, Byron zenith RAMS, and Ringwood zenith RAMS systems will measure downwelling solar fluxes at the SGP CART central facility (Lamont) and the two extended facilities indicated. These flux measurements will be augmented by Lamont SIROS, Byron SIROS, and Ringwood SIROS measurements at these three sites; Coldwater SIROS and Vici SIROS radiation measurements will be primary at these two additional extended facilities over which flights will be made. Radiosondes launched from the central facility and selected extended facilities will characterize atmospheric water vapor and temperature. Similarly ozone sondes will gauge atmospheric ozone levels. Several additional measurements will be made near the central facility, including (1) an augmentation of the sondes' water vapor by a Raman LIDAR system, (2) cloud extents and DD or ID with a scanning cloud Doppler radar, (3) lower troposphere wind, temperature, and humidity conditions with both a 915 MHz profiler with RASS and 50 MHz profiler with RASS and (4) LW path with a MWR.
Satellite:The primary instrument is the IMAGER on GOES-8 for measurement of narrowband radiances in the visible, mid-near-infrared, and near-infrared bands. Secondarily, narrowband data will be used from the IMAGER (GOES-9), AVHRR (NOAA-12 and NOAA-14), and the ?? (METEOSAT-4) whenever satellite views match the area of operations.
Three strategies will be used for evaluating cloudy sky atmospheric absorption relative to models. These can be variously adapted to use data from the five levels of ARESE measurements: (1) surface, (2) 0.5 km DHC-6, (3) 12 km Egrett, (4) 20 km ER-2, and (5) TOA (satellite.A direct way of evaluating cloudy sky shortwave absorption, relative to that for clear skies, is to compare cloud radiative forcing at the surface to that at the TOA. Cloud radiative forcing is the difference between all-sky and clear sky net downward shortwave radiation. Models typically give a value near 1 for the ration of cloud radiative forcing at the surface to cloud radiative forcing at the TOA although some recent measurements indicate a value near 1.5 might be more appropriate. Since model simulations of cloud radiative forcing are easily performed for the DHC-6 and Egrett altitudes, this approach can be used to compare measurements taken on board those platforms with model results.
During ARESE upwelling surface shortwave flux will not be measured with the same instruments used on board the aircraft since the zenith RAMS used on the surface has no nadir viewing components. A second analysis strategy overcomes this deficiency by substituting surface insolation for the surface net flux used in the above strategy. Models typically give a value near 1.25 for the ratio of surface cloud insolation forcing to TOA cloud radiative forcing, compared to some recent measurements indicating a value neared 1.75 for this same ratio.
The ratio of cloud radiative forcing at the surface to that at the TOA can be shown to be mathematically equal to -(delta T)/(delta alpha) where alpha is the TOA albedo and T is the atmospheric transmittance. This leads to a third approach that evaluates (delta alpha)/(delta T) from a linear regression of the measured quantities of albedo and transmittance. Since this approach does not require clear sky identification it serves to remove cloud shortwave absorption from broken cloud effects.
As mentioned above, this experiment is the same as ARESE and it is fully developed in the separate document - "ARESE (ARM Enhanced Shortwave Experiment) Science Plan" by Francisco P. J. Valero, et. al.
The operations plans for conducting this experiment are well established, and the number of available flight hours (< 100 for each aircraft) defines the amount of airborne data that will be collected. Whether this is sufficient is an open question.There is some controversy concerning the interpretation of various cloud effects and their influence on the data interpretation strategies above. These issues are being addressed by the ARESE science team.
