Abstract

Recently, there has been growing emphasis on improving surface flux inputs to mesoscale models and general circulation models. Since there is presently no operational network providing this information, we have conducted a feasibility experiment to determine whether the Bowen ratio (and indirectly surface heat and moisture fluxes) can be reasonably and accurately derived from thermodynamic measurements obtained from balloon-launched radiosondes.

The experiment took place during July 1988 at the Regional Airport in Tallahassee, Florida using an Atmospheric Instrumentation Research, Inc. (AIR) airsonde system and a surface radiation and energy budget station (SREBS) developed at Florida State University. The AIR system consists of a balloon-launched airsonde, which measures vertical profiles of atmospheric pressure, temperature, and relative humidity, and an automatic data acquisition system, which receives and records sensor output from the airsonde package. The SREBS is a compact, self-contained, battery-powered system used to measure approximately 100 surface parameters. For this experiment, the system was used to monitor in situ surface energy fluxes at the time of the radiosonde flights. The data recorded from the airsonde launches were used to create mixing-line profiles for each launch. Using the profiles, an objective technique for choosing the appropriate surface-layer mixing lines was developed, and from these the associated Bowen ratios within the surface layer were deduced.

Intercomparisons were made between Bowen ratios derived from the airsonde profiles and the Bowen ratios measured directly by the surface radiation and energy budget station. The results show that this technique produces estimates of the Bowen ratios within 9% of measured values, and a sensitivity analysis indicates that estimates of sensible and latent heat fluxes have root-mean-square differences of less than 6 W m⁻².

Abstract

Surface, aircraft, and satellite observations are analyzed for the 21-day 1989 intensive field campaign of the First ISLSCP Field Experiment (FIFE) to determine the effect of precipitation, vegetation, and soil moisture distributions on the thermal properties of the surface including the heat and moisture fluxes, and the corresponding response in the boundary-layer circulation. Mean and variance properties of the surface variables are first documented at various time and space scales. These calculations are designed to set the stage for Part II, a modeling study that will focus on how time–space dependent rainfall distribution influences the intensity of the feedback between a vegetated surface and the atmospheric boundary layer. Further analysis shows strongly demarked vegetation and soil moisture gradients extending across the FIFE experimental site that were developed and maintained by the antecedent and ongoing spatial distribution of rainfall over the region. These gradients are shown to have a pronounced influence on the thermodynamic properties of the surface. Furthermore, perturbation surface wind analysis suggests for both short-term steady-state conditions and long-term averaged conditions that the gradient pattern maintained a diurnally oscillating local direct circulation with perturbation vertical velocities of the same order as developing cumulus clouds. Dynamical and scaling considerations suggest that the embedded perturbation circulation is driven by surface heating/cooling gradients and terrain effects rather than the manifestation of an inertial oscillation. The implication is that at even relatively small scales <30 km), the differential evolution in vegetation density and soil moisture distribution over a relatively homogenous ecotone can give rise to preferential boundary-layer circulations capable of modifying local-scale horizontal and vertical motions.

A simulation of the appearance of an intense hailstorm in the passive microwave spectrum is conducted in order to characterize the vertical sources of radiation that contribute to the top-of-atmosphere microwave brightness temperatures (T_B ) which can be measured by satellite-borne radiometers. The study focuses on four frequencies corresponding to those used on the USAF Special Sensor Microwave Imager (SSM/I), a recently launched payload flown on the U.S. Air Force DMSP satellites. Computation of the microwave brightness temperatures is based on a vertically, angularly, and spectrally detailed radiative transfer scheme that has been applied to the highly resolved thermodynamical and microphysical output from the three-dimensional Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The RAMS model was used to carry out a 4-h simulation of an intense hailstorm that occurred on 11 July 1986 in the vicinity of Eldridge, Alabama. Initial conditions for the cloud model run were developed from the 1986-COHMEX data archive.

Two types of vertically resolved radiative structure functions referred to as a “generalized weighting function” and an “emission source weighting function” are used to describe the process by which radiation originates and reaches the satellite radiometer. In addition, these weighting functions are subdivided into individual contributions by the various hydrometeor species generated by the cloud model. Along with the surface contribution and cosmic background radiation, these weighting functions provide a normalized description of where radiation originates and how it ultimately reaches the satellite. It is emphasized that this information provides an indepth understanding of how precipitation retrieval algorithms should be designed vis-à-vis the passive microwave problem.

Abstract

A methodology is developed for deriving atmospheric water vapor transports over the World Oceans from satellite-retrieved precipitation (P) and evaporation (E) datasets. The motivation for developing the method is to understand climatically varying properties of transports, that is, year-to-year changes of the seasonally averaged divergent transport distribution fields, over regions where conventional data, in particular, winds, are sparse. Ultimately, the method is intended to take advantage of the relatively complete and consistent coverage, as well as continuity in sampling, associated with E − P datasets obtained from satellite measurements. Separate P and E retrievals from Special Sensor Microwave Imager (SSM/I) measurements, along with P retrievals from Tropical Rainfall Measuring Mission (TRMM) measurements, are used to obtain the transport solutions.

In this opening study, a 7-yr climatological normal is derived for the January–February–March (JFM) period for years 1988–94, providing the basis for comparing vapor transport anomalies from the 1997/98 El Niño and 1999/2000 La Niña events. These are derived from JFM-averaged transport solutions for 1998 and 1999, respectively. These two periods correspond to times when the Multivariate ENSO Index (MEI) provided by the NOAA Climatic Data Center (CDC) was first at a relative maximum and then at a relative minimum in conjunction with back-to-back west Pacific warm and cold events. Because the El Niño–La Niña events produce such highly contrasting behavior in the transports, shifting from a largely meridionally oriented solution to a largely zonally oriented solution, focusing on this pairing, helps to explain why the methodology is reliable and effective in capturing important details embedded in full-coverage E − P fields.

The analysis includes a sensitivity study of the transport solution technique based on 20 combinations of four precipitation datasets (two satellite based and two model based) and five evaporation datasets (two satellite based, one in situ observation based, and two model based), which helps to explain the reliability of the method. The analysis also includes a comparison to water vapor transports derived with the same method, but applied to E − P datasets obtained from global analysis products prepared by the National Centers of Environmental Prediction (NCEP), again to help explain the reliability of the method. The study concludes by first showing how the anomaly JFM 1998 El Niño solution behaves in close correspondence to associated SST anomalies and is generally more realistic in comparison to the corresponding NCEP solution. Finally, its reliability is discussed in terms of the implications of the vapor transport features for the El Niño–La Niña transition, vis-à-vis north–south and east–west circulations and their accompanying impact on the atmospheric hydrological cycle.

Abstract

A technique is presented for discriminating different cloud types through an image subtraction of visible and infrared SMS/GOES picture pairs. The technique emphasizes how one could separate snow from clouds and identify cirrus by the subtraction method. Quantitative threshold values are shown which can be used in an objective manner to make this separation.

Use is made of an all-digital image display device allowing such mathematical operations to be performed on satellite data. Techniques such as this can be made operational through the interfacing of the image analysis system with a direct-readout SMS/GOES ground station and distribution network.

Abstract

This study highlights infrared sensor technology incorporated into the global positioning system (GPS) dropsonde platforms to obtain sea surface temperature (SST) measurements. This modified sonde (IRsonde) is used to improve understanding of air–sea interaction in tropical cyclones (TCs). As part of the Sandy Supplemental Program, IRsondes were constructed and then deployed during the 2014 hurricane season. Comparisons between SSTs measured by collocated IRsondes and ocean expendables show good agreement, especially in regions with no rain contamination. Surface fluxes were estimated using measurements from the IRsondes and AXBTs via a bulk method that requires measurements of SST and near-surface (10 m) wind speed, temperature, and humidity. The evolution of surface fluxes and their role in the intensification and weakening of Hurricane Edouard (2014) are discussed in the context of boundary layer recovery. The study’s result emphasizes the important role of surface flux–induced boundary layer recovery in regulating the low-level thermodynamic structure that is tied to the asymmetry of convection and TC intensity change.

Dual-polarization weather radars have evolved significantly in the last three decades culminating in operational deployment by the National Weather Service. In addition to operational applications in the weather service, dual-polarization radars have shown significant potential in contributing to the research fields of ground-based remote sensing of rainfall microphysics, the study of precipitation evolution, and hydrometeor classification. Microphysical characterization of precipitation and quantitative precipitation estimation are important applications that are critical in the validation of satellite-borne precipitation measurements and also serve as valuable tools in algorithm development. This paper presents the important role played by dual-polarization radar in validating spaceborne precipitation measurements. Examples of raindrop size distribution retrievals and hydrometeor-type classification are discussed. The quantitative precipitation estimation is a product of direct relevance to spaceborne observations. During the Tropical Rainfall Measuring Mission (TRMM) program substantial advancement was made with ground-based polarization radars collecting unique observations in the tropics, which are noted. The scientific accomplishments of relevance to spaceborne measurements of precipitation are summarized. The potential of dual-polarization radars and opportunities in the era of the global precipitation measurement mission is also discussed.

Abstract

Rainfall rate estimates from spaceborne microwave radiometers are generally accepted as reliable by a majority of the atmospheric science community. One of the Tropical Rainfall Measuring Mission (TRMM) facility rain-rate algorithms is based upon passive microwave observations from the TRMM Microwave Imager (TMI). In Part I of this series, improvements of the TMI algorithm that are required to introduce latent heating as an additional algorithm product are described. Here, estimates of surface rain rate, convective proportion, and latent heating are evaluated using independent ground-based estimates and satellite products. Instantaneous, 0.5°-resolution estimates of surface rain rate over ocean from the improved TMI algorithm are well correlated with independent radar estimates (r ∼0.88 over the Tropics), but bias reduction is the most significant improvement over earlier algorithms. The bias reduction is attributed to the greater breadth of cloud-resolving model simulations that support the improved algorithm and the more consistent and specific convective/stratiform rain separation method utilized. The bias of monthly 2.5°-resolution estimates is similarly reduced, with comparable correlations to radar estimates. Although the amount of independent latent heating data is limited, TMI-estimated latent heating profiles compare favorably with instantaneous estimates based upon dual-Doppler radar observations, and time series of surface rain-rate and heating profiles are generally consistent with those derived from rawinsonde analyses. Still, some biases in profile shape are evident, and these may be resolved with (a) additional contextual information brought to the estimation problem and/or (b) physically consistent and representative databases supporting the algorithm. A model of the random error in instantaneous 0.5°-resolution rain-rate estimates appears to be consistent with the levels of error determined from TMI comparisons with collocated radar. Error model modifications for nonraining situations will be required, however. Sampling error represents only a portion of the total error in monthly 2.5°-resolution TMI estimates; the remaining error is attributed to random and systematic algorithm errors arising from the physical inconsistency and/or nonrepresentativeness of cloud-resolving-model-simulated profiles that support the algorithm.

Abstract

An Advanced Microwave Precipitation Radiometer (AMPR) has been developed and flown in the NASA ER-2 high-altitude aircraft for imaging various atmospheric and surface processes, primarily the internal structure of rain clouds. The AMPR is a scanning four-frequency total power microwave radiometer that is externally calibrated with high-emissivity warm and cold loads. Separate antenna systems allow the sampling of the 10.7- and 19.35-GHz channels at the same spatial resolution, while the 37.1- and 85.5-GHz channels utilize the same multifrequency feedhorn as the 19.35-GHz channel. Spatial resolutions from an aircraft altitude of 20-km range from 0.6 km at 85.5 GHz to 2.8 km at 19.35 and 10.7 GHz. All channels are sampled every 0.6 km in both along-track and cross-track directions, leading to a contiguous sampling pattern ofthe 85.5-GHz 3-dB beamwidth footprints, 2.3 × oversampling of the 37.1-GHz data, and 4.4 × oversampling of the 19.35- and 10.7-GHz data. Radiometer temperature sensitivities range from 0.2° to 0.5°C. Details of the system are described, including two different calibration systems and their effect on the data collected. Examples of oceanic rain systems are presented from Florida and the tropical west Pacific that illustrate the wide variety of cloud water, rainwater, and precipitation-size ice combinations that are observable from aircraft altitudes.