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TRMM-LBA Overview
A Science Overview for the NSF Component of TRMM-LBA/Brazil
Submitted to The National Science Foundation, Division of Atmospheric Sciences
Prepared by
Prof. Steven A. Rutledge
Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523
With contributions from R. Avissar, V. Bringi, L. Carey, W. Petersen,
E. Williams, J. Wilson and E. Zipser
TABLE OF CONTENTS
Abstract
Under support from NASA/TRMM, a major ground validation program, known as TRMM/Brazil, will be carried out in Amazonia from 1 November 1998 to 28 February 1999. This program will focus on the dynamical, microphysical, electrical and diabatic heating characteristics of tropical convection in this region. Data collected in the program will be used in part to validate products from the TRMM satellite as it repeatedly overflies this region. This field program presents an enormous opportunity to address several NSF-based scientific objectives in the study of tropical convection in this region. Indeed, the characteristics of convection in this climatically-important region are only poorly known. We are requesting partial support for the S-pol radar from NSF for use in TRMM/Brazil. NASA intends to pay for a significant portion of the S-pol radar deployment costs. This Science Overview details the science to be carried out in association with several funded or pending NSF proposals. This research will capitalize on NASA-funded deployments of a second Doppler radar, atmospheric sounding systems, multiple aircraft, a lightning location and detection network, raingauges and a profiler. The NSF investigators consider this project to be a once in a lifetime opportunity to study tropical convection in Amazonia, and to compare the characteristics of convection in this region to other regions in the tropics and mid-latitudes. TRMM/Brazil will be conducted in parallel with the wet season component of the Large Scale Biosphere-Atmosphere Experiment in Amazonia.
I. Introduction
The Tropical Rainfall Measuring Mission (TRMM) is a joint satellite project between NASA and the National Space Development Agency of Japan (NASDA). TRMM is designed to measure tropical precipitation from a low-inclination orbit using a suite of sensors, including the first spaceborne application of a precipitation radar. One of the greatest challenges standing in the way of improved numerical model simulations of the global climate is knowing when the model accurately simulates the hydrological cycle. Underpinning validation of the simulated hydrological cycle is knowledge of actual rainfall, especially tropical rainfall, since the release of latent heat in tropical precipitation is one of the principal energy sources in the atmospheric general circulation. TRMM will provide the first opportunity to diagnose not only the integrated latent heat release in the tropics, but its vertical profile. The tropical general circulation is most sensitive to the vertical divergence of this heating. The products derived from the TRMM data will therefore be particularly important for studies of the global hydrological cycle, for testing the performance of general circulation climate models, and their ability to simulate and predict climate accurately on the seasonal time scale. There are many closely interconnected issues, such as the effects of the El Nino - Southern Oscillation (ENSO) on climate which can be addressed with a reliable, extended time series of tropical rainfall estimates like TRMM will provide. Indeed, TRMM data are now allowing coverage of the present El-Nino following TRMM's launch on 27 November 1997.
The TRMM observatory for rainfall observations consists of a precipitation radar (PR, provided by Japan), a multi-frequency microwave radiometer (TMI) and a visible and infrared radiometer (VIRS). In addition, the lightning imager (LIS) and an earth radiation budget sensor (CERES) accompany these sensors.
The Precipitation Radar (PR) will provide information on the 3-D radar reflectivity distribution over both land and ocean. More specifically, this instrument will define the layer depth and structure of the precipitation and provide information about the rainfall reaching the surface, the key to determining the latent heat input to the atmosphere. This instrument will provide a pathway for estimating rain over land where passive microwave channels have considerable difficulty. The PR is an electronically scanning radar operating at 13.8 GHz with horizontal polarization. The horizontal resolution is 4.3 km at nadir, the range resolution is 250 m and the scanning swath width is 220 km.
The multi-channel microwave radiometer, designated as the TRMM Microwave Imager (TMI), is designed to provide information on the integrated column precipitation content, its areal distribution, and intensity. The TMI operates on 5 frequencies, of which 4 have dual polarization. The 5 frequencies are 10.65, 19.35, 22.235 (single polarization), 37.0 and 85.5 GHz. The horizontal resolution of the TMI will range from 5 km at 85.5 GHz to 45 km at 10.65 GHz. The swath width of this instrument is 760 km.
The Lightning Imaging Sensor (LIS) is designed to investigate the global incidence of lightning, its relationship with the global electric circuit, and, in conjunction with the PR, TMI, and VIRS, its correlation with rainfall. The presence of lightning provides important clues about the convective vigor, microphysical processes, and vertical distribution of latent heating. LIS will be optimized to detect the lightning location, mark the time of occurrence, and measure the radiant energy. LIS is a calibrated optical sensor operating at 0.7774 microns and will observe the distribution and variability of lightning over the Earth as viewed by TRMM. The horizontal resolution at nadir is 5 km and the swath width is 590 km.
TRMM's unique combination of sensor wavelengths, coverage's, and resolution capabilities together with the low-altitude, non-Sun-synchronous orbit provide a sampling capability that will yield monthly precipitation amounts to a reasonable accuracy when averaged over areas equivalent to a 500- by 500-km grid. The highly precessing orbit also will provide insight into diurnal variability of rainfall over the oceans. Such a climatology will go a long way toward meeting the scientific requirements for studying latent heating of the atmosphere for modeling and diagnostic studies. The scientific uses of TRMM data, however, are not restricted to climatological studies. The combination of radar profiles, multi-channel microwave, visible and IR radiances, and lightning from 35° N to 35° S is a unique and powerful database for fundamental studies of cloud and precipitation processes within both tropical oceanic and tropical continental precipitation systems.
A substantial and well-defined ground validation (GV) program is running in parallel with the space mission, at several validation sites representing tropical rainfall regimes. A better understanding of the characteristics of rainfall, improving techniques for direct measurement and estimation of rainfall from remote methods, and validating satellite-derived products are key research objectives of the GV program.
The basic products from both the satellite and GV programs are accumulated rainfall, along with the characteristic horizontal and vertical structure of precipitation. However, TRMM's ultimate product is the quantification of the vertical diabatic heating profile associated with rainfall. Cloud resolving models will provide the necessary linkage between rainfall and latent heating profiles. These models must generate realistic cloud systems when initialized with accurate environmental conditions. Rainfall, broken down into convective and stratiform components, will serve as weighting factors to the respective convective and stratiform heating profiles generated by the cloud models. The heating profile in stratiform precipitation (heating above the melting level and cooling below) shows a large degree of repeatability from region to region. However, a large degree of variability exists among convective heating profiles. In order to gain confidence in the cloud-resolving models, for example, the Goddard Cumulus Ensemble Model, specific field campaigns are planned under the TRMM umbrella to investigate the characteristics of convective clouds in order to validate the cloud models. In addition to model validation, the satellite and GV algorithms that produce rainfall statistics and cloud structure need to be validated against field data. Two specific field campaigns are planned: a campaign in west-central Brazil during the wet season of 1998-99 and a program at Kwajalein Atoll, Marshall Islands during the late summer of 1999.
Both field campaigns will provide unprecedented dynamical, microphysical, thermodynamical and electrical measurements of tropical convection. Indeed, the campaign in Brazil, known as TRMM/Brazil, will provide detailed sampling of tropical continental convection in Amazonia for the very first time. Previous convective experiments have been conducted in Amazonia, most notably ABLE (Amazon Boundary Layer Experiment; Garstang et al., 1994), however that campaign had very limited radar observations, consisting of a single X-band non-coherent radar only. Radar observations proposed for TRMM/Brazil will be far superior to those available in ABLE. The TRMM/Brazil field campaign will be conducted in parallel with the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) wet season campaign. LBA is a large, multi-country, multi-agency program intended to examine the effects of land use change on Amazonian and regional weather patterns. Further information on LBA and couplings between LBA and TRMM/Brazil are provided in Sec. 6.
The broad array of instrumentation planned for the TRMM/Brazil (see Sec. 4) will provide a unique opportunity for NSF-funded PI's to contrast the dynamical, microphysical and electrical aspects of Amazonian convection to the mid-latitudes, and other regions of the tropics, most notably monsoon and continental convection over the Maritime Continent region and open-ocean convection studied in TOGA COARE. The field program will also provide an avenue to further explore the use of S-band multiparameter radar to remotely assess the microphysical characteristics of tropical convection. In addition to studying the physical properties of the convection, the Brazil field campaign provides a unique opportunity for GEWEX/GCSS (GEWEX Cloud System Study Working Group 4), specifically research directed towards developing improved convective parameterizations of precipitating convective cloud systems for use in General Circulation Models. The NSF-based scientific objectives of this work are detailed in Sec. 3.
The TRMM/Brazil field campaign will be conducted from 1 November 1988 until 28 February 1999. Single-Doppler radar, atmospheric sounding measurements and extensive raingauge measurements will begin on 1 November. During January and February 1999, dual-Doppler observations are planned when the NCAR S-pol polarimetric radar will join the other Doppler radar in the program. Frequent radiosonde launches will occur within a mesoscale sounding network to be operational during January and February 1999. Dual-wavelength profile observations will supplement the dual-Doppler observations. A lightning network (developed by NASA/MSFC) will provide coverage of cloud-to-ground lightning locations and flash polarities throughout the campaign. Aircraft will be an integral part of the field campaign and will provide important measurements for the NSF-based research. A NASA ER-2 aircraft will overfly systems in W. Amazonia and obtain passive microwave data (from AMPR - Airborne Microwave Profiling Radiometer) and vertical profiles of radar reflectivity from EDOP (ER-2 Doppler radar, operation at the same wavelength as the TRMM radar, 13.8 GHz; 2.14 cm). The second aircraft will be either the University of North Dakota Citation II or Aeromet Lear jet. This platform will provide in-situ microphysical measurements in stratiform precipitation and developing convection. Unique instrumentation is proposed for the Citation II, including a CPI (Cloud Particle Imager) and HVPS (High Volume Particle Spectrometer). The CPI uses a new camera imaging system to image particles in the 10 - 2000 micron size with unprecedented detail and resolution, as compared to current 2D-C probes. The HVPS has a sample volume approximately 8 times that of 2D-P probes and samples particles from a few hundred microns to several millimeters. Both instruments will provide important measurements for the NSF-based research.
II. Meteorological Overview
The Brazilian tropical rainfall region is easily identified when examining seasonal mean OLR (Outgoing Longwave Radiation) patterns. This region is one of three major regions of minimum OLR situated along the equator over South America, Africa and Indonesia/W. Pacific Ocean (the latter region referred to as the Maritime Continent). These three regions each provide strong thermal forcing of the global tropical atmosphere. Surface temperature contrasts accompanied by rising motion and latent heat release, drive east-west circulation cells in the equatorial plane. The largest of the three thermally-direct zonal circulations is of the course the Walker Circulation, associated with ascent over the Maritime Continent and descent over the eastern Pacific. In order to improve simulations of the tropical large scale atmosphere, convection in each of these ascending branches must be better understood, from mass flux, momentum transport and diabatic heating perspectives. TRMM/Brazil will undoubtedly add to our knowledge base concerning tropical convection over the S. American upwelling region.
Studies by Horel et al. (1989), Hastenrath (1997) and others indicate a sharp onset of the wet season in equatorial Brazil (Amazon basin) during austral spring. The abrupt transition is associated with the southerly migration of the equatorial trough and heating of the continent, the former providing the requisite forcing and moisture transport for copious amounts of convection in this region. Monthly rainfall statistics for the center of the TRMM/Brazil field network (town of Ji-Parana, state of Rondonia) are shown in Fig. 1.
Satellite-based climatologies of tropical South America and Amazonia reveal that Mesoscale Convective Systems are ubiquitous in this region (Velasco and Fritsch, 1987; Mohr and Zipser, 1996; and others). The Mohr and Zipser (1996) SSM/I based climatology for January reveals frequent MCS structures over western Amazonia. Moreover, the MCSs in this region, along with MCSs in Africa and Central America contain the lowest brightness temperatures in their sample, suggesting these MCSs develop from deep and intense convective cells. A recent study by Garreaud and Wallace (1997) indicates a strong diurnal modulation of convection in this region (they also identified a strong diurnal modulation of deep convection to the west of this region over the Andes Mountains). These regions exhibit a sharp peak in convective cloudiness that is spatially organized in bands in the late afternoon and evening hours. This pattern is consistent with the development of isolated, deep convection in the late afternoon and evening hours, followed by the development of mesoscale precipitation structures during the late night and early morning hours.
Relatively little is known about the lightning flash rates in Amazonia. However, based on our experience in other tropical continental locations (Darwin and vicinity), we expect significant lightning in this region. Preliminary data from the VLF lightning network operated by the UK does suggest that there is a seasonal dependence on flash rates, but less variability in seasonal rainfall (J. Weinman, personal communication). Petersen and Rutledge (1998) have studied the relationship between areal rainfall and cloud-to-ground flash rate in terms of a rain yield (ratio of rain mass over a given area to total CG lightning within the same area). Petersen and Rutledge found that the rain yield was reduced during tropical continental rainfall relative to oceanic or monsoonal rainfall. These results suggest that rainfall during the first part of the wet season in western Amazonia (October-December) has "continental" characteristics. Rainfall during the latter half of the wet season (January-March) is accompanied by less lightning, suggesting more monsoon-like conditions.
III. NSF Objectives in TRMM/Brazil
The integrated observation of cloud microphysical, kinematic, and electrical properties with an S-band polarization radar, a dual-Doppler radar network, in-situ aircraft, a sounding network and a lightning detection network will provide for a comprehensive study of Amazonian convection. In accord with this unique opportunity, there are several scientific objectives under the umbrella of NSF-supported research that can be addressed, all made possible by the first-time deployment of the S-pol polarimetric radar to the deep tropics. S-pol is capable of measuring a host of linear polarimetric variables (Zh, Zdr, Kdp, rhoHV, and Ldr) which are related to the shape, size, orientation, and thermodynamic phase of precipitation-sized hydrometeors. As demonstrated in several recent studies (Carey and Rutledge, 1996; Jameson et al., 1996; French et al., 1996; Ramachandran et al., 1996; Lopez and Aubagnac, 1997; Bringi et al., 1997; and Carey and Rutledge, 1998), these polarimetric radar measurables can be used to infer the bulk hydrometeor type and estimate the precipitation amount throughout most of the radar echo volume. These measurements are especially critical in areas of intense convection where in-situ measurements are not possible. We plan to improve current polarimetric radar techniques for estimating precipitation types and amounts in tropical continental convection by using in-situ aircraft microphysical measurements from the HVPS and CPI probes coupled with theoretically-based particle scattering techniques. Following these observationally and theoretically based improvements to the bulk hydrometeor estimation techniques, we will compile statistics of bulk hydrometeor types (e.g., rain, graupel, supercooled drops, etc.) in many convective systems to describe and quantify precipitation processes in these systems. Knowledge of the microphysical processes will be placed in the context of air motions provided by dual-Doppler observations from the S-pol and TOGA (C-band Doppler) radars. Merged with electrical observations, we will study the coupling between kinematics, microphysics and electrification in these tropical systems. These measurements will include total lightning flash rate (both in-cloud and cloud-to-ground [CG] lightning) from a network of Advanced Lightning Direction Finders (ALDF) and field change meters (or flat plate antennas).
The following scientific objectives are addressed through the NSF proposals of S. Rutledge (Colorado State University); V. Bringi and J. Hubbert (Colorado State University); R. Avissar (Rutgers University), E. Williams (MIT), and J. Wilson and E. Brandes (NCAR). Additional scientific collaboration is expected with Prof. Steven Kreuger of the University of Utah through GEWEX/GCSS/Working Group 4 (cloud-resolving modeling studies of convective systems in the tropics). We will also collaborate with S. Goodman (NASA/MSFC), R. Ravihagan (Univ. of Alabama-Huntsville), and J. Dye of NCAR/MMM in the areas of cloud physics and cloud electrification. Furthermore, E. Zipser, who is the team leader for the TRMM field campaigns, will also play a strong role in the experiment. We also expect a significant number of TRMM Science Team members to utilize S-pol and other datasets obtained in TRMM/Brazil.
Specific objectives to be addressed in the field program include:
1. Formulate conceptual models of the kinematic, microphysical, and electrical properties of tropical Amazonian convection for comparison with studies of tropical oceanic (e.g., TOGA COARE and GATE), tropical continental (e.g., DUNDEE and CaPE) and maritime continental convection (e.g., MCTEX). This work will broaden our knowledge base regarding tropical convection. These observations are expected to serve as important validation datasets for cloud resolving model simulations of tropical convection, for example, those being done under GEWEX/GEWEX Cloud Systems Studies (GCSS). The primary goal of the GCSS studies is to develop better parameterizations of convection in GCM's in order to improve simulations of global climate. Indeed, detailed observations of the convection over Amazonia have not been undertaken despite its large scale importance as established by cloud radiative forcing and GCM diagnostic studies. The data collected in the TRMM/Brazil campaign (especially data collected by the NCAR S-pol and TOGA Doppler radars) will be used by GCSS to validate their cloud resolving models in this tropical continental regime. These results will complement parallel GCSS simulations of tropical oceanic systems observed in TOGA COARE.
The conceptual models will be contrasted to the structure and organization of mid-latitude convective systems, for example, findings from the PRE-STORM experiment. The modes of convective organization will be identified (linear line with trailing stratiform, asymmetric Mesoscale Convective System, etc.) and compared to these other geographical regions. This work will formulate the basis for further studies of the momentum and heat transports by Amazonian convective systems, and comparison of these transports to results in other geographical locations. The observed convective structures will be analyzed in the context of environmental parameters, including shear and CAPE.
2. Develop improved techniques for hydrometeor identification from polarimetric radar data (for example, the five parameter classification scheme described by Doviak and Zrnic, 1993). Further progress in this area can only be made with supporting in-situ microphysical observations which will be a strong component of the TRMM/Brazil campaign. TRMM/Brazil expects to deploy the new Cloud Particle Imager (CPI) manufactured by Spec, Inc. of Boulder. This probe gives vastly improved images of cloud and precipitation particles compared to conventional 2D PMS probes. For larger precipitation sized particles the High Volume Spectrometer (HVPS) will be used. The combination of the new CPI probe and the HVPS on the University of North Dakota Citation II aircraft will yield an unprecedented characterization of particle types and particle size distributions. (Funding for the CPI instrument has been requested from NASA/TRMM. Costs to mount the HVPS on the Citation II aircraft have been requested in the Bringi/Hubbert proposal to NSF. New Mexico Tech will loan the HVPS instrument to us at no charge.) In combination with coordinated S-pol measurements we will have an unprecedented opportunity to improve/validate polarimetric-based hydrometeor classification schemes in tropical precipitation. We expect that a portion of these results will be transferable to the study of mid-latitude storms as well. We will also apply T-matrix scattering theory to compute all polarimetric measurands from the observed particle type/size distributions. These calculations will allow us to examine the sensitivity of the polarimetric observations to assumed size distributions as well as mixtures of various hydrometeor types.
3. The proposal to NSF by Bringi and Hubbert requests support for the deployment of a 2-D video disdrometer manufactured by Joanneum Research. This instrument uses two line scan cameras to provide highly accurate estimates of drop fallspeed, shape, and drop size distributions (dsd's) . Considerable experience has been gained with this instrument in successful studies with the CSU-CHILL multiparameter radar. Multiparameter measurands can be computed from the disdrometer data and compared to radar-measured parameters (Zdr, Kdp, etc.). This instrument has also been used recently in New Guinea to collect data on tropical rainfall. Drop size distributions observed in Papua, New Guinea were observed to deviate significantly from mid-latitude drop size distribution models (Marshall-Palmer, Joss thunderstorm, Joss drizzle) at the same rain rates. In particular, the slope of the dsd at the large drop end (2.2 to 3 mm) is much larger (slope = 6.5 mm-1) than the Marshall-Palmer fit (2.08 mm-1) and tends towards numerically-simulated equilibrium dsd slopes (Hu and Srivastava, 1995). The differential reflectivity, Zdr, is well known to be strongly weighted by the slope of the large drop end of the dsd. We will perform a direct comparison of Zdr to 2-D video disdrometer measurements in tropical rainfall to examine the importance of transient drop shapes. A direct comparison of Kdp (specific differential phase) between radar and disdrometer will also be carried out to examine sensitivities to dsd/drop shape in estimating rainfall from Kdp (R-Kdprelationships). Recent work by Keenan et al. (1998) in tropical N. Australia showed that R-Kdp estimates were highly sensitive to the form of the drop size-shape relationship used in the scattering calculations, when measured at C-band. TRMM/Brazil will allow us to study this sensitivity in tropical rainfall at S-band. We will also evaluate the possible microphysical origins (either eddy shedding or drop collisions) of high Ldr (linear depolarization ratio) in tropical rain (Jameson and Durden, 1996).
4. Deduce the role of collision-coalescence, mixed phase microphysics, and a hybrid process involving both of the above mechanisms in the production of tropical continental rainfall as a function of convective lifecycle. This goal will be supplemented with the use of a 1-D microphysical model with a detailed 4-class ice scheme (Petersen, 1997). In carrying out this objective, we will be able toinvestigate the role of mixed-phase ice microphysics and convective vigor in the production of lightning in tropical continental convection using polarimetric radar, dual-Doppler, and in-situ observations. We will compare and contrast these observations to those from CaPE (e.g., Jameson et al., 1996; Bringi et al., 1997) and MCTEX (Rutledge and Carey, 1997). Recent observations in CaPE and MCTEX suggest that the ice phase develops rapidly in association with the lofting and subsequent freezing of large drops produced by coalescence in the convective updrafts. These observations suggest a vertical velocity threshold for electrified tropical convection, specifically a vertical velocity equal to the terminal fallspeed of large drops (6-8 m s-1). Indeed, Zipser and Lutz (1994), and Petersen (1997) postulated this same dependence in their studies of lightning-producing oceanic convection. The ice phase develops rapidly once the drops freeze. Evidently these frozen drops play a critical role in rapidly electrifying the cloud via non-inductive charging mechanisms.
5. Determine the validity of the H5 dependency (e.g., Williams, 1985) of the total lightning flash rate on echo top height (H) for tropical continental convection in Amazonia. Parameterizations of lightning flash rates in global models used to estimate the world-wide source of lightning-produced NOx follows this H5 dependency (Price and Rind, 1992). This relationship needs to be validated against a broader set of observations than is currently available, especially in regions with copious amounts of tropical convection like Amazonia. There is competing evidence that suggests that lightning flash rates are dependent on the vertical structure of the convective clouds (e.g., the height obtained by a particular radar reflectivity threshold, Petersen et al., 1996) or the ice mass content in the mixed phase region of the storm (Rutledge and Carey, 1997), as opposed to the overall echo height. We intend to carry out a test of these hypotheses in TRMM/Brazil.
6. C-band attenuation studies. TRMM/Brazil will field both a non-polarimetric C-band Doppler radar, and the S-pol 10 cm polarimetric radar, in addition to a dense network of raingauges and several disdrometers. This field design will allow us to investigate propagation effects as a function of both drop size distribution and wavelength in order to refine attenuation correction procedures in tropical rain (for example, the reflectivity-based relationship developed by Geotis for GATE rainfall). All subsequent C-band radar rainfall datasets since GATE, for example, TOGA COARE, have used this GATE relationship. TRMM/Brazil will afford the opportunity to compare attenuation estimates from the gauge network as well as estimated from the S-pol polarimetric data. These studies will be of importance in future studies of tropical rainfall using C-band non-polarimetric radars.
7. By merging the TRMM/Brazil observations (specifically lightning flash rates/polarities and observations of storm structure) with similar datasets from the mid-latitudes (CSU-CHILL radar and lightning datasets) and tropical W. Pacific (from COARE and MCTEX radar and lightning observations), construct a multi-dimensional parameter space analysis of storm and environmental characteristics (e.g., rain and/or hail mass flux, peak rain and/or hail rates, large hail amount, reflectivity tilt, environmental shear, and CAPE) versus lightning parameters to investigate differences in CG flash density and polarity, IC flash rate and IC/CG ratios as a function of storm intensity and structure. This work will not only take an important step in the direction of improving lightning flash rate parameterizations for use in global models, but will address the fundamental physical differences between high flash rate, intense mid-latitude convection, and predominantly low flash rate, weaker tropical convection.
8. Data collected by the S-pol radar, in conjunction with the second Doppler radar, radiosonde network and instrumented flux towers (provided by LBA), will be used to validate model simulations in this geographically-diverse region. Simulations with the CSU Regional Atmospheric Modeling System (RAMS, including the RAMS-Large Eddy Simulation version) will be carried out to simulate land-atmosphere interactions in deforested regions. Furthermore, the simulations will seek to determine land-atmosphere interactions in forested regions and to compare these interactions with those over the deforested regions.
9. Under sponsorship of the U.S. Weather Research Program, NCAR is studying the use of polarimetric radar to improve remote estimation of precipitation. To date NCAR/ATD researchers have carried out field programs with S-pol in Kansas and Colorado. A third field campaign will be conducted in Florida during the summer of 1998. The addition of data from TRMM/Brazil will provide a very comprehensive dataset for examining the transferability of results between greatly varying rainfall regimes. Specific tasks include:
i) Sensitivity of polarization measurements to temperature.
The scattering characteristics of raindrops depend on shape, size and dielectric constant of the particle. The S-band dielectric constant of water varies as much as 10 to 20% between 0 and 20 C (Ray, 1972). At higher frequencies such as C-band, the variation is even larger. We will examine this sensitivity and effects on R-Zdr and R-Kdp relationships for the warmer rainfall events in Amazonia. The effects of tropical drop size distributions and drop shape effects will be included (results from Objective 3 above).
ii) Area-time Integration Technique:
The area-time integral (ATI) based rain rate estimation is a statistical relationship that varies from region to region. ATI is linearly proportional to the volume of rain (i.e., rain rate integrated over a time interval). Traditionally, reflectivity has been used to estimate both ATI and volume rainfall (Rosenfeld et al., 1990). We propose to use propagation phase for estimating volume rainfall. The resultant rainfall estimate is less sensitive to variation in drop size distribution, ground clutter and radar system calibration. Being a range cumulative measurement, propagation phase at the far end of the storm is the total phase and it is directly proportional to the total precipitation along the particular radial. The total precipitation for each radial can be integrated over an azimuthal sector to derive the ATI relationship. This ATI result can then be compared to the reflectivity-based ATI result. The propagation phase ATI result from TRMM/Brazil will also be compared to propagation phase ATI results from Kansas and Florida.
10. Beginning with Madden and Julian (1972), the great focus of attention on this dominant intraseasonal variation has been over ocean regions, most notably the Western Pacific region. Very recent studies with remote TOVS satellite and ELF (Schumann resonance) observations (Anyamba et al., 1998) indicate strong MJO signals in enhanced deep convection in both Africa and South America. TRMM/Brazil will provide the first opportunity for detailed studies of thermodynamics and deep convection on the MJO time scale in a tropical continental region. Efforts are already underway to organize an extensive set of surface observations in Brasilia to compare with radar and satellite data on this phenomenon collected in TRMM/Brazil. The modulations in rainfall on 30-50 day times scales over this region will likely be an important focus for TRMM itself.
11. MIT in currently monitoring electromagnetic signals in the Earth-ionosphere cavity known as Schumann resonances from a receiving station in West Greenwich, RI. The broad objectives of this project are twofold: further exploration of the background SR intensity as a diagnostic for global change, and the elucidation of relationships among mesoscale lightning, transient excitations of the Earth-ionosphere cavity (Q-bursts), and mesospheric sprites. Radar and lightning observations in TRMM/Brazil will be used to ground-truth the meteorological and electrical characteristics of tropical convection that influence the Rhode Island SR signals. As one specific example, we have recently developed a method for measuring the vertical charge moment associated with mesoscale lightning over South America based on analysis of calibrated transient signals in Rhode Island. The moment change is fundamental in modulating the optical intensity of sprites. Local electrostatic measurements in TRMM/Brazil together with the proposed cloud-to-ground lightning measurements proposed by NASA's Marshall Space Flight Center will enable rigorous comparisons of an electrostatic and an electromagnetic measurement of charge transfer.
IV. Experimental Design
TRMM/Brazil will field a comprehensive array of observational sensors to study the physical characteristics of convection in Amazonia. These sensors include multiple radiosonde sites (funded by both Brazil and NASA), a four-station lightning detection network (funded by NASA), a network of flat plate antennas, a dense raingauge and disdrometer network (funded by NASA), two Doppler radars (including the S-pol polarimetric S-band radar; requested funding from both NSF and NASA and the TOGA radar, to be funded by NASA), and a dual-wavelength profiler from the NOAA/Aeronomy Lab (funded by NASA). In addition to these sensors, two aircraft are planned for the experiment. The first aircraft, a twin jet (either the U. of North Dakota Citation II or Aeromet Learjet will be used; proposals have been submitted to NASA), will provide in-situ sampling of stratiform, developing and dissipating convective clouds. These data will be of critical importance to our planned research on improving hydrometeor identification algorithms, explaining high Ldr values in tropical rain and identifying microphysical processes in tropical convection (and relationship to storm electrification; Objectives 1, 2, 3 and 4). The twin jet will operate out of Porto Velho, resulting in a very short ferry time to the prime operational area (see Fig. 2). The second aircraft is the high altitude NASA ER-2 operating out of Brasilia. The ER-2 will carry the EDOP radar (ER-2 Doppler, X band radar) and AMPR (Airborne Microwave Profiling Radiometer), a multi-frequency radiometer similar to the TMI instrument on TRMM. The ER-2 will provide important data for the NSF objectives stated in Sec. 3. For example, the EDOP radar will provide high resolution information on the vertical structure of convection which is needed in the study of lightning flash rates vs. cloud top height (Objective 5). Both aircraft are expected to be operational during the months of January and February 1999. Each will fly approximately 100 research flight hours. Plans are uncertain at this time but it is also possible that the NASA DC-8 aircraft will operate in TRMM/Brazil. If so, the DC-8 would carry the NASA/Jet Propulsion Lab cloud radar (94 GHz), a cloud lidar and the ARMAR (X-band Airborne Rain Mapping Radar). This aircraft would operate for the primary purpose of observing tropical cirrus clouds. However data from these instruments would be particularly valuable in documenting the horizontal and vertical structure, and kinematics of convection in Amazonia (Objective 1). The DC-8 would most likely operate from Brasilia. The planned experimental design is shown in Fig. 2. This network will provide some synoptic level information, but is primarily designed to sample both the meso- and convective scales. Synoptic information will be provided by two sources: two per day soundings at the Porto Velho and Vilhena sites. These sites are existing Brazilian Air Force operational sites that are planned to be supplemented during the TRMM/Brazil-LBA field project. The funding for these additional launches will be obtained from either LBA, Brazilian or NASA resources, or a combination of these sources. These two sites will be operational during the entire field project, 1 November 1998 through 28 February 1999. We will also have access to surface synoptic observations in this region during the field project. A mesoscale sounding array will be formed by three sounding stations deployed for TRMM/Brazil and LBA. This mesoscale sounding array will operate during the intensive months of the campaign, planned for January and February 1999. Support for the mesoscale sounding network (four launches per day are planned for each site) is to be obtained from both Brazilian and NASA funding. These soundings will be used to describe the kinematic and thermodynamic environment for developing convection, and subsequent modification of the environment by mature convective systems. We plan to use VIZ ground stations and sondes at the mesoscale sounding array sites. These sondes are 3-D GPS units with excellent humidity measurement performance.
Widespread coverage of rainfall is expected within the 150 km radius circle centered on the S-pol radar location (see Fig. 2; actual radar sites for both the S-pol radar and the TOGA radar have already been secured during a site survey trip to this region in October 1997). Approximately 30 raingauges will be deployed by NASA in the 150 km range circle centered on S-pol, as well as several disdrometers. The 2-D video disdrometer will also be deployed within 20 km of S-pol, at a site to be determined. The dual-Doppler coverage area will be nested within the mesoscale sounding array. We plan a radar baseline of 50 km, which is approximately 10 km shorter than the baseline used in PRE-STORM, and about 10 km longer than the baseline used at the NSSL Doppler radar pair for many years. The PRE-STORM baseline provided excellent coverage of the mesoscale structure of convection, but was not designed to resolve individual convective drafts. The NSSL baseline did however provide adequate coverage of the convective scale, by virtue of its shorter baseline. Hence we will resolve the mesoscale structure and organization of convection in TRMM/Brazil, and resolve individual convective circulations with reduced resolution. The dual-Doppler data will be critical for satisfying Objectives 1, 4, 5, 7 and 8. We will collect dual-Doppler data during the January-February 1999, and single-Doppler data from 1 November 1998 through February 1999 (using the TOGA radar). Dual-Doppler data will be collected by coordinated 360 degree volume scans primarily, interleaved with polarimetric scans from S-pol (for rain mapping and hydrometeor identification purposes) and 360 degree volume scans from the TOGA radar. We will also use coordinated dual-Doppler sector scans (< 180 degree sectors) in order to top deep convection at close to moderate ranges, to maintain adequate temporal resolution (6 minutes or less between volume times). The NOAA/Aeronomy Lab dual-wavelength profiler (915 MHz and S-band) will be located near the dual-Doppler baseline (restrictions in available sites prevent locating the profiler near the center of either dual-Doppler lobe). The S-band reflectivity from the profiler will be helpful in examining the vertical structure of the precipitation systems in a time-height sampling mode. These data will be an important complement to the pursuit of Objectives 1, 4, 5, and 7. Doppler velocity data in the vertically-pointing mode will be obtained from the 915 MHz profiler data as a complement to the vertical velocity data derived from the dual-Doppler network.
Coverage of cloud-to-ground lightning ground strike locations, flash polarity and peak currents will be detected by a four station LLP network, consisting of advanced lightning detection stations manufactured by LLP, Inc. Funding for this network has already been obtained by NASA/Marshall Space Flight Center. Installation of this network is planned for May-June 1998 with operations to begin shortly thereafter. Data from the lightning network will be critical for Objectives 1, 4, 5 and 7. A network of flat plate antennas (3-4 sensors) will also be deployed in the experimental region for detecting total lightning. We have used these antennas with good success in the past. In-cloud flash rates can be derived from total lightning measurements provided by the flat plates by subtracting the CG flash rates detected by the LLP network. Costs for the flat plate network will likely be covered by NASA funds.
V. Field Operations and Logistics
Operations control for the experiment will be carried out from the S-pol radar site near Ji-Parana. Here we expect to have access to synoptic, satellite, and data from various model simulations. Radar, aircraft, and sounding network operations will be coordinated at this site via direct radio links and cellular phone. University of Sao Paulo (USP) will install a high speed phone communications line to the S-pol site to obtain access to Internet and therefore model output, satellite data, etc. USP personnel will run the CSU/RAMS model in a forecast mode and transmit these products via the Internet to the operations center at S-pol. It is also likely that the NCAR MM5 model will also be used in a forecast mode by USP and those products made available over the Internet as well. A satellite link back up communications system will also be available at the S-pol site (provided by NASA/TRMM).
A site survey trip carried out in late October 1997 shows that radar and sounding operations are entirely feasible in the Ji-Parana region. Radars will be run continuously on diesel power. Diesel fuel is readily available in the region as are crane services (for erecting and disassembling antennas) and heavy truck transport (for hauling radars and other equipment from Porto Velho). Current plans call for transporting the radars by ship to Manaus via the Amazon River, then by barge to Porto Velho. From there the radars will be hauled to the site by truck over paved roads. Local politics in the region appear to be very stable and we anticipate great interest in the project from the locals and support from them. Hardware supplies are readily available in Ji-Parana. Adequate housing exists in and around Ji-Parana for scientific and support staff. Memorandums of Understanding are presently being developed with Brazilian officials to establish permission for the use of the various observational platforms in TRMM/Brazil. Personnel from Colorado State University, NCAR, NASA, MIT/Lincoln Lab, University of Sao Paulo and other Brazilian and European agencies (through their involvement in LBA) will staff the field sites.
VI. Coupling with LBA
The TRMM/Brazil field campaign will be conducted in parallel with the wet season component of the Large Scale Biosphere-Atmosphere Experiment in Amazonia, known as LBA. LBA is a large, international field program designed to further our understanding of the climatological, ecological, biogeochemical and hydrological processes in Amazonia, and the impact of land use change on these processes. Studies of the interaction between Amazonia and the Earth system are also planned, through regional and global scale numerical model simulations. Studies of physical climate processes, carbon storage and exchange, biogeochemistry, atmospheric chemistry, land surface hydrology and water chemistry, and land use and land cover change are specific components of LBA. Central to the studies of physical climate in Amazonia, LBA will focus on studying the spatial and temporal variations of energy and water fluxes. A key element of these fluxes is of course transport associated with moist convection. TRMM/Brazil can expect to benefit from various numerical modeling studies, atmospheric sounding support, surface flux measurements and logistical support provided by LBA. In return, TRMM/Brazil will provide rainfall measurements (from satellite, polarimetric radar and raingauges) for the Rondonia hydrological component of LBA. It is also possible that LBA will extend the coverage of the TOGA C-band radar from 1 March to 1 September 1999. If this extended coverage is realized, we would be able to extend our study of convective systems and their lightning properties over this period as well.
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