Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 https://doi.org/10.1186/s40623‑019‑0985‑y FRONTIER LETTER Open Access Characterization of atmospheric structures observed by a VHF MST‑type radar in the troposphere over Santa Cruz, Costa Rica Marcial Garbanzo‑Salas1,2* and Wayne Hocking2 Abstract Results from the first VHF profiler research radar in Costa Rica, operating at a central radar frequency of 46.6 MHz, are presented. Emphasis has been on studies of scattering layers detected in the altitude range 1–6 km, with the main goal being to identify regions with radar echoes and observe the temporal evolution of the echoes. Data were obtained over the course of a full year using a vertical resolution of better than 100 m. Layers of strong scatter were observed regularly, often with simultaneous broad spectra, which may indicate enhanced turbulence. Similar layers have been observed over equatorial Indonesia, and these have been associated with the planetary boundary layer. The presence of echo layers was more common during the dry‑season months (December–April); in fact during March, two layers were observed in the lower troposphere for more than 35% of the time. Stable pattern structures often occurred for extended periods, but at times the layers could also vary drastically in behavior from 1 day to the next. After sunset, strong echo layers could persist for several hours. Some examples of regularly observed layer behavior are given. Keywords: MST type radar, Planetary boundary layer, Layers of turbulence Introduction only one receiver cluster was used, but this was subse- During the past few years an initiative to create a radar quently upgraded into a three-receiver system. Several research center has been undertaken in Costa Rica. The aspects of Costa Rica’s radar design and location enhance location chosen for the radar site is a satellite campus of its research capabilities (Hocking et  al. 2014). In North the University of Costa Rica located in Santa Cruz (Lat: America, bandwidth allocations to radar operators are 10.283720, Lon: −85.595166, Elevation: 60 m a.s.l). Santa usually limited to typically 250 kHz for radars operating Cruz provides the project with a quiet radio-frequency at central frequencies of ∼ 20–60 MHz. However, for the environment and sufficient land to expand the arrays or Costa Rica radar, a bandwidth allocation of 5  MHz was deploy other instruments if the need arises. In Costa Rica assigned to the project. An immediate advantage of the there were no tropospheric radars before this project large bandwidth is that the spectral content of the pulse started. Consequently, this was the first time that a VHF sent into the atmosphere can contain a wide range of radar could be used to obtain detailed information about frequencies. In the case of the radar in Santa Cruz, this tropospheric behavior and structure. bandwidth was used by creating a several MegaHertz The radar operates at a central frequency of 46.6 MHz wide chirp frequency inside the pulse. and was designed for low-altitude (1–6  km) studies. Pulses were coded using a chirped frequency which It comprises a group of 9 Yagi-antennas for transmis- varied linearly from the start to the end of the pulse by sion, and clusters of 4 antennas for reception. Initially 2  MHz. Deconvolution was used to greatly improve the range resolution (Hocking et  al. 2014). In order to carry out the deconvolution, the signal of the transmit- *Correspondence: marcial.garbanzo@ucr.ac.cr ted pulse was recorded along with the received signal. A 1 CIGEFI, University of Costa Rica, 2060 San Pedro, San José, Costa Rica Full list of author information is available at the end of the article radar pulse with a length of 1 km (3.336 µ s ) should have © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licens es/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 2 of 10 a resolution of 500  m, but by introducing the deconvo- are then estimated from the time series using a fast Fou- lution, the resolution was improved to less than 100  m. rier transform (FFT). Doppler peaks can be related to This improvement in resolution, coupled with the rela- properties of atmospheric targets. tively long pulse, resulted in strong, well-defined echoes Details about spectral peak location and determina- from the atmosphere, even with a low power transmitter. tion can be found in many references, e.g., Yamamoto The radar used a 1 kW peak-power transmitter to gather et  al. (1988), Hocking (1997), so we will not discuss information satisfactorily up to 6 km. The pulse repeti- these procedures in detail here. We have used a Gauss- tion frequency was 3 kHz, and a 64-point coherent inte- ian fitting method, in which we fit a function of the form gration scheme was also used. The total data-length per −(v−v )2o f (v) = A e 2σ2 + D to the spectra. The variables A, v , dataset was 20 s. o Another feature of this radar was that the hardware σ and D were determined using a nonlinear least squares used in the detection was minimized. Only one detec- fitting method. A is the peak value of the fitted Gaussian tor per channel of digitization was used; normally two function, and vo is the offset of the peak from zero Dop- are required for in-phase and quadrature signals, but pler shift (in units of meters per second). The value of σ is our use of high sampling rates (comparable to the radio associated with the width of the associated spectral peak frequency) and deconvolution procedures means that and is related in part to the velocity distribution of tur- the in-phase and quadrature components could be gen- bulence and hence the turbulent energy dissipation rate, erated in software after digitization on a single channel e.g., Jacoby-Koaly et al. (2002), although consideration of (Hocking et  al. 2014). Each detector consisted of just 4 the impact of so-called spectral beam-broadening is also amplification stages and one filtering circuit. No mixing very important, e.g., Hocking (1983). The value of D rep- or beating took place in hardware, minimizing addition resents the noise level. The total integrated power under of extra electronic noise and artificial frequency content. the atmospheric contribution to the spectrum was also Several different experiments took place during the determined in our analysis (after removal of noise) and years 2013–2014 and more than 190 complete days cov- was used as the main measure of backscattered power in ering almost a full year were recorded and later analyzed. this paper. Here we present results of analysis of powers, spectral After the spectral analysis was finished, the informa- widths and other related parameters. tion was compiled and studied in order to observe the Results are presented in two main categories, these atmospheric behavior for each day, since the radar echoes being (1) category I events, which refer to quasi-striated and their temporal evolution can teach us about different (layered) atmospheric echoes, and (2) category II events, atmospheric processes. The sequence used for the analy- which refer to echoes likely to be associated with the sis of each daily dataset consisted of several basic steps: planetary boundary layer (PBL from now on). Typical examples as well as more detailed statistics are presented 1. Observe general aspects of the day (long lasting for each category. echoes, strong echoes, isolated patches of intense returns, and their time evolution). Methodology 2. Identify echo layers based on integrated power, spec- In this section, different methodologies are described. tral width, and radial velocity. Our spectral analysis is described first, followed by dis- 3. Measure layer durations. cussion of the processes of obtaining information about 4. Match the layers observed with other phenomena the atmospheric echoes, their evolution, and their observed on the current (and even previous) day(s). classification. 5. Identify possible PBL behavior based on integrated We will refer to “atmospheric targets”, which will be power, spectral width, and radial velocity. taken to be any radar-scattering entity generated by natu- 6. Characterize the boundary layer by its maximum ral atmospheric events which result in radio-backscatter depth and growth rate. detectable by the radar. Aircraft and man-made targets 7. Identify regions of strong isolated backscattering are not considered, and clearly signals that are too weak (also referred to as “isolated patches of turbulence”, or to be detected will not be included either. Lightning IPoTs). is also excluded in our case. We also exclude meteors 8. Measure the horizontal extent of the IPoTs to quan- (which can appear in our data due to range-aliasing) and tify their presence in the atmosphere. transient phenomena. 9. Note any interesting events, transitions, or patterns. To begin, we look at Doppler spectral analysis. Initially the received signal is deconvolved with the signal of the This methodology was carried out for each of the avail- transmitted pulse (Hocking et al. 2014). Doppler spectra able daily datasets. In this manner the information was Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 3 of 10 quantized and statistics were calculated about the pres- the association of these echoes with turbulence seems ence of echo layers, characteristics of the PBL and justifiable. isolated patches of turbulence (IPoTs). The IPoTs (char- acterized by a lack of structure as they were not layers Category II: PBL or organized events) will not be treated in this paper, as Turning our attention to the top of the PBL, it can be more research is needed in order to better understand expected that this region should be a region of enhanced their morphology, time evolution and to provide possible turbulence and consequently should be detectable by interpretations. radar. The PBL grows in depth throughout the day, so Moving our focus on to regions of persistent returns, the associated echoes should ascend from low heights the methodology used to quantify scattering layer occur- in mid-morning to greater heights as the day unfolds. rence will now be discussed. Of course it is possible that We did indeed observe echo layers at heights consistent layers did occur which were too weak to be seen by the with the likely tops of the PBL, in line with this assump- radar, either due to weak turbulence, or due to the layers tion. These often rose in altitude quickly with time and being very well mixed, leading to weak backscatter. The could persist for several hours, in line with expected PBL latter were referred to as “ghost layers” by Hocking et al. behavior. We also note here that we use the term PBL in (2016), who also discussed the reasons for their exist- a general sense: a PBL can be either convective or stably ence, and such layers have been modeled by Fritts et al. stratified, but either has some potential to produce radar (2012). However, we do note that data recorded by Luce backscatter as long as suitable refractive index variations et al. (2002) do indicate that such layers are likely to be exist. So while we expect that many of the layers are con- rare—usually most turbulent layers produce some degree vective PBL (often denoted CBL); we will not make that of backscatter. In order to be specific, we will tend to use distinction here. the name “echo layers” to discuss radio-detected scatter- In such cases where the behavior and movement of the ing layers, to help emphasize the types of layers we are layers were consistent with this expected behavior, we detecting. used the echo layers as a proxy for PBL behavior. Such In regard to our classification scheme, we have based it behavior is consistent with the observations of Hashigu- on the number of echo layers measured and the fractional chi et al. (1995), for example. portion of the day during which they were present. We The echoes associated with the PBL do not appear so will discuss category I echoes (layered, or striated echoes) much as a “layer” in height-time contour plots, but rather first, then layers associated with the PBL (category II). as rising echoes, but of course in physical space they Within each category, various classes will be defined. would have extended over many tens or hundreds of km, and so would in fact be a physical layer. At times, addi- Category I: layered echoes tional category I echoes could be seen to evolve from the We first discuss category I echoes. These scattering residual motions of the PBL, and these were counted as layers generally formed and died at roughly the same layers in our category-I analyses. height, showing little sustained ascent or descent. They Just as with category I echoes, studies of the spectral are also at times referred to as “striated” echoes. These widths were important in helping determine whether the were subdivided into 3 sub-categories (referred to as scatter was from turbulence. Category II echoes generally “classes”), based on the number of occasions when one, appeared to be turbulent. two, and three or more quasi-horizontal echo layers were Two classes of echoes associated with the PBL were observed simultaneously. These classes were denoted L1, created. The first class included those cases in which the L2, and L3, respectively. This type of analysis was car- layering and temporal behavior could be considered as ried out for all the available days in the radar dataset. The relatively “typical”. The second class incorporated ech- length of time for which each layer was observed was also oes which exhibited behavior that was uncommon. The recorded. uncommon cases are not shown here but can be found However, radar scatter may not always be isotropic, and elsewhere (Garbanzo-Salas 2015). could even be anisotropic (possibly due to wind-shear These different echoes are discussed in the next section. effects and production of quasi-specular reflectors), thus leading to narrower spectra. For this reason we also care- fully determined the spectral widths associated with the Results and discussion echoes, and used wide spectral widths as a proxy for Layered echoes enhanced turbulence. In general, spectral widths were Figure  1 shows examples of integrated power, spectral quite wide for category I echo layers, so that in the main width and radial velocity for a single day (April 13th, 2014), highlighting category I-type echoes. Below about Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 4 of 10 a b c Fig. 1 Radar product used for analysis. Information obtained by Gaussian fitting to the Doppler spectra. Shown are a integrated power, b spectral width, and c radial velocity for April 13th, 2014. This type of product was generated for each day and used to categorize and classify the atmospheric echoes by recognizing various different characteristics and their associated time evolution 1.2 km altitude some receiver saturation is observed, but with a layer below. Also of interest is the fact that the it has been included because it helps show the lower lim- vertical velocities [panel (c)] alternated between posi- its of the useable data. tive and negative values at several times [particularly in Two strong and persistent category I echo layers, layer (1)], indicating either wave activity or some sort of labeled (1) and (2) in the figure, can be seen around mean organized vertical convection with large up-welling and altitudes of approximately 2.5 and 3.5  km, respectively, down-welling. The oscillations have a mean period of though varying in height throughout the day by ± sev- 7.65 min between 05:00 and noon (encircled by the long eral 100  m. Echo layer (1) is prominent throughout the flat ellipse), of 11.47  min just prior to noon (first verti- full 24-h period; layer (2) starts well before sunrise and cal ellipse), and changing to about 12.08 mins toward persists for a significant portion of the remaining day. the end of the oscillatory period (second vertical ellipse). Both layers are associated with enhanced spectral width These latter two values are close to the expected Brunt– [see panel (b)], suggesting significant sub-pulse-scale Väisälä period. We will not dwell specifically on these vortical and/or fluctuating motions (convection and/or oscillations, but they are certainly of interest and worthy turbulence). Spectral widths can reach as high as 1 m s− 1 of more detailed future study. They might be of value in in these cases. Just prior to noon, and persisting till just determining mean temperature gradients, e.g., see (Rött- after 2 pm, the whole region from 1 to 3.5 km altitude ger 1980). produced strong backscatter [labeled (3)], with layer (1) Below the altitude of 2.0  km, enhanced echoes are broadening substantially in depth and almost merging observed between ∼  22:00 and 02:00 LT and between Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 5 of 10 ∼ 08:00 and 12:00 [labeled (4)], but since no data are evi- turbulence associated with the growth of a convective dent below 1 km altitude, it is unclear if these are layers, boundary layer or a shallow PBL. or the top of a convective region extending up from the We now turn to a longer dataset consisting of nine con- ground. The vertical extrusions in region (4) are sugges- secutive days. This is shown in Fig.  2. Multiple striated tive of vertical plumes. echo layers were observed regularly during this period. The enhanced scatter just before 12:00 [labeled (5)] Data from April 6th to the 8th clearly show multiple seems distinct from regions (1) and (4). It apparently striated quasi-horizontal layers, with more than three merges with the turbulent layer (1) at around 2.5  km. layers visible simultaneously at different heights. For There is a possibility that it may be the signature of example, the layers on April 7th persisted for ≈ 60% of Fig. 2 Radar products for nine consecutive days, from April 6th to April 14th (2014). Three days per panel are shown with the dates specified at the top. Each day contains three graphs, these being (from top to bottom) (1) Integrated power, (2) Spectral width, and (3) Radial Velocity. The amount of time when no echo layer is observed is scarce. On occasions as many as five simultaneous echo layers could be observed (e.g., April 14th) Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 6 of 10 the day (14.2  h), and they were allocated to class L3 in were used as backup information in determining things our scheme. A case with one layer can be observed at the like echo lifetime and confirming that the character of 12 h mark of April 12th. This case was classed as an L1 the layer had not changed during its duration. case with a duration of 0.46 of a day (11.04 h). It is clear that the presence of striated echo layers is Note in the data for April 7th, a region of strong echoes greater during the dry season. The minimum percent- can be seen rising from 1 km altitude just prior to noon age of time during which only one layer was found dur- and reaching heights of almost 4 km by mid-afternoon. ing the dry-season months occurred in May, with 15.6% This is not a category I echo, but rather a category II echo, occurrence. A maximum of 32.1% temporal coverage of and will be discussed shortly. Other examples appear on all type L1 layers was observed during February. The L2 April 8th and 13th. class had largest occurrence during March with 37.2%, The data displayed in Fig.  2 are representative of the while the L3 class maximized in the following month of dry season (typically December to April). April, reaching a monthly maximum of 24.2%. Figure  3 Results of the statistics gathered about echo layers shows that there seems to be a local minimum in echo will now be presented. However it is worth first clarify- layers observed in July, although it must be remembered ing that June 2014 was missing in the analysis due to data that there were no data in June. storage issues. Furthermore, July, August and Novem- ber (2014) registered less than 10 days of useful data per month. However, information is presented as a fraction PBL echoes of the total measured time, so the values given below for We now turn our attention to category II echoes. As these months are still considered to be representative, noted above, examples could be seen on April 7th, 8th albeit with larger uncertainties than the other months. and 13th, but better examples are shown in Fig.  4. Fig- Statistics obtained with the L1/L2/L3 classifica- ure  4a shows a very clear example, which has been tion scheme are now presented in Fig.  3. Here, (and emphasized by adding a broken line to guide the eye. It also Fig.  5—to come later), all relevant statistics were is also labeled as “(1)”. The echoes rise out from the low- obtained using all three parameters of power, spectral est heights and ascend throughout the day. All graphs in width and vertical velocity. The primary parameter used Fig. 4 show similar events, except for Fig. 4c. was the power, but spectral width and vertical velocity Fig. 3 Echo layer results. Year‑long statistics of atmospheric layers. One (L1), two (L2), or more than two (L3) echo layers were observed, and the percentage of time that each type of layering persisted within each month (relative to available radar time) is presented. The maximum value is reached during the dry‑season months. The rainy season shows a decrease in the number of measurable layers. Error bars represent ± 1 standard deviation Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 7 of 10 a b c d e f g h Fig. 4 Typical common echo layer structures. Eight different cases of likely PBL evolution are shown, labeled (1) to (7). Panels a and b [cases (1) and (2)] correspond to continuous growth with full development of the PBL echoes as observed on February 18th, 2014 and December 28th, 2013, respectively. Panel c shows the case of February 8th, 2014 where no observable PBL echoes were found. Panel d) contains a case of truncated linearly growing PBL echoes (3) observed on April 7th, 2014. Cases for January 9th, May 4th, March 11th and February 13th (2014 for all the cases) are presented in panels e, f, g, and h, as (4), (5), (6), and (7), respectively, and exemplify linear growth of the echoes at the top of the PBL top. Note that the layers generally have different ascension rates Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 8 of 10 Cases of category I echoes in Fig.  4 are indicated 4th, March 11th and February 13th (in all cases for as “non-PBL echo layers”; these have been discussed the year of 2014). The four cases are presented to above and will not be considered further here. cover the most common scenarios observed for this Although all PBL echoes show similar general behav- type of growth. The echo layers for the first three ior, appearing in the mid-morning at around 1 km alti- cases (e–g) correspond to linear growth with verti- tude and rising steadily, they do vary in detail. Figure 4 cal ascent speeds of 9.1, 10.1 and 9.6 cm s−1 . A large shows examples of different types of evolution, labeled difference between the three cases is evident under- from (1) to (7). Strong quasi-linear growth can be neath the layers. Figure 4e) presents a region of weak observed during April 7th, while more modest growth scattering (red coloring) under the layer top, in the rates can be observed during other days (e.g., April 6th, body of the PBL. The cases presented in Fig. 4f and 8th and 9th). Examples of the PBL maximum heights g do not show such a “radar-quiet” region below are also marked in Fig.  4. For example, in Fig.  4e, the the layer top. Instead strong echoes are observed in maximum is marked as 4.2  km, this being the upper the f panel before sunset and less intense echoes are level of the echoes associated with the PBL. How- observed in the g panel, both with similar behavior. ever, one could argue that this might be too high, and These differences will be discussed shortly. The last some of these echoes, while strong, might actually lie case presented in Fig. 4h contains a linear but rather above the PBL top. However, we also examined spec- weak growth of the layer top. An ascension rate of tral widths for these echoes, and they were quite wide, 3.3 cm s−1 was estimated for this case, which rep- indicating turbulence. They could of course be small resents close to one third of the estimated value for plumes rising out of the PBL, but at present we cannot the other three cases just described. In this case, even distinguish that. Hence we will persist with our deter- with a slow growth, a region of only weak scattering minations of height in this way, using the upper lev- below the layer top is observed, similarly to Fig. 4e. els of enhanced turbulence associated with these PBL echoes, in order to at least maintain a consistent defi- As discussed above, while the top of the PBL seems to be nition. We see annual variability of over 1.5 km, which associated with enhanced echoes, the region below the is greater than any uncertainty associated with meas- top, and down toward the ground (referred to here as urements on any 1 day, thus making these data still of the “body” of the PBL) often generates little to no back- value. scatter, especially when the growth is continuous. Pan- Over 80% of days showed PBL-type layers. The classes els a and b in Fig. 4 are examples, but this feature occurs contained in Fig.  4 (primarily category II echoes) are at times for all types of category II echoes. Yet on other described in more detail here: occasions strong scatter from the body of the PBL can be seen. When scatter is weak from the body of the PBL, it • Continuous growth with full development Figure  4a may suggest weak turbulence, but it is also possible that contains data for February 18th, 2014. At the maxi- the air in this region has been homogenized by the tur- mum rate of growth the PBL top rises in altitude bulence, resulting in weak backscatter despite enhanced by ∼ 12 cm s−1 and reaches a maximum height of turbulence (e.g., see earlier discussions about “ghost lay- 3.8 km. Figure 4b displays the case recorded during ers”). This is an area for future study, but it is important December 28th, 2013. A clear PBL top is observed to to remark that the PBL echo layer growth rates are not grow, but with weaker backscatter strength than the particularly dependent on the absence or otherwise of previous case. After reaching a maximum of 2.9 km scatter from the body of the PBL. the height remains approximately constant until it In the previous paragraphs, several representative decays away near sunset (18:00 h). cases of echo layers were described. Of course not all the • Absent On some occasions there are no echo layers cases observed during the year-long experiment were as growing from the lower heights. Figure 4c contains a clearly defined or as simple to describe as the ones just representative case of this situation, showing data for discussed, but for the well-defined cases we had sufficient February 8th, 2014. data to allow determination of useful statistics. • Truncated Figure  4d, taken from data correspond- Minimum, maximum and average heights of the ing to April 7th, 2014, shows a clear echo layer with ascending echo layer top were calculated for each month linear growth that rose steadily in height at a rate of and are graphed in Fig. 5. The monthly means represent 14 cm s−1 for several hours and then suddenly disap- averages of daily means, and standard deviations of the peared at 3.75 km altitude at a time of 01:35 pm. daily values were also found and plotted on the graph as • Linear growth Figure  4e–h shows different cases of vertical “error bars”, these representing ± 1 half standard linear growth corresponding to January 9th, May deviation. As seen, the average height was at a minimum Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 9 of 10 Fig. 5 PBL echo layer results. The heights of the tops of the PBL as a function of month, using our echo layers as a proxy for PBL. The average value of the PBL per month, as well as the maximum and minimum heights observed for individual days, is shown. The average PBL top is at the greatest heights during May and August. A local minimum is observed during July, while the yearly minimum value was registered during January. The variability is shown as black vertical line bars which represent ± half standard deviation during the dry season and maximized during the rainy Other scatterers season. The maximum height of the layer top often Our focus in the last sub-section has been (to a large reached the 5  km maximum measurable height during extent) on cases where the layers actually appear to be the rainy season months. These cases were often associ- markers for the PBL and convective boundary layers (and ated with discrete layers or convection (not shown). especially their tops). However, we have already discussed The month of July (as observed in Fig.  5) shows a other types of scatterers (e.g., iPOTs) and of course the decrease in the average height of the ascending layer striated echoes, which may be largely independent of top. This decrease could be due to the limited data avail- the PBL. However, the various layers can also be mixed. able for this month, but there is also the possibility that For example, we have discussed the occurrence of cases the July dataset is impacted by a phenomenon known in where category I layers have “spun off” the PBL. In other Central America as veranillo. Also known as Midsum- cases, even PBL-type layers can show odd behavior. For mer Drought (Magaña et al. 1999), it is a multi-compo- example, on April 12th (see the data on the bottom panel nent event in which atmospheric conditions imitate the of Fig.  2), a weak category II layer could be interpreted dry-season months (December-April). This similarity to as occurring at midnight. Later on the same day, at noon, dry-season atmospheric behavior could cause the average a striated layer located at 3  km is displaced upwards to height to tend more to dry-season altitudes in July. 3.5 km and falls back to 3 km after 2.5 h. These types of Figure 5 shows that the average height reached a yearly activity seem to have nothing to do with the PBL, which maximum of 3.93 km during August. The minimum was was lower down so certainly other dynamics are at play registered during January, which showed a mean monthly there. height of 2.34  km. The ≈ 1.6  km difference between We have concentrated on events which can be fairly rainy and dry season average is large and represents a net clearly defined to satisfy one of our two categories: these increase of 68% penetration higher into the troposphere. other events represent areas of research for future study. This increase in depth can be caused by more (and/or stronger) ascending air currents and less wind shear gen- Conclusions erated by weakened easterly winds from the Caribbean The VHF profiler radar located in Santa Cruz, Costa sea. The minimum (yellow line) observed in Fig. 5 seems Rica, has been used to probe the lower troposphere. less conclusive regarding the observed maximum during The three parameters of backscattered power, spec- the rainy season. tral width and radial velocity have been combined Garbanzo‑Salas and Hocking Earth, Planets and Space (2019) 71:6 Page 10 of 10 to identify and characterize the atmospheric echoes. Author details 1 2 Strong backscatter was observed regularly in the form CIGEFI, University of Costa Rica, 2060 San Pedro, San José, Costa Rica. Uni‑ versity of Western Ontario, 1151 Richmond Street, London, Canada. of layers. These layers generally were stable in height with time (to within ± 1 km or so) and were referred to Acknowlegements as category I echoes. Thanks to the University of Costa Rica for the funds assigned to the Projects B1107 and B1108, the University of Western Ontario for the time and support However, on many days echoes were observed ascend- for the field trips, and Mardoc for providing some equipment and antennas. This ing in time after mid-morning, and decaying near sunset. work was made possible by the facilities of the Shared Hierarchical Academic These echoes seem to be well associated with the PBL, Research Computing Network (SHARCNET: www.sharcn et.ca) and Compute/ Calcul Canada. and in particular seem to highlight the top of the PBL. We have referred to them as category II echoes, and have Availability of data and materials taken them to be proxies for PBL height. Further verifi- Due to the large size of the dataset it would be provided on demand to the corresponding author. cation should nonetheless be undertaken using other instruments like radiosondes, and such radar/in situ Competing interests comparisons will be the subject of future research. The authors declare that they have no competing interests. Figures  1, 2, and 4 show representative layers, while Funding results of statistical analyses are given in Figs. 3 and 5. The funds for this research were provided by the University of Western Ontario, Figure 3 considers category I echoes (layers) and shows The Natural Sciences and Engineering Council of Canada, and the University of Costa Rica. frequencies of occurrence of days in which (i) 1, (ii) 2 and (iii) 3 or more layers (referred to as L1, L2, and L3 Publisher’s Note classes, respectively) existed simultaneously. The most Springer Nature remains neutral with regard to jurisdictional claims in published frequent occurrence of such layers was observed to be maps and institutional affiliations. in the middle of the dry season, during February [for Received: 28 February 2018 Accepted: 4 January 2019 the one layer (L1) case] and March [for the two layer (L2) case]. The least frequent occurrence of layers in our data occurred during July (for all the categories, though recognizing that we had no data for June). 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