Barros and Lang (2003) - University of California, Santa Barbara

JULY 2003 BARROS AND LANG1411FIG. 2. (a) Time series of daily rainfall during Jun 2001, for selected network stations. (b) Jun 2001 meandiurnal cycle of rainfall for high- and low-altitude gauge stations.pography forms part of the footslopes of the AnnapurnaRange, to the northwest. The main advantages of Besisaharwere its proximity to the core of the meteorologicalnetwork (e.g., a gauge station in the town ofKhudi lies just a few kilometers up the Annapurna trekkingtrail), and that it had access to good infrastructure(e.g., paved roads, commercial power, and reasonableaccommodations for project workers). Tansen lies 90km to the southwest (SW) of Besisahar, providing anintermediate location between the mountains and theplains of northern India. The launching site was locatednear the top of a small, relatively isolated mountain at1453 m MSL.The basic frequency of flights, observed at both sites,was five per day: 0000, 0600, 1200, 1800, and 2100UTC. See Table 2 for launch frequency at both sites.Launches were done on UTC time to better match operationalradiosonde observations and model analyses.Nepal is 0545 ahead of UTC time, so this correspondsto 6-hourly flights with one additional nocturnal flight(2100 UTC, or 0245 LNT) to provide more resolutionaround the time of the postmidnight peak in rainfall. Inthis case, the 1800 UTC launch precedes this peak, andthe 2100 UTC follows it. Launch times periodicallywere adjusted to match overflights by the TRMM satellite,and at times more than five launches per day (atBesisahar) were made as interesting weather conditionsdeveloped. When monsoon onset occurred (later shownto be after 10 June) launch frequency was increased to3 hourly (0000, 0300, 0600 UTC, etc.) at Besisahar tobetter resolve the incoming monsoon flow. This alsoprovided more detailed resolution of the atmosphericdiurnal cycle.Nepal provided many challenges in order to completea successful field project. Apart from the typical problemsassociated with developing countries, such as extralogistical strains, poor accommodations, and illness,there were difficult political barriers. The governmentmandatedmourning period following the tragic demiseof the king and queen of Nepal just as MOHPREX wasslated to start delayed the project. For the first few days,all launches had to be done late at night, resulting in alow frequency of at most two flights per day (Table 2).Information on sounding data quality control can befound in appendix A.3. Monsoon onsetDaily rainfall at five network stations is shown in Fig.2a. Monthly and seasonal totals can be found in Table1. Unlike the previous 2 yr, there was no major rainevent associated with the monsoon onset, where 100mm day 1 of rainfall fell at a majority of the stationsover 2 days (Lang and Barros 2002). Indeed, after some

1412 MONTHLY WEATHER REVIEWVOLUME 131FIG. 3. Time series of Besisahar and Tansen winds at four different atmospheric layers.significant early rainfall, the weather was relatively dryuntil 13 June. The most intense storms after this date(15, 19, and 24 June) produced rainfall accumulationsup to 100 mm in 1 day, but this was a local phenomenonrestricted to one to two gauges at most. However, rainfallwas persistent after 12 June, with most gauges receivingat least some rainfall each day.Average daily rainfall during June 2001 was 17.0 mmat high altitudes and 19.3 mm at low altitudes. Ten ofthe 17 functioning network gauges during this monthreceived over 500 mm of rain in June (Table 1). Thewesternmost ridge station (Pasqam Ridge) received 974mm, the most of any gauge. Khudi (closest to Besisahar)received 696 mm, but the Tansen rain gauge receivedonly 301 mm. The observed rain totals during June 2001were overall lower when compared with previous years(1999 and 2000). For example, Khudi received 771 mmin June 1999 and 981 mm in 2000, while Tansen had456 mm in June 1999 and 608 mm in 2000. The primaryreason for this was the lack of a major onset rainfallevent in 2001 (Lang and Barros 2002).Figure 3 shows the temporal evolution of horizontalwinds at Besisahar and Tansen from four atmosphericlayers, centered on 900, 700, 500, and 200 hPa. Nomenclatureand sign convention are standard, with positiveU being westerly flow and positive V being southerlyflow. The data show that the wind profile underwentconsiderable layer-dependent evolution during June asthe monsoon established itself. For instance, being dominatedby the diurnal cycle, near-surface winds (900hPa) at Besisahar exhibited no obvious change throughoutthe 3 weeks of the project, routinely switching betweensoutheast (SE) flow during the day to northwest(NW) flow at night. Tansen also showed considerablevariability but no long-term trends.At 700 hPa, winds were extremely light during earlyJune, but showed more fluctuations in intensity duringthe middle of the month. Eventually, ESE flow settledin Besisahar, though with strong variability in intensity[east (E) flow being strongest in the morning]. At Tansen,however, NW winds became increasingly weaker.Note that Tansen is more exposed at 700 hPa than Besisahar,which is situated very close to several 2000–3000-m ridges on the foothills of the Himalayan range(Fig. 1). These ridges serve as an obstacle to flow thatdoes not have a significant southerly component. Whenmesoscale depressions originating from the Bay of Bengalinteract with easterly vertical shear forced by themountains, an asymmetric secondary circulation developswith strong near-surface upslope (southerly) flow

JULY 2003 BARROS AND LANG1413FIG. 4. (a) Time series of Besisahar and Tansen precipitable water. (b) Same as in (a) but for CAPE.and return flow (northerly) at midlevel (see also Fig. 6in Lang and Barros 2002).The largest wind changes occurred aloft at 500 and200 hPa at Besisahar (nearly perfect agreement withTansen at 200 hPa). At 500 hPa, winds originally witha significant westerly component vanished after 10 June,becoming more easterly. A short-duration return to lightwesterly flow occurred after 15 June, but this soon wasreplaced by well-developed SE flow. The most dramaticreduction in westerly flow occurred at 200 hPa with adecrease in wind velocity from U 20ms 1 at thebeginning of the project to near zero after 11 June. Lightand variable westerly flow eventually prevailed towardthe end of the month. The north–south component at200 hPa was variable throughout MOHPREX. The overallimpression given by Fig. 3 is that of a reduction inthe strength of the westerly component of flow, if notan outright switch to SE winds as the monsoon establisheditself.The time evolution of precipitable water (PW) at Besisaharand Tansen is shown in Fig. 4a. While the diurnalcycle was a major component of PW variability, a significantincrease in the mean value of PW between 10and 15 June, remaining more or less constant after that.A similar pattern was found in the values of convectiveavailable potential energy (CAPE; Fig. 4b), a measureof atmospheric instability (Moncrieff and Miller 1976).Finally, the atmosphere warmed during the project, withan increase in freezing-level altitude on the order of 500m prior to 15 June (not shown).Based on all these data, it is apparent that monsoononset occurred during the period 10–15 June, manifestingitself in all notable fields (Figs. 2–4). While theonset process was a multiday phenomenon, we set theapproximate date for onset as 13 June, since by this timeCAPE and precipitable water had reached their maxima,while upper-level westerlies had become nearly quiescent.In addition, 13 June was the date after which rainfallwas observed on a daily basis in the Marsyandiregion.This time period is consistent with the developmentand onshore movement of a depression that originatedfrom the Bay of Bengal (Fig. 5), which placed centralNepal under a regime of moist SE flow, consistent withmonsoon onset conditions originally discussed by Langand Barros (2002). Specifically, onset monsoon depressionstend to form over the easterly notch of the monsoontrough over the Bay of Bengal, and propagate westwardon the northern flank of the westerly jet over theIndian subcontinent: the more southward the jet, theweaker the interaction with the easterly vertical shearalong the Himalayas, keeping the depressions awayfrom the Himalayan range, and favoring disorganizedconvection over northern India. Accordingly, the 2001onset depression moved mostly west from the bay, anddid not interact as strongly with the mountains as did

1414 MONTHLY WEATHER REVIEWVOLUME 131FIG. 5. NCAR–NCEP reanalysis 850-hPa winds at four different times during the 2001 monsoon onsetin central Nepal.the onset depressions from 1999 and 2000 (Lang andBarros 2002). Therefore, although the atmosphericstructure exhibited definite change, rainfall totals duringthe 2001 onset period were not as dramatic as in theprevious 2 yr because convection associated with thedepression did not reach the mountains. That is, localprocesses facilitated by the synoptic SE monsoon flowwere more important for rainfall during the 2001 onset.4. Diurnal cycleWhile the month-long evolution of the atmospherewas significant during the MOHPREX project, the previousfigures also demonstrate the importance of thediurnal cycle. In fact, for many fields (e.g., CAPE) thediurnal cycle was the strongest component of variability.Thus, further analysis of this variability is warranted.The diurnal cycle of rainfall during June 2001 isshown in Fig. 2b. High- (2000 m MSL) and low-(2000 m MSL) altitude stations were combined andaveraged together in 30-min bins to create this plot.Clearly demonstrated is the near-midnight peak in rainfallat both types of stations. Also, there is a lull inrainfall at both stations during the time period 0600–1200 local Nepal time (LNT). Average rain rates increaseduring the afternoon hours, although there is asignificant depression in rainfall near 1800 LNT at highaltitudes. The secondary afternoon peak is not as strongduring June 2001. This peak tends to be more apparentat the seasonal scale (Barros et al. 2000), where dominanttrends better overcome day-to-day variability.The methodology employed to calculate the diurnalcycle of sounding-derived variables is reviewed in appendixB. The diurnal cycle of precipitable water isconsistent with a late-morning/midday minimum inrainfall and a postmidnight maximum (Fig. 6a). Themaximum occurs during the period 2100–0000 LNT andthe minimum during 0900–1200 LNT. Soundings thatoccurred when there was no rainfall (see appendix B)show some differences, with a slight dip of 2 mm at0000 LNT compared to the all-weather soundings, anda secondary peak of 54 mm at 0300 LNT (not shown).Figure 6b shows the diurnal cycle of CAPE, whichhas a similar pattern to precipitable water, reaching amaximum over the period 1800–0000 LNT. This is the6-h period immediately preceding the nocturnal rainfallpeak in the Marsyandi network. The minimum in CAPEoccurs during the period 0600–0900 LNT, when observedcloud cover and rainfall were near their minimum.Considering only soundings with no observedrainfall (not shown), there is no significant deviationfrom this general pattern. Overall, the observations areconsistent with a gradual buildup of convective instabilityduring the day, which is then released after midnightleading to a stable atmosphere at the start of anew daytime period. Total column moisture and instabilityreinforce one another, and from a thermodynamicperspective the nocturnal rainfall peak is no surprise.

JULY 2003 BARROS AND LANG1415FIG. 6. (a) Diurnal cycle of Besisahar precipitable water. (b) Same as in (a) but for CAPE.In order to understand this thermodynamic behavior,the diurnal evolution of the vertical profiles of temperatureand mixing ratio were explored (not shown). At0000 LNT, absolute moisture is well above averagethroughout the depth of the troposphere, while temperaturesare slightly cooler than the daily mean, particularlyat high altitudes. This is an excellent combinationto produce high instability and precipitable water. Bycontrast, at 1200 LNT, moisture is well below averagein the troposphere and temperatures are average to aboveaverage. Intermediate times show some variability, butoverall reflect the trends between high (0000 LNT) andlow atmospheric instability (1200 LNT). What the datasuggest is normal diurnal heating and cooling of theatmosphere, which is perhaps lagged a bit by the highmoisture levels. Also, there is a daily replenishment ofmoisture occurring at all levels after the nocturnal depletion.The timing of these two processes leads to theobserved diurnal evolution of CAPE and precipitablewater.Note from Fig. 6a that the net daily depletion of atmosphericmoisture is 6 mm on average. This moisturesupply must be restored daily through local evapotranspirationand through advection by the monsoonflow. Figure 7 shows time series of relative humidity,evapotranspiration, and rainfall at Telbrung (station 19,Fig. 1) during June 2001. The hydrometeorological towerat Telbrung is equipped with sensors that fully monitorthe water and energy cycles including soil moisture,soil temperature, soil heat fluxes, and incoming and outgoingshortwave and longwave radiation fluxes. Evapotranspiration(ET) estimates are obtained indirectly fromthe latent heat flux, which is estimated as the residualof the energy balance equation. During the monsoon,ET varies between 0.5 and 6 mm day 1 , correspondingto an average daily evaporative fraction (ET/rainfall) onthe order of 0.15. Telbrung is a ridge location; at lowelevations, which do not see rainfall during the day, weestimate that the evaporative fraction correspond to averagepremonsoon values about 0.35 when soil moistureavailability is not a limiting factor. Although spatiallyvariable, these data imply that 15%–35% of the dailysoil moisture supply is locally or regionally recycled.The remainder must be restored daily through moistureadvection by the monsoon flow. Thus, analysis of thediurnal cycle of tropospheric winds is warranted.Let us consider the diurnal evolution of U and V inJune 2001 for three surface stations (Fig. 8). The Besisaharstation was a temporary station established nearthe sounding station to support the flights, whereas Koprungand Rambrong are high-altitude stations along theeastern and central ridges, respectively, of the Marsyandinetwork (Table 1, Fig. 1). There is a diurnalreversal of wind direction at Besisahar (SE during daytime,NW at night) indicative of normal valley flow,although the strongest winds are observed during thedaytime. By contrast, Rambrong sees no such reversal,only a slight veering of the enhanced daytime winds

1416 MONTHLY WEATHER REVIEWVOLUME 131FIG. 7. Time series of (top) relative humidity (RH), (middle) evapotranspiration (ET), and (bottom)rainfall observations at the Telbrung tower (station 19 in Table 1 and Fig. 1) during Jun 2001.FIG. 8. Daily cycles of surface winds at three network stations.

JULY 2003 BARROS AND LANG1417FIG. 9. Daily cycles of vertical profiles of winds at Besisahar.from SE to SW in the late afternoon. Koprung, at anintermediate altitude, has a very weak reversal in theN–S plane, but once again afternoon winds are strongestand mostly from the S to SE. As noted earlier, the asymmetrybetween day and night winds is observed elsewherein the Himalayas (Egger et al. 2000; Ueno et al.2001; Bollasina et al. 2002).Figure 9 shows the diurnal evolution of the verticalprofiles of U and V winds at Besisahar. Mean windspeeds are fairly light (5 ms 1 ) in this region, particularlyin the lower troposphere with behavior similarto the surface observations at Besisahar (Fig. 8), thatis, daytime upvalley winds and nocturnal downvalleyflow. The low-level flow is less southerly/more northerlywhen results from rain soundings are considered separately(not shown), suggesting that cool outflow fromrainstorms increases the downvalley tendency of windsin accord with the analysis of Steiner et al. (2003) inthe Alps. The diurnal variability of middle- and uppertroposphericwinds is less clear, mostly showing shortdurationfluctuations around the mean trend of increasingwesterly flow (i.e., veering of winds) with altitudeaway from SE flow in the midtroposphere.The main reason for the lack of an upper-level diurnalcycle is the evolution of the wind fields during thecourse of the project, which was much stronger than thediurnal cycle amplitude except near the surface (Fig. 3).That is, the most trust should be placed in the diurnalcycle results from the lowest parts of the troposphere,and regions aloft should be viewed skeptically sincetemporal evolution of the wind field was not removedwhen computing diurnal cycle.These results show that a southerly component of flowis present during much of the day (also observed at highaltitudesurface stations; Fig. 8), which is strengthenedby the daytime valley wind at low levels. Given thenegative gradient in atmospheric moisture between Indiaand the Tibetan Plateau during the monsoon, near-constantadvection of moisture into the Marsyandi regionis expected under these wind conditions (Peixoto andOort 1992). Increased near-surface southerly flow duringthe afternoon enhances this advection and, in concertwith the weak nighttime wind reversal, sets up the nocturnalmaximum in precipitable water.5. Case study: 13 June 2001An excellent example of nocturnal convection is thethunderstorm case of 13 June 2001 (local time). Whilenot the most significant rain event during MOHPREX,a TRMM overpass occurred during this storm, providinginsight into the vertical structure of nocturnal convectionin this region. This storm occurred during the monsoononset period, when the Bay of Bengal depressionwas slowly progressing westward across central India.Soundings at 1800 (prior to event) and 2100 UTC (duringevent) contained 2051 and 2276 J kg 1 of CAPE,respectively, which is suggestive of intense convection(Bluestein 1993). Figure 10 shows rain rates spread outduring the night and into the early morning hours for

1418 MONTHLY WEATHER REVIEWVOLUME 131FIG. 10. Time series of rainfall at select network gauges for the nocturnal precipitation event of 13Jun 2001. The time of the TRMM overpass is noted by the vertical dotted–dashed black line.available network stations. (Also noted is the time ofthe TRMM overpass.) The Ganpokhara (central ridge)and Pasqam Ridge stations received the heaviest rainfall.However, rain rates remained less than 10 mm h 1 atmost gauges.Although the TRMM overpass occurred during a timeof reduced rainfall at the network, there was still intenseconvection in the region (Fig. 11). In fact, during theentire 2100 UTC Besisahar flight, which matched thetiming of the TRMM overpass, observers noted light tomoderate rainfall. Two vertical cross sections are shownin Fig. 12: one through a core just east of Besisahar(bottom-left panel), and another through the bent lineof convection to the SE (bottom-right panel). The Besisaharcell demonstrates the typical vertical structureexpected of electrified tropical convection (Petersen etal. 1996). Peak radar reflectivities exceed 45 dBZ, butthe highest reflectivities are situated mostly below thefreezing-level altitude (5.5 km MSL). However, reflectivitiesin excess of 30 dBZ do extend up to 10 km,well above the 10C layer altitude at 7 km. Thethreshold of 30 dBZ above the 10C layer altitude isgenerally regarded as sufficient for electrification (Petersenet al. 1996). Not surprisingly, frequent lightningand thunder were observed at Besisahar.The SE convective line is much more intense thanthe Besisahar cell, consistent with its larger size. Indeed,40 dBZ extends several kilometers above the freezinglevelaltitude. While a time history of reflectivity structureis not available, the bent nature of the convectiveline is suggestive of retarding effects on storm motionby the orography.Meteosat-5 infrared imagery (not shown) from thistime reveals a classic pattern of nocturnal convectionin the Himalayas that is often observed during the monsoon.Strong convection associated with the depression(cf. Fig. 5) is present over central India, while northernIndia is very clear. However, multiple cells are linedagainst the Himalayan Mountains, with clear conditionsover the Tibetan Plateau.This case study shows that intense and organized convectivesystems, isolated from the major monsoon systemsover India, can occur along the footslopes of theHimalayas during the monsoon. Such systems can bevertically developed and produce frequent lightning,reminiscent of monsoon break convective storms thathave been studied in other regions of the world [e.g.,northern Australia; Rutledge et al. (1992), Williams etal. (1992), Cifelli and Rutledge (1998)]. This is consistentwith a monsoon that is in its nascent stages, aswas the case on 13 June.6. Regional-scale variabilityCurrently, there are no other radiosonde stations inNepal, although there are several in northern India, withina few hundred kilometers of the project site. In addition,there are two active radiosonde stations in Tibet

JULY 2003 BARROS AND LANG1419FIG. 11. Horizontal and vertical cross sections of TRMM-measured radar reflectivity for the 13 Jun 2001 event.(see MB for a map of radiosonde stations in this region).However, soundings from these regions are well knownto be among the most error prone in the world (Collins2001). During our analysis, we found many clearly erroneouswinds and temperatures, but by far the largestproblem was with the humidity measurements. In thelower troposphere, the air often was measured to be nearsaturation in the Indian soundings, despite the presenceof temperatures in the range 25–35C. This led to clearlyunrealistic mixing ratios greater than 25 g kg 1 .Wesuspect that wetting of the relative humidity sensor, particularlyduring rainstorms, is a chronic problem for theIndian flights. The other problem with the soundingswere the low frequency of flights (at best two per day;0000 and 1200 UTC) and frequent early terminationbefore reaching the tropopause, both of which hamperthe acquisition of statistically relevant numbers for comparisonwith the MOHPREX data.Therefore, in this study we decided to use the NationalCenters for Environmental Prediction–National Centerfor Atmospheric Research (NCEP–NCAR) reanalysis(Kalnay et al. 1996) for comparison with MOHPREX.Reanalysis data are available at 6-h intervals, 2.5 horizontalresolution, and 12 vertical levels in the range1000–100 hPa. The caveat is that over the Himalayas,the reanalysis data of interest (temperature, moisture,winds) are of course primarily derived from the aforementionedsoundings. However, this is after rigorouserror checking and balancing of any physically unrealisticvalues by the reanalysis model. The other advantageof using the reanalysis as a basis for comparisonis that the scientific community uses it extensively, andthus the MOHPREX data provide an excellent independentcheck on its ability to capture monsoon conditionsnear the extreme Himalayan barrier.Because of the coarse horizontal resolution of thereanalysis data (2.5), we do not expect good agreementwith the MOHPREX data below the local terrain envelope.The reanalysis cannot possibly resolve the steepmesoscale and microscale terrain features in the vicinityof the MOHPREX launch stations, which would havestrong effects on the MOHPREX winds through slopeflows and blocking. For example, ridges in excess of2000–3000 m MSL are located near the valley Besisaharsite. Tansen is more exposed near a hilltop, but ridgeson the order of 1000–2000 m MSL are nearby. However,above 700 hPa (3000 m MSL; the local terrain envelope)there is a better basis to compare the NCEP andMOHPREX data, since local terrain would have less ofan effect on the dominant large-scale flow at these altitudes.Thus, we place most of our focus on middleandupper-tropospheric data and determine how well thereanalysis matches the large-scale environment in theHimalayas.The first step is to compare the Besisahar and Tansensoundings themselves, to understand variability on the

1420 MONTHLY WEATHER REVIEWVOLUME 131FIG. 12. Vertical profiles of mean temperature, mixing ratio, and wind differences between Besisaharand Tansen.mesoscale (physical separation of the sites is 90 km).For all comparisons in this section soundings were firstinterpolated to a vertical grid with 5-hPa spacing. Figure12 shows the mean profile differences between the twosites for all relevant parameters. The sites had 22 soundingsin common with one another. On the whole, agreementis fairly good, suggesting not much atmosphericvariability is occurring on the mesoscale near the mountains.There is a tendency for the Besisahar profile tobe slightly cooler (1C difference) than Tansen.Tansen saw greater moisture in the lower troposphere,possibly due to the presence of surface vegetation at theelevated launch site (850 hPa altitude). However, therewas significantly more moisture (2 gkg 1 difference)in the midtroposphere above Besisahar. One reason forthis may have been the larger percentage of rain flightsTABLE 3. Specifics of the NCEP–NCAR reanalysis grid points usedin the analyses. Note that the elevation is for model topography only,and does not necessarily match the true elevation at that location onEarth.Letter Lat (N) Lon (E) Elev (MSL, m) DescriptionABCDE30.030.027.527.525.082.585.082.585.085.0435153978581966115PlateauPlateauHimalayasHimalayasPlainsat Besisahar, leading to its sondes traveling throughmore cloud layers. However, another possibility couldbe enhanced moisture convergence at midlevels due tothe blocking effects of the Himalayan range. At Besisaharthere is a tendency for stronger low-level easterlywinds, as well as more southerly flow throughout thetroposphere (except near the surface). The low-levelwind effects are most likely the result of the Besisaharvalley wind.In comparing the NCEP–NCAR reanalysis to MOH-PREX, we decided on five grid points for this intercomparison(Fig. 1; Table 3): two at 30.0N in the TibetanPlateau (points A and B), to the NW and NE; twoalong the Himalayas at 27.5N (points C and D), to theeast and west of the launch sites; plus one at 25.0N onthe Indian plains SE of the project site (point E). Table3 shows the relevant information for these grid points.There is good agreement in the wind time series athigh levels in the atmosphere (Fig. 13a). At 500 hPa(Fig. 13b), above the tips of the ridges by Besisahar, Uwinds match well with nearby grid points (points C andD). However, the reanalysis shows more variability thanthe MOHPREX data. Note how the late June increasein V wind at Besisahar is not captured as well by thereanalysis. Perhaps the best agreement between MOH-PREX and the reanalysis is seen at 200 hPa (Fig. 13b),well above the tallest ridges. Here the changes at Be-

JULY 2003 BARROS AND LANG1421FIG. 13. (a) Time series of 500-hPa winds for Besisahar, Tansen, and five surrounding NCEP–NCARreanalysis grid points (Table 3). (b) Same as in (a) but for 200 hPa.

1422 MONTHLY WEATHER REVIEWVOLUME 131sisahar and Tansen reflected the large-scale variabilityoccurring in the NCEP data. In particular, note how oncethe monsoon was fully established after 15 June, differencesin the U data occurred among the grid points.A reintensification of the westerlies occurs in the TibetanPlateau, while in northern India upper-level easterlieswere dominant. This pattern is indicative of thefamiliar upper-level monsoon high over the plateau(Murakami 1987). In between, weak E–W flow becomesestablished in the Himalayan grid points (C and D) andBesisahar.Mean differences in temperature, moisture, and windprofiles are shown in Fig. 14a (Besisahar-NCEP) andFig. 14b (Tansen-NCEP). We display characteristic terrainelevations in these plots to assist in understandingterrain influences. Most ridges in the immediate vicinityof the MOHPREX radiosonde sites do not exceed 2000–3000 m MSL, although 4000–5000 m are common elevationsin the nearby high Himalayas and Tibetan Plateau.Note how both Besisahar and Tansen tended tohave more midlevel moisture than the NCEP grid points.In addition, temperature profiles disagreed the most forthe Tibetan grid points as expected since the plateau isan elevated heat source relative to the surrounding atmosphere(e.g., Ye 1981). As seen in the wind timeseries, the best agreement between MOHPREX and thereanalysis occurred for the Himalayan grid points. However,there still tended to be stronger southerly flow atBesisahar, throughout much of the troposphere. Thiscould have been due to the N–S valley in which the sitewas located. Flow in the N–S plane through this valleywould be less obstructed than flow over the coarse reanalysistopography. There also could have been somechanneling of the flow by the narrow valley, increasingwind speeds. This would increase moisture advectionand may be a partial cause of the observed large differencesin midlevel moisture.The midlevel moisture differences have profound impactson total column moisture. Figure 15 shows timesseries of precipitable water for the reanalysis points andfor MOHPREX (cf. Fig. 4a). While PW at Tansen agreesrelatively well (within 25%) with the moistureamounts at the closest NCEP grid point (point C), Besisaharhas much more moisture, at times even exceedingthe precipitable water at the upstream NCEP gridpoint over northern India (point E). Most NCEP profilescapture the decreasing moisture in early June, but theincrease associated with monsoon onset comes up to 4days later than during MOHPREX. In addition, there isa late June moisture decrease in most of the NCEP datathat was not observed at Besisahar. Finally, despite havingfour time points per day the NCEP data show adiurnal cycle that is extremely weak compared to theone observed at Besisahar. Overall, there appear to beprofound disagreements between the MOHPREX andreanalysis data in terms of atmospheric moisture.According to the reanalysis, average June 2001 dailyrainfall in the vicinity of the Marsyandi network (usingpoint D) was 12.3 mm, compared to the 17–19 mmobserved on an average day at the network proper. Indeed,the observed range of rainfall totals at the networkis comparable to reanalysis-predicted rainfall on the outskirtsof the Bay of Bengal. This is also true for precipitablewater values. Clearly, significantly more moistureexists along the Himalayas than previously estimated.7. ConclusionsThe MOHPREX project has been described, and majorresults shown. The project was a great success consideringthe difficulties experienced. Good coverage ofthe diurnal cycle of the atmosphere (temperature, moisture,winds), as well as the onset and development ofthe early monsoon, were obtained for two sites alongthe Himalayas in central Nepal.The onset of the monsoon manifested itself as a gradualincrease in total column moisture and convectiveinstability, and a weakening of middle- and upper-troposphericwesterly winds. The lower-level winds weremore strongly affected by the diurnal cycle of slope andvalley winds. However, daytime upslope/upvalley windswere more intense than their nocturnal downslope/downvalley counterparts. This result is consistent withobservations in other parts of the Himalayas (Egger etal. 2000; Ueno et al. 2001; Bollasina et al. 2002) andsupports the theory of diurnal winds along tropicalslopes in opposing flows (Fitzjarrald 1984). In this case,the opposing flow is the monsoon winds. Also, the observationsare consistent with the presence of the Tibetansurface low pressure (Murakami 1987), whichwould tend to favor upslope flow over downslope (Zänglet al. 2001).The diurnal cycles of moisture and instability supportthe observations of a postmidnight maximum in rainfallin this region. A case study of a nocturnal storm wasshown, with intense vertical structure as revealed byTRMM. The data suggest that vertically intense breakconvection was prevalent during the early monsoon,consistent with a monsoon flow that had not fully establisheditself by the end of the project in late June.Based on the results of the MOHPREX project, apossible mechanism to explain the nocturnal peak inrainfall is shown in Fig. 16. Large-scale monsoon flowis presumed to be roughly constant during the day andnight, though it is of course subject to the variability inthe monsoon trough over northern India and the appearanceof monsoon depressions in the Bay of Bengal.The Besisahar wind data show weak diurnal variationwell above the surface, which supports this assumption.However, the mountains provide a huge barrier to theseflows, tending to block them and cause convergence,particularly at low levels. This convergence is reducedduring the daytime because diurnally forced upslope andupvalley flow reduce the spatial gradients in wind velocities.The upslope flow aids the formation of con-

JULY 2003 BARROS AND LANG1423FIG. 14. (a) Vertical profiles of mean temperature, mixing ratio, and wind differences between Besisaharand five surrounding NCEP–NCAR reanalysis grid points. (b) Same as in (a) but for Tansen and the reanalysisgrid points. The horizontal dotted lines note the pressure-coordinate heights of characteristic terrain elevationsnear Besisahar and Tansen (2000–5000 m MSL).

1424 MONTHLY WEATHER REVIEWVOLUME 131FIG. 15. Time series of precipitable water for Besisahar, Tansen, and five surrounding NCEP–NCAR reanalysis grid points.FIG. 16. Schematic diagram of the proposed mechanism to explainnocturnal rainfall in the Himalayas.vection at higher elevations, leading to the observedsecondary peak in rainfall there.At night, however, wind speeds become relativelyweak near the surface, even reversing direction in valleyssuch as the Marsyandi. Thus, near-surface windconvergence increases, leading to upward motion thatcan force convection. This convection is aided by thesteady increase of moisture during the day from advectionby the monsoon flow. In addition, atmospheric instabilityis maximized during nocturnal hours, due tointeraction between the steady increase in surface moistureand diurnal heating and cooling of the atmosphere.The near-coincident availability of moisture, instability,and surface forcing leads to the nocturnal peak in rainfall.This peak will be strongest at low elevations whereblocking of flow is most active, as observed.This is qualitatively similar to the processes that causepredominantly nocturnal rainfall over tropical islands,such as Hawaii (Chen and Nash 1994). On tropical islandsand in the Himalayas, the thermally forced diurnalflow interacts with the predominant environmental flow(trade winds in the case of Hawaii, the monsoon flowfor the Himalayas), causing nocturnal convergence andsubsequent precipitation.The nocturnal convergence could be aided by the interactionbetween mountain-forced gravity waves andthe thermodynamics of the Himalayan atmosphere,which MOHPREX shows is most favorable for noctur-

JULY 2003 BARROS AND LANG1425nal convection. In a numerical simulation of monsoononset in this region, Barros and Lang (2003) showedthat spatially fixed gravity waves form over the terrain,with downward motion strongest over ridges and upwardmotion strongest between ridges. Under normalmonsoon conditions, the SE flow that creates this gravitywave pattern is not as strong, but it should be enoughto force some waves. The waves occur as long as theforcing flow holds, and the upward-moving antinodesshould fire off convection when the thermodynamics arefavorable (near midnight).In summary, the proposed mechanism suggests a robustatmospheric system that strongly favors nocturnalconvection and rainfall. It also explains why this nocturnalpeak does not occur elsewhere, such as the TibetanPlateau or the northern Indian plains, and whythis nocturnal peak only occurs during the monsoon.The interaction of the ambient monsoon flow with thesouth slopes of the Himalayas, modulated by the diurnalvariability of atmospheric state, is the primary cause ofthe nocturnal peak. Future numerical modeling work isplanned to provide more support for this hypothesis.This hypothesis has advantages over one based on alow-level jet (LLJ), which is used to explain nocturnalconvection in the high plains of the United States (e.g.,Bluestein 1993). Although LLJs are expected to occurnear large mountain barriers at latitudes characteristicof the Himalayas (30), we find no evidence for diurnallyvariable barrier jets in either the MOHPREX orNCEP–NCAR reanalysis data. The Himalayas are adata-poor region, but strong LLJs may be preventedfrom forming in the vicinity of the Marsyandi network(and other portions of the middle Himalayas) due to thesteep and variable topography in this region. In addition,the long-term presence of the monsoon trough overnorthern India may play a role in preventing or alteringLLJs. Synoptically forced barrier jets, however, can beimportant for moisture and precipitation when monsoondepressions approach the Himalayas (Fig. 5; also Langand Barros 2002). However, under these conditions thelarge-scale flow tends to overwhelm any terrain-forceddiurnal effects (Lang and Barros 2002).The MOHPREX data provided an excellent independentcheck on the performance of the NCEP–NCARreanalysis in this normally data-poor region. Overall,the reanalysis data captured well upper-level winds thatwould not be as affected by the steep topography thatthe model is too coarse to resolve. However, moisturewas consistently underestimated by the reanalysis, havingprofound impacts on simulated rainfall amounts,which undercut observed values by about one-third. Thelow model resolution and lack of reliable observationsin this region are the most likely candidates to explainthis underestimation of moisture.Overall, it appears that the south slopes of the Himalayasstill are not well resolved by existing datasets,despite the presence of prodigious monsoon rainfall andlatent heating. The results of MOHPREX and from theMarsyandi network in general should go a long waytoward addressing these concerns. These data are beingused currently to evaluate simulations produced by acloud-resolving atmospheric model under typical monsoononset conditions in the central Himalayas. Theobjective is to elucidate the orographic effects on thespatial distribution of winds and rainfall, and to characterizeorography–convection interactions and theirimpact on the diurnal cycle of rainfall in the Himalayas.Acknowledgments. NSF provided funding for MOH-PREX under Grant EAR-9909498. Several participantswere also funded by NASA/TRMM under Grant NAG5-7781. Dr. Ana Barros was the principal investigator forMOHPREX. Dr. Timothy Lang was the lead scientistin the field during the project. The authors extend theirgratitude to the Nepal Department of Hydrology andMeteorology for the opportunity to conduct the MOH-PREX project. This project could not have been possiblewithout the dedicated efforts of Tank Ojha, our Nepaleseliaison, and his network of interpreters, porters, anddrivers. Dr. Jaakko Putkonen of the University of Washingtonparticipated in the first 2 weeks of the field phaseof MOHPREX, and manages the periodic collection ofMarsyandi network data at other times. Timothy Limof NCAR was the lead technician in the field for thefirst 2 weeks of the project, and was responsible fortraining project participants in the launching of radiosondes.We thank the graduate and undergraduate studentswho assisted in the field: Dr. Steve Nesbitt (Universityof Utah) and Casey Brown (Harvard University),as well as Elizabeth Korb and Martha Fernandes (bothof Tufts University). Dr. Rit Carbonne and Ned Chamberlainof NCAR, and Dr. Douglas Burbank and BethPratt of University of California, Santa Barbara, assistedin the planning of MOHPREX. Dr. Ramata Magagi ofHarvard University provided a very useful Meteosat-5perspective via satellite phone during the project (satellitedata were obtained from the University of Wisconsin/SSEC).Dr. Edward Zipser of University of Utahgave excellent feedback and guidance in the interpretationof results. Finally, the staff and ownership of theHotel Sayapatri in Besisahar are commended for theirexceptional hospitality toward project workers.APPENDIX ASounding Data Quality ControlThe radiosonde system used was the National Centerfor Atmospheric Research (NCAR) GPS/Loran AtmosphericSounding System (GLASS). The radiosondemodel used was the Vaisala RS 80-15GH. Both of thesesystems are standards within the meteorological researchcommunity, and more information on them (includingspecifications and measurement accuracy) canbe found at the NCAR Atmospheric Technology DivisionWeb site (http://www.atd.ucar.edu/rtf/facilities/

1426 MONTHLY WEATHER REVIEWVOLUME 131class/class.html). The GLASS system provides excellentvertical resolution with a time resolution of 1 s. Windsare measured by the GPS method.There were 150 sondes originally supplied to theMOHPREX project. However, as is typical of similarfield projects, some attrition in the form of launch failuresand data loss occurred. Nevertheless, launch failureswere typical to below average as compared to otherfield experiments with the NCAR GLASS system (T.Lim 2001, personal communication). Specifically,launch failures were about 10%–15% of all sondes, mostof these being partial failures with some (often most)data available. There were 121 sounding flights of analyzablequality for Besisahar (117 with at least somewind data), and 22 for Tansen (21 with at least somewind data; Table 2). Corrupted data were removed bya combination of standard hydrostatic checks and checkson erratic raw data at the timescale of 1 s. Correctedprofiles were carefully checked against less noisy flightsat similar times in order to confirm that the proper datawere retained. No obvious biases in observed fields werenoted.APPENDIX BDiurnal Cycle AnalysisThere were not enough soundings to compute a diurnalcycle at Tansen with acceptable statistical confidencein the results. Diurnal cycles for Besisahar soundingswere calculated in the following way. First, the dayin local time was split up into eight 3-h bins (0000,0300, 0600, 0900, 1200, 1500, 1800 and 2100 LNT).Then, soundings were placed in the appropriate bin ifthey occurred within 1.5 h of the bin time. Once allsoundings were distributed, mean values of all fieldswere calculated for each bin. Because Nepal is nearly6 h ahead of UTC and launches were based on UTCtime, flights did not deviate significantly from the centerof the bin period except when they were adjusted tomatch TRMM overpasses. Note from Table 2 that only11 of 23 project days had more than five launches, andonly 6 days had seven or more launches. The most commonoff hours were 0900, 1500, and 2100 LNT, soresults from these time periods should be viewed withextra caution, as fewer flights were available for analysis.Diurnal cycle analyses do not take into accountthe month-long evolution of the analyzed fields, whichwas discussed in section 3.Daily cycles of vertical profiles were calculated byfirst dividing up each vertical profile into 11 segments.Resolution of each segment was 50 hPa between 900and 700 hPa, and 100-hPa resolution above this layerup to 100 hPa. For any specific flight, all data withineach segment were averaged together. Then, each segmentwas binned in time and the daily cycle calculatedin the manner discussed above.In order to determine the extent to which the presenceof rain affected results, rain and no-rain flights also weredefined. 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